Testing strategic pluralism: The roles of attractiveness and competitive abilities to understand conditionality in men’s short-term reproductive strategies

Oriana Figueroa; Jose Antonio Muñoz-Reyes; Carlos Rodriguez-Sickert; Nohelia Valenzuela; Paula Pavez; Oriana Ramírez-Herrera; Miguel Pita; David Diaz; Ana Belén Fernández-Martínez; Pablo Polo

doi:10.1371/journal.pone.0237315

. 2020 Aug 31;15(8):e0237315. doi: 10.1371/journal.pone.0237315

Testing strategic pluralism: The roles of attractiveness and competitive abilities to understand conditionality in men’s short-term reproductive strategies

Oriana Figueroa ^1,², Jose Antonio Muñoz-Reyes ², Carlos Rodriguez-Sickert ³, Nohelia Valenzuela ^2,⁴, Paula Pavez ², Oriana Ramírez-Herrera ², Miguel Pita ⁵, David Diaz ⁶, Ana Belén Fernández-Martínez ⁵, Pablo Polo ^2,^*

Editor: Alex Jones⁷

PMCID: PMC7458284 PMID: 32866153

Abstract

The decision to allocate time and energy to find multiple sexual partners or raise children is a fundamental reproductive trade-off. The Strategic Pluralism Hypothesis argues that human reproductive strategies are facultatively calibrated towards either investing in mating or parenting (or a mixture), according to the expression of features dependent on the individual's condition. This study seeks to test predictions derived from this hypothesis in a sample of 242 young men (M ± SD = 22.12 ± 3.08) from Chile’s 5th Region (33֯ south latitude). Specifically, two predictions were considered that raise questions about the relationship between traits related to physical and psychological attractiveness (fluctuating facial asymmetry and self-perception of attractiveness) and competitive skills (baseline testosterone and self-perception of fighting ability) with short-term reproductive strategies. Our results indicate that psychological features related to the self-perception of physical attractiveness are related to short-term reproductive strategies. However, no evidence was found that fluctuating facial asymmetry, basal levels of testosterone and self-perception of fighting ability were related to short-term reproductive strategies. These results support the existing evidence of the importance of physical attractiveness in calibrating men’s reproductive strategies but cast doubts about the role of fluctuating facial asymmetry. They also suggest that traits related to physical attractiveness, in comparison to competitive capabilities, play a more important role in calibrating men’s short-term reproductive strategies.

Introduction

Reproductive strategies can be defined as an integrated set of adaptations that constitute solutions to different reproductive compromises or trade-offs that the individual faces [1]. The Strategic Pluralism Hypothesis seeks to explain inter- and intra-individual variation in human reproductive strategies based on the expression of traits dependent on the condition of the individual in interaction with the environment. This hypothesis emphasizes the costs and benefits for men and women concerning the resources invested in seeking partners versus providing parental care [1, 2]. In mammals, the trade-off for males between the search for a partner and parental care is particularly relevant given that males invest less than females in obligatory parental care while having a higher potential reproductive rate [3]. This means that for males, including men, maximizing reproductive success is mainly constrained by the degree of access to multiple reproductive partners [3, 4]. However, biparental care in humans may represent an important factor that affects offspring survival and, then, constrains men's reproductive success as well [5]. According to the Strategic Pluralism Hypothesis, individuals would display a mixture of short and long-term reproductive strategies, reflecting different degrees of investment in mating versus parenting effort according to individual phenotypic features and ecological and social conditions [1]. In this sense, the ability to attract partners and to compete with individuals of the same sex are two factors that affect the degree of investment in short-term reproductive strategies, given that they reduce the costs-benefits balance of investing in mating [6]. Consequently, the integrated study of these two factors is key to understanding their influence on short-term reproductive strategies.

Physical attractiveness is directly related to the capacity of being chosen as a mate [6–8]. Research in this field has identified a series of bodily features associated with attractiveness, facial symmetry being one of these [9, 10]. Fluctuating facial asymmetry has been proposed as an indicator of individual quality that reflects the capacity of an individual to maintain a symmetric pattern of stable development [9, 11]. However, there is mixed evidence supporting the notion of fluctuating asymmetry as a reliable indicator of the degree of developmental stability and individual quality [12–14]. In turn, fluctuating facial asymmetry is assumed to underlie individual differences in facial symmetry and, consequently, in attractiveness, but this assumption is rarely tested. Despite this, some studies suggest that men with relatively lower levels of fluctuating facial asymmetry are more attractive to women, especially for short-term relationships, are more economically successful, less faithful, and less inclined to invest in their progeny [15–18]. These results are consistent with evidence that men with lower levels of fluctuating facial asymmetry have more sexual partners and tend to be more direct in approaching the opposite sex in courtship, a characteristic that is related to short-term or unrestricted reproductive strategies [1, 8, 14, 19, 20]. Nevertheless, other investigations [10, 21] have not been able to replicate the association between the number of sexual partners and fluctuating facial asymmetry in men. These contradictory findings indicate the need to generate new studies in the field including psychological variables that may influence the relationship between fluctuating facial asymmetry and reproductive success to further test the relevance of fluctuating facial asymmetry as a signal of quality. Particularly relevant for our study is the relationship that has been observed between self-perception of physical attractiveness and the prevalence of short-term reproductive strategies since there is an association between self-perceived characteristics and received social signals, which together affect behavior. That is, the assessment of one's attractiveness is associated with short-term strategy because it reflects the preference of women for certain traits [6, 22, 23].

At the level of competitive abilities, intrasexual competition is another component that influences access to partners of the opposite sex [6, 24]. Testosterone is related to the development of the traits and behaviors related to intrasexual competition, as the display of direct physical aggression [25]. Testosterone is an androgenizing hormone with two main types of effects: organizational and activational. At the organizational level, it has an androgenizing effect during the prenatal stage and at puberty. At the activational level, baseline testosterone levels, as well as changes in circulating testosterone levels, have been associated with behavioral changes related to intrasexual competition and reproductive effort [26–28]. Focusing on baseline testosterone, there is evidence that baseline testosterone levels are positively related to dominance, competitiveness [28, 29], and especially aggression [30], although this effect depends on the specific context [27], and some studies reported a lack of evidence for the mentioned relationships [31]. Lower baseline testosterone levels have been associated with men’s relational status and paternity; that is, baseline testosterone levels are lower in men involved in long-term relationships, and especially in those that are fathers [32, 33, for a review see 34]. However, these levels can be expected to remain high in men who, although involved in a relationship, are interested in having extramarital relationships [35–37]. Puts et al. [38] studied the relationship between testosterone levels and three dimensions of sociosexuality: sociosexual desire, sociosexual behavior, and sociosexual attitudes. Their results indicate that testosterone levels are positively related to unrestricted sociosexual psychology (desires and attitudes), which results in a larger number of reproductive partners (behavior). However, they found that the number of reproductive partners has a negative effect on testosterone levels. They interpret these findings as a negative feedback mechanism that prevents maintaining high testosterone levels once sociosexual desires have been satisfied [38]. This suggests that testosterone plays an important role in the willingness of individuals to compete for reproductive partners, which in turn implies searching for short-term strategies. This relationship is complex because there are negative regulatory mechanisms. At the psychological level, testosterone increases competitive behavior and reduces cooperation in determined contexts [29] suggesting that testosterone levels influence intrasexual competition and reproductive success [28, 36, 39]. In addition, the self-perception of having traits related to resource holding potential, like being a good fighter, may be positively related to signs of dominance in competitive contexts [40] suggesting that this psychological feature plays a role when competing for mating.

Competitive abilities and physical attractiveness do not act independently. Lukaszewski et al. [23] examined the effect of body strength as an indicator of both fighting ability and attractiveness, and the self-perception of physical attractiveness, as well as the assessment by third parties of physical attractiveness and sociosexual attitudes and behaviors. Their results show that self-perception of physical attractiveness mediates the positive effect between physical strength, unrestricted socio-sexual attitudes, and the number of sexual partners. This indicates the need to explore how indicators of physical attractiveness and fighting ability explain unrestricted human reproductive strategies, using a larger number of morphological, physiological and psychological indicators.

Considering all of the above, we can establish that attractiveness and competitive abilities are important elements that according to the Strategic Pluralistic Hypothesis are expected to play a major role calibrating unrestricted reproductive strategies, especially when a combined effect of the two is displayed. However, there have been few studies that consider biological (morphological and physiological) and psychological variables in an integrated manner in order to understand how reproductive trade-offs are dealt with. The objective of this study is to investigate how the features of physical attractiveness and competitive abilities influence short-term male reproductive strategies and how psychological features may act as moderators of these effects. To do this, fluctuating facial asymmetry and the self-perception of physical attractiveness are considered as anthropometric and psychological features of physical attractiveness. Likewise, the levels of circulating baseline testosterone and self-perception of fighting ability are considered physiological and psychological features associated with competitive capacity. Considering the postulates of the Strategic Pluralism Hypothesis, we should expect a positive association between features signaling physical attractiveness and competitive capacity with short-term reproductive strategies because both sets of traits decrease the costs associated with seeking and competing for mates and satisfy the short-term mating preferences of women. Our particular predictions are as follows: (1) fluctuating asymmetry should be negatively associated with short-term reproductive strategies, especially in individuals with high levels of self-perceived physical attractiveness, whereas basal levels of testosterone should be positively associated with short-term strategies, especially in those individuals with high levels of self-perceived fighting ability; (2) the individual's competitive abilities are expected to moderate the effect of attractiveness on short-term reproductive strategies. In this way, attractiveness has a positive effect on short-term reproductive strategies that is greater for individuals who display higher competitive abilities.

Materials and methods

Ethics statement

The research was approved by the ethics committee of Universidad de Playa Ancha. Participants signed a written informed consent before they participated in the study.

Participants

The initial sample was composed of 246 young men. However, three individuals were rejected because they failed to complete some of the questionnaires and one individual failed to provide a photo, so the sample was reduced to 242 men between 18 and 35 years of age (M ± SD = 22.12 ± 3.08). The participants were recruited using ads posted in universities in the 5^th Region of Chile (33֯ south latitude). In terms of sexual orientation, 97.5% stated they were heterosexual and 3.5% stated they were homosexual. In terms of relational status, 53.8% stated they were in a couple at the time of participating in the investigation.

The participants received an economic reimbursement of five thousand Chilean pesos (approximately seven US dollars) as an economic reimbursement for participating, plus up to 30,000 pesos (approximately 43 USD and almost twice of the daily minimum wage) conditional on their performance in the economic games that were played as a part of a wider experimental procedure. These games were introduced and played after the measurement of basal testosterone and after the participants answered the questionnaires. Consequently, these games were not expected to affect responses and measures considered in this study.

Psychological measurements

Sociosexual Orientation Scale (SOI)

We used a multidimensional version of the SOI developed by Jackson & Kirkpatrick [41] that had been applied previously with Chilean subjects [see 42]. The scale is divided into attitudinal and behavioral dimensions. There are two attitudinal factors that measure sociosexual orientation in the short-term (e.g. “I can easily imagine being comfortable with and enjoying casual sex with different women”, 10 items) and long-term (e.g. “I am interested in maintaining a long-term romantic relationship with a special woman”, 7 items). These factors are in the format of 7-point Likert scale responses in which 1 indicates “strongly disagree” and 7 indicates “strongly agree”. The behavioral dimension consisted of 5 items of open-ended responses that included questions about the number of sexual partners in the past (3 items) (e.g. “Over your entire life, how many women have you had complete sexual relations with?), a question about sexual fantasies (How often do you fantasize about having sexual relations with women other than your current partner?) and a question about the expected number of sexual partners in the future (How many women do you think you will have sexual relations within the next five years). This study only considered the items referring to the attitudinal factor in short-term relationships. Polo et al. [42] obtained a Cronbach α value of .95 for the aforementioned factor, while a Cronbach α of .70 was obtained in the present study, indicating that the instrument is sufficiently reliable.

Self-perceived fighting ability questionnaire

We used a version of the self-perceived fighting ability questionnaire developed by Muñoz-Reyes et al. [25], which had been applied previously with Chilean subjects [see 43]. This is a short 4-question questionnaire that assesses the self-perception of fighting skills (1. How good a fighter am I? 2. How do others perceive my abilities as a fighter? 3. How much fear can I provoke in someone who is about to fight me? 4. What are my odds of winning a fight if I have to fight someone?). The responses are on a seven-point Likert scale in which 1 indicates "well below average" and 7 indicates "well above average". Muñoz-Reyes et al. [43] obtained a Cronbach α score of .84 in the original study. In the present study, an α coefficient of .87 was obtained, indicating adequate reliability for the studied sample.

Self-perception of attractiveness

This consists of a single question to assess self-perception of physical attractiveness (How attractive do you think you are?). The response is on a 7-point Likert scale in which 1 indicates “not attractive at all” and 7 indicates “very attractive”.

Anthropometric and physiological measurements

Fluctuating facial asymmetry

This indicator of attractiveness is measured according to the protocol of Sanchez-Pages & Turiégano [44]. Frontal photographs were taken of the participants with a Nikon D-90 camera under constant conditions of light, head orientation, focal length (3 m), shutter speed (1/60 s) and aperture (f/5.6). Participants were asked not to wear any form of facial adornment and to maintain as neutral an expression as possible. Photos where the subject smiled or inclined his head were rejected and we selected the best photo of each participant. Fluctuating facial asymmetry was calculate based on 106 facial points or landmarks (LM), which were obtained with the program FACE ++ [see 45 for a similar procedure, 46] from the selected photos. This software identifies high-precision facial reference points, like facial contours, eyes, eyebrows, and nose. The use of this software was automated with a MatLab software algorithm connected to the interface of programming applications of FACE ++. Fluctuating facial asymmetry was determined with the software MorphoJ [47] (also see http://www.flywings.org.uk/MorphoJ_page.htm) based on the Procrustes distances between each LM the corresponding mirror image LM. These distances reflect both directional and fluctuating asymmetry but can be decomposed in these two components employing a Procrustes ANOVA analysis since Procrustes coordinates are based on the algebra of sums of squares [48, 49]. In this sense, the variance attributable to the variable “side of the face” corresponds to directional asymmetry, whereas the variance attributable to the interaction between “side of the face” and “individual” corresponds to fluctuating asymmetry. In other words, fluctuating asymmetry was calculated as the deviation of each individual’s asymmetry from the overall average asymmetry in units of Procrustes distance. Accordingly, higher values represent higher levels of individual fluctuating asymmetry than lower values. In addition, we calculated the distribution of the differences between each LM and the corresponding mirror LM from each individual and each coordinate in order to characterize the nature of the variation in the asymmetry component. We found that differences between LM in the horizontal axis were normally distributed in 44 out of 48 pairs of LM, whereas differences between LM in the vertical axis were normally distributed in 45 out of 48 pairs of LM.

Baseline testosterone

Baseline testosterone was measured with a 1-ml sample of saliva of the participants, who were asked not to eat or drink anything other than water for at least one hour before the sample was taken. The samples were taken at approximately noon to avoid alterations in testosterone levels as the result of circadian hormonal variation. A passive saliva collection method (Salimetrics®) was used to collect samples. After being collected, the samples were centrifuged, frozen and stored at -20°C in cyrotubes (SalivaBio®) for 20 days. All the samples were analyzed with the Testosterone Enzyme Immunoassay Kit (Salimetrics®) in accordance with manufacturer’s instructions. Due to a problem with storing the saliva samples (freezing rupture and consequent increase in temperature), 136 individuals were not included in the baseline testosterone analysis because the intra and interplate variation coefficients were very high. Thus, the sample for all the baseline testosterone analyses was 106 individuals between 18 and 35 years of age (M ± SD = 22.34 ± 3.08). The coefficients of intraplate and interplate variation were respectively equal to and less than 15%.

Statistical analysis

To test our predictions, we fitted one general linear model in three successive steps. The two first steps tested our first prediction and the third step tested our second prediction. Fluctuating facial asymmetry, basal levels of testosterone, self-perception of physical attractiveness and self-perception of fighting ability were considered independent variables, and short-term socio-sexual orientation was considered the dependent variable. The age and relational status of the subjects were also considered as control variables. Independent and control variables were centered on their means. In a first step, we fitted only the main effects in the model. In the second step, we added the interaction terms between fluctuating asymmetry and self-perception of physical attractiveness and between basal levels of testosterone and self-perception of fighting ability. In this way, we tested our first hypothesis taking into account that nonsignificant interactions may preclude to assess main effects. Finally, we added the interaction term between fluctuating asymmetry and basal levels of testosterone and between self-perception of physical attractiveness and self-perception of fighting ability to test our second prediction. Because of the reduced sample size with the introduction of the baseline testosterone variable and because it did not have a statistically significant effect on the model, the variable was eliminated to restore the complete sample. A similar analytical strategy was used to test our predictions but without basal testosterone levels and using the full data set. Consequently, we finally fitted two models, one with the reduced data set and the other one with all the data available, in three successive steps.

The normality of residuals was verified for the two models and IBM SPSS 21.0 software was used for the general linear models. The level of significance was set at α = .05.

Results

Table 1 shows the descriptive statistics for the morphological, physiological and psychological variables. First, we show the results considering the reduced data set, that is, the model that considers baseline testosterone levels. We did not find any significant main effect of our variables (see Table 2). When introducing the predicted interactions between physical and psychological variables, we found that self-perception of physical attractiveness did not moderate the predicted relationship between fluctuating facial asymmetry and unrestricted sociosexual orientation (B = -236.910, t = -.685, p = .495). Similarly, the self-perception of fighting ability did not moderate the predicted relationship between basal testosterone levels and unrestricted sociosexual orientation (B = .002, t = .473, p = .638). Finally, when we considered the interaction between attractiveness and competitive traits at both levels (physical and psychological), we did not find that the interaction terms were significant, that is, basal testosterone levels did not moderate the predicted relationship between fluctuating facial asymmetry and unrestricted sociosexual orientation (B = 1.783, t = .435, p = .665), and self-perception of fighting ability did not moderate the predicted relationship between self-perception of physical attractiveness and unrestricted sociosexual orientation (B = .296, t = 1.125, p = .263). Overall, our results with the reduced data set showed that no variable was related to unrestricted sociosexual orientation neither when considering main effects nor the predicted interactions.

Table 1. Descriptive statistics for the variables employed in this study.

	Reduced sample (N = 106)		Full sample (N = 242)
Variable	M ± SD	Range (min, max)	M ± SD	Range (min, max)
Age	22.34 ± 3.08	18, 35	22.12 ± .3.08	18, 35
FFA	.016 ± .005	.008, .033	.016 ± .005	.008, .033
Basal testosterone	212.18 ± 75.91	94.31, 506.80
SPPA	4.77 ± .99	2, 7	4.68 ± .97	1, 7
SPFAQ	17.26 ±.4.78	5, 28	16.80 ± 4.88	4, 28
Short-term SOI	44.63 ± 13.62	10, 70	44.45 ± 13.42	10, 70

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Fluctuating facial asymmetry (FFA), self-perception of physical attractiveness (SPPA), self-perception of fighting ability (SPFA), short-term sociosexual orientation inventory (Short-term SOI).

Table 2. General linear model for short-term sociosexual orientation considering the reduced data set (N = 106).

Model	R²_adj	p		B	t	p	η²_p
Main effects	.041	.116	Intercept	47.568	22.937	< .001	.842
			RS = Paired	-5.107	-1.814	.073	.032
			Age	.306	.708	.481	.005
			Basal testosterone	.011	.611	.543	.004
			FFA	190.731	.691	.491	.005
			SPPA	1.594	1.106	.271	.012
			SPFA	.535	1.759	.271	.012
Interaction terms (1)	.028	.213	FFA * SPPA	-236.910	-.685	.495	.005
			Basal testosterone * SPFA	.002	.472	.638	.002
Interaction terms (2)	.037	.167	FFA*Basal testosterone	1.783	.435	.665	.002
			SPPA*SPFA	.296	1.125	.263	.013

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Relational status (RS), fluctuating facial asymmetry (FFA), self-perception of physical attractiveness (SPPA), self-perception of fighting ability (SPFA).

When considering the full data set, that is, setting aside the basal levels of testosterone, we found a positive relationship between the self-perception of psychical attractiveness and unrestricted sociosexual orientation when fitting main effects (B = 2.074, t = 2.215, p = .028; see Table 3 and Fig 1). However, neither fluctuating facial asymmetry (B = 208.547, t = 1.104, p = .271) nor self-perception of fighting ability (B = .195, t = 1.053, p = .293) were related to unrestricted sociosexual orientation. Moreover, self-perception of psychical attractiveness did not moderate the predicted relationship between fluctuating facial asymmetry and unrestricted sociosexual orientation (B = -203.254, t = -.945, p = .346). And finally, self-perception of fighting ability did not moderate the predicted relationship between self-perception of psychical attractiveness and unrestricted sociosexual orientation (B = .309, t = 1.742, p = .083). Overall, our results with the full data showed a positive effect of self-perception of psychical attractiveness over unrestricted sociosexual orientation being the other predicted effects non-significant.

Table 3. General linear model for short-term sociosexual orientation considering the full data set (N = 242).

Model	R²_adj	p		B	t	p	η²_p
Main effects	.050	.004	Intercept	46.532	37.430	< .001	.856
			RS = Paired	-3.870	-2.271	.024	.021
			Age	.493	1.772	.078	.013
			FFA	208.547	1.104	.271	.005
			SPPA	2.074	2.215	.028	.020
			SPFA	.195	1.053	.293	.005
Interaction terms (1)	.049	.006	FFA* SPPA	-203.254	-.945	.346	.004
Interaction terms (2)	.058	< .001	SPPA*SPFA	.309	1.742	.083	.013

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Relational status (RS), fluctuating facial asymmetry (FFA), self-perception of physical attractiveness (SPPA), self-perception of fighting ability (SPFA).

Fig 1 — Dots represent observed values. Full line represents expected values across the observed range of short-term sociosexual orientation. Dotted lines represent 95% interval confidence bands of the predicted values.

Discussion

The Strategic Pluralism Hypothesis explains the conditionality of human reproductive strategies and the resolution of the trade-off between investment in multiple partners and investment in parental care [1]. This hypothesis considers that there are biological, psychological and anthropometric factors that calibrate reproductive behavior according to the context in which the individual faces the aforementioned trade-off. This study proposes two predictions that were mainly not sustained as only an effect of self-perceived physical attractiveness on short-term sociosexual orientation was found. Our results emphasize the role of physical attractiveness in men on the unfolding of unrestricted reproductive strategies (short-term strategies at the scale of sociosexual orientation). The main result indicates that the traits of attractiveness have an effect on unrestricted reproductive strategies whereas fighting abilities do not.

The first prediction sought to determine if there is a positive relationship between traits associated with physical attractiveness and traits associated with competitive abilities with unrestricted reproductive strategies. Also, we postulated that these effects should be moderated by psychological variables related to self-perception of physical attractiveness and fighting ability, respectively. Our results suggest that only self-perceived attractiveness does affect unrestricted sociosexual orientation when evaluated with the full data set. However, we failed to show the expected effect of fluctuating facial asymmetry on unrestricted reproductive strategy either as a main effect or moderated by self-perception of physical attractiveness. Conversely to our results, some studies found evidence that fluctuating facial asymmetry is associated with the implementation of short-term reproductive strategies [20], with the number of sexual partners over one’s lifetime [50, 51], and with the perception of attractiveness [14]. Several lines of evidence may explain our contrasting results. First, despite that symmetry is associated with attractiveness, this association is weak and other facial features like averageness may play a more important role in perceived attractiveness [52]. Also, facial fluctuating asymmetry is assumed to underlie variation in facial symmetry between individuals being an indirect measure of overall symmetry. This circumstance may lessen its relationships with attractiveness. And more importantly, there exists controversy about the relationship between evolutionary relevant features and levels of fluctuating facial asymmetry [21]. If it is the case that fluctuating asymmetry is not an accurate proxy of developmental instability, the rationale about the importance of fluctuating facial asymmetry as a trait related to short-term mating strategies weakens and other variables as muscularity or strength could be better predictors of an unrestricted sociosexual orientation than fluctuating facial asymmetry [42, 53]. However, this is an unresolved issue as positive evidence about the importance of fluctuating asymmetry as a proxy of health and mating success was also reported [14]. An alternative explanation of our results is that fluctuating facial asymmetry affects unrestricted sociosexual orientation but this effect is mediated rather than moderated by psychological features. In this regard, previous investigations have found that the effects of morphological features on the psychology of unrestricted male sociosexual behavior were mediated by self and third-party perceptions of physical attractiveness [23, 54]. However, our cross-sectional design precluded us to investigated mediation relationships in an accurate way [55].

Self-perception of fighting ability was not related to unrestricted sociosexual orientation. In this regard, other studies have established a relationship between fighting and mate value [43], which is defined as “the complete set of characteristics that an individual has in a given moment and in a particular context that affects his capacity to successfully find, attract and keep a partner” [56]. According to Muñoz-Reyes et al. [43], fighting ability is associated with the mate value of a partner, which implies a positive relationship between this variable and men’s assessments of their chances of finding partners, and therefore of employing intrasexual competition strategies, which implies a high degree of self-confidence in the search for partners. It has been established that the self-perception of fighting ability is also associated with aggressive behavior [25]. These findings indicate that it is plausible to support that fighting ability is a conflict resolution mechanism in situations of intrasexual competition, which is consistent with studies that have found a positive association between traits associated with fighting abilities and reproductive success [23, 43, 57]. Despite the above evidence, our null results may indicate that self-perception of fighting ability when evaluated jointly with self-perception of attractiveness is not an important factor related to unrestricted strategies. That can be explained if we assume that mate choice or indirect competition through showing off attractive features may be more important in industrialized societies rather than the direct competition through fights.

No effect was found for baseline testosterone levels on short-term reproductive strategies. Studies have associated testosterone with the search for social status [58], self-confidence in competitive situation [39] and the adoption of dominant roles in economic environments [29]. Consequently, testosterone can be considered a social hormone associated with status-seeking and not so much with aggression in itself. Status in turn could be related to different reproductive strategies according to the way it is acquired. The relationship between testosterone and reproductive strategies has been explored in other studies and evidence has been found that favors the role of testosterone as a promoter of short-term strategies. For example, Edelstein et al. [32] found an interaction between unrestricted sociosexuality and the relational status of men and established that men in relationships with partners, but that have interest in extramarital relationships, have similar testosterone levels as those of single men, producing a positive attitude about unrestricted strategies. Puts et al. [38] established that there is a negative relationship between the number of sexual partners and baseline testosterone levels, and a positive relationship between high levels of baseline testosterone and unrestricted sociosexual psychology (desire and attitudes). Although this investigation employed a reduced version of the sociosexual orientation questionnaire [59], a relationship was found between baseline testosterone levels and an orientation toward short-term strategies. The reduced sample in the model that assesses the effect of baseline testosterone on reproductive strategies could explain the null result with respect to this variable.

Based on the second prediction, a relationship was expected between attractiveness and competitive abilities on unrestricted strategies. We failed to find that association as the effect of self-perception of attractiveness was not moderated by self-perception of fighting abilities. This result further suggests that fighting abilities do not play a major role in unrestricted sociosexual orientation both directly or moderating the effects of physical attractiveness. In addition, it is important to consider that self-perception of fighting ability may not necessarily be related to the willingness to compete for new mates, but may be also associated with the willingness to protect a current mate and the offspring. In this regard, that feature is expected to be related to more restricted sociosexual orientation reflecting a higher inversion in parental care [6].

Among the limitations of this research is the inclusion of only one anthropometric measurement (fluctuating facial asymmetry), which, although a common measurement to study physical attractiveness, could be complemented with others that are also considered attractive features and, in some cases, more important in explaining facial attractiveness [52]. Another limitation was the loss of data due to the storage of samples and handling of the testosterone kit, despite following protocols tested in other investigations. In addition, our null results of the effect of basal testosterone on sociosexuality do not preclude a potential relationship between testosterone changes elicited on a mating context and sociosexuality. Changes in testosterone levels and additional anthropometric variables associated with unrestricted strategies should be included in future research, such as facial masculinization [35], height [e.g. 60] and body mass [e.g. 42]. Finally, individuals in our study expected to participate in a competitive game and to be paid according to their performance. Despite that these tasks were performed after the measures taken for the current study; they may introduce some noise in the study.

In conclusion, the present study contributes some evidence that supports the Strategic Pluralism Hypothesis as we found that psychological features of attractiveness are related to unrestricted reproductive strategies among men. However, our results are not conclusive about the potential role of competitive skills (measured by basal levels of testosterone and self-perception of fighting ability) and the role of fluctuating facial asymmetry in explaining unrestricted reproductive strategies. These findings encourage further research on traits that may be affecting the cost-benefits balance in the reproductive trade-off that men have between maximizing the number of sexual partners and investing in parental care, and designs that allowed to investigate mediation relationships considering the importance of the relationship among anthropometric features on the self-perception (that is, psychological features) of subjects when the reproductive trade-off is solved.

Supporting information

S1 Dataset

(XLSX)

Click here for additional data file.^{(44.2KB, xlsx)}

S1 File. Normality and t tests.

(PDF)

Click here for additional data file.^{(141.5KB, pdf)}

Acknowledgments

We thank two anonymous reviewers and the editor for their valuable comments on earlier versions of the manuscript.

Data Availability

All relevant data are within the manuscript and its Supporting Information files.

Funding Statement

PP was founded by a FONDECYT Iniciación number 11181293 from the Chilean Government. OF was founded by a scholarship for doctoral studies from the Centro de Investigación en Complejidad Social of the Universidad del Desarrollo (number 20170806005S002). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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PLoS One. doi: 10.1371/journal.pone.0237315.r001

Decision Letter 0

Alex Jones

4 Mar 2020

PONE-D-19-27652

Testing strategic pluralism: The roles of attractiveness and competitive abilities to understand conditionality in men’s short-term reproductive strategies

PLOS ONE

Dear Dr. Polo,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

==============================

Dear Dr Polo,

Thank you for submitting your work to PLOS ONE, and apologies for the delay in returning the manuscript to you. I have now secured the opinion of two reviewers in the field. As you will see from their comments, they generally find much to like in the manuscript, commenting on the large sample size, the number of theoretically driven measures, and the testing of an influential theory in the area. They also highlight different concerns about the manuscript, which I agree with. The concerns of Reviewer 1 centred around the presentation of the hypotheses of the SPH, which was presented as an essentially dichotomous outcome for a given male, when in reality it is likely to be more continuous. The bearing on the results and interpretation should be considered. Reviewer 1 also raised concerns over the measurement, definition, and accuracy of the use of facial asymmetry as a proxy for attractiveness perception. This is something I also agree with, and have researched myself (e.g. Jones and Jaeger, 2019; Symmetry). I do not think that these are insurmountable issues and careful discussion and interpretation of the measures is required.

Reviewer 2 pointed out some more statistical issues with the manuscript, as well as the use of economic games which was puzzlingly mentioned once, and then is not touched upon again. I agree with Reviewer 2 in that much more detail is needed about the background of the work here. In my opinion, and after reading the manuscript, perhaps the most problematic part of the manuscript were the analyses and the modelling procedure. Reviewer 2 also pointed this out and I am in full agreement with them on this point; that there are too many models here testing too many hypotheses.

The main issue is that while you have sufficient data and variables to test a model of short-term SOI approaches, splitting variables across into different models obviously ignores the fact that in reality, these variables were measured from, and exist within, the sample of males you obtained. The data-generating process (DGP) is not being accurately tested here. In my view, you have the capacity to run build two models - one which includes all predictors and your full sample, which includes any theoretically important interactions that you wish to test, and another which is includes testosterone as a measure on the reduced sample for which those measures are available for. This way, you can be more certain in your conclusions about what predictors affect what in the presence of other variables, rather than splitting things up. Alternatively, you may wish to first account for the variation explained by age and partner status, and fit your models in a stepwise fashion. Either way, I do not think the current analysis is convincing in its conclusions as it fails to approximate the DGP. I also wondered about the transformations used here. Squaring a variable if it is somewhat non-normal will not make it more normal, but will lead to a kind of gamma-distributed variable (most values bunched up to one side of the distribution), but in any case the mathematics of ordinary least squares does not mind non-normally distributed variables for the predictors or the response; only that the residuals are normally distributed. This is mentioned implicitly but I couldn't tell if the residuals were normal or something was wrong with the predictors.

I was unsure about the use of the mediation analysis here. There are significant reservations about the use of mediation analysis in cross sectional designs - for mediation to be conclusive multiple time points are required, or at least with a cross sectional design heavily theoretical arguments and multiple measures used as mediators should be used to build confidence in the results. I think a far safer conclusion could be made by specifying an interaction between FA and SPA - for example, do men with lower levels of FA perceive themselves to be particularly attractive, and thus have a specific SOI score? To test this would require a simple interaction and centring of your variables.

The manuscript was well written and has a high quality data set (which I would encourage the authors share via the OSF, if they can), and I look forward to receiving a resubmission that addresses these concerns.

==============================

We would appreciate receiving your revised manuscript by Apr 18 2020 11:59PM. When you are ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

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Alex Jones

Academic Editor

PLOS ONE

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Reviewer #1: This study gathers together evidence in support of the Strategic Pluralism Hypothesis, the idea that males can allocate time and resources to either mate selection or the raising of children. The study is well done and believable, but the paper can be improved especially in the presentation of the Strategic Pluralism Hypothesis and its predictions.

The Strategic Pluralism Hypothesis is presented first, followed by a set of the sub-hypotheses. These sub-hypotheses are actually predictions of the Strategic Pluralism Hypothesis, but is unclear how they are related. They also need to be portrayed as predictions. For example, if SPH is true, then we expect a, b, and c to be true. Finally, what are the alternatives to the SPH? Science functions best when there are alternative explanations for phenomena.

The Strategic Pluralism Hypothesis is posed as an either/or affair, when it is really an entire spectrum. The two ends of the spectrum are 100% of the time and energy spent trying to mate with every available partner versus 100% of the time spent raising children with one partner. But the time and energy allocations might be 50/50 or 70/30. If the optimal allocations for males are 50/50, how would that effect the predictions? Would one be able to detect an effect? And although it isn’t mentioned in the text, sexually transmitted diseases might enter into the balance of selective forces as well.

Facial symmetry is employed as an indicator of facial attractiveness. The authors portray this symmetry/asymmetry as fluctuating asymmetry. But fluctuating asymmetry is a population parameter. Individual asymmetry may collectively represent fluctuating asymmetry, or it may represent directional asymmetry or antisymmetry or a mixture of all three kinds of asymmetry. The authors mention directional asymmetry when they mention the morphometric analysis and Procrustes distance, but that is the last time it appears. Was there detectable directional asymmetry?

The other two forms of asymmetry (directional asymmetry and antisymmetry) may be better indicators of male quality than fluctuating asymmetry. If low fitness males exhibit fluctuating asymmetry, the modal low fitness males will still be perfectly symmetrical. But if low fitness is associated with a transition from fluctuating asymmetry to directional asymmetry (or antisymmetry), then the predictive value of individual asymmetry is much better. There is a literature for this and evidence for transitions among the three forms of asymmetry in populations.

Throughout the text, the authors mention that fluctuating facial asymmetry is associated with attractiveness. This is a misleading way to portray this association, because it is symmetry, not asymmetry, that is considered attractive. Moreover, it is “individual” symmetry (or asymmetry) that potential mates respond to. Fluctuating asymmetry is assumed to underlie the variation in individual asymmetries. This is an assumption that must be tested, but it rarely is. At the very least it needs to be mentioned.

In addition, the evidence that fluctuating asymmetry is an indicator of genetic quality has been long discussed, and without resolution. It would pay to study (and cite) some of the papers by population geneticists. They generally aren’t sympathetic to the idea of “genetic” quality. How would you measure it? In what sense might a male have “quality” genes? This brings us to Darwinian and inclusive fitness. There are very few studies that have actually looked at fluctuating asymmetry and all possible fitness components (ability to find a mate, fertility, etc.).

For the measurement of fluctuating asymmetry, what is the measurement error associated with the approach? All studies of FA need to assess measurement error, because it can inflate estimates of FA. To do this, the authors need to take more than one photo of each person and then use FACE ++ and MorphoJ to estimate overall asymmetry and follow that with a variance component ANOVA. The methodology in this section is extremely unclear. I presume that a single, unitless number is produced (Procrustes distance), with larger numbers indicating more asymmetry.

For the statistical analysis, the relationship between the Strategic Pluralism Hypothesis and its predictions is unclear. You need to make clear, for example, that the SPH predicts a relationship between socio-sexual orientation and perception of attractiveness and facial symmetry. Moreover, there are no alternative hypotheses mentioned in the paper. If the results didn’t support SPH, what would they support?

The English is grammatically good, though somewhat wordy. In several places, I found the meaning of a phrase or sentence to be obscure. This is usually attributable to wordiness and passive voice.

In the figures, it would be more effective to spell out SOI, FFA, and SPA. I was finding myself having to look these up every time I came across them.

I have also included many comments on the manuscript itself.

Reviewer #2: The authors have conducted a novel test of a well-established theory that adds to knowledge in this area, contains different measures (e.g., biological, psychometric) and is conducted on a decent sized sample that may be deemed ‘non-WEIRD’. The manuscript is of interest to scholars in different fields. The authors find relationships between self-perceived attractiveness, self-perceived fighting ability and openness toward short-term sexual relationships, albeit one that may differ according to the model used (e.g. attractiveness appears to be more stable predictor).

Based on the information reported, I deem there no ‘fatal flaws’ in the work, but would suggest the work requires ‘major revision’ in light of the points below.

General method

I was a little bit concerned that this study appears to be part of an experiment involving economic games - this clearly functions as a competitive task but does not seem to be anywhere in the manuscript (unless I’ve missed something). Personally it seems like something that should be part of the current paper (regardless of whether it constitutes a ‘failed manipulation’) as it could feasibly alter testosterone levels (particularly with the size of the potential reward offered)? A good justification would be required for why this is treated as a ‘separate study’ and/or explanation of how it was part of the general study session. On a second point, you control for relationship status in your analyses but have a sample of men who have responded to items such as ‘….other than your current partner’ when around half the sample are single. I’m not sure if this represents an issue to consider/explain (see next point).

Statistical models and their relation to hypotheses

The way you setup your argument, I’m expecting to see a single model to test the relative/unique contribution of ‘mate quality’ and ‘resource holding potential’ on short term sexual strategies. You sort-of do this (and take into account the different sample sizes when including testosterone in the model). But at no point do you include symmetry in the same model as the predictors related to competitive ability, and I’m not really sure why. The use of the term ‘competitive’ (when describing perceived fighting ability as a proxy for competitiveness, like a personality trait) is a bit odd as people who consider themselves fighters might not be ‘competitive’ per se (i.e., they might not have to try as hard because they are good fighters). Finally on this issue, given the SOI questionnaire (see above) and effects of your control variable (relationship status), I did wonder why the predicted effects were not explored to see if they interacted with partnership status (i.e., stronger among single men, who are more likely to be competing for mates).

Stylistically, I found the analyses hard to follow, which was a problem as there are multiple models, and some effects altered beyond conventional significance in different models/with different control variables. I would suggest reporting the analytical strategy as each model is reported in the results. Then, depending on your response to my earlier point, I would suggest omitting tables and reporting all stats within-text, moving the stats from Table 1 to the appropriate point in the methods (or at the very least, provide a single table with sub-headings to show how the model is ‘built’).

Discussion

Based on above, I would revisit this section for clarity, so that the conclusions match the models, particularly your point on line 307 and line 330 (i.e., you don’t measure reproductive success – only one subscale of the SOI is applicable here and even this could be deemed ‘reproductive potential’ rather than ‘reproductive success’). ‘Three partially supported hypotheses’ is a bit vague/subjective in light of my reading of your results thus far. Line 348 is an important oversight (i.e., why did you examine testosterone if you did not expect it to be related to ‘reproductive strategies’). You then say on line 363 that there is an effect of testosterone! I would also relate the concluding paragraph more directly to the study data.

Minor points.

• Second sentence of abstract (better wording?)

• Line 46 – ‘highly valued’ implies a strong relationship between asymmetry and attractiveness (it exists across studies, but some papers find the relationship tends to be small in effect size).

• Around line 96 – I would suggest brief discussion of some null findings for balance, e.g. work by Michal Kandrik. I would suggest briefly discussing Quist et al. 2012 in the Discussion (as she finds that high SOI women prefer male facial symmetry).

• Line 100 – ‘more interested’ rather than ‘highly interested’ (the latter may imply a strong effect).

• Page 5, first paragraph – I was struggling to follow the logic of the argument here – why many sexual partners would reduce testosterone (and any accompanying citation), why you are arguing that these men are only competing for short-term partners, and why citation # 26 is then followed by discussion of self-perceived fighting ability. Please could you unpack/clarify?

• Just as a general thought - it’s a shame self-rated attractiveness wasn’t anchored to an ‘average’ in the same way that the fighting scale was.

• In the tables, I haven’t heard of the phrase ‘typical error’ – do you just mean ‘standard error’?

• First paragraph of discussion – a bit ‘wordy’ (please clarify/unpack).

General proofreading

Lines 33 (optionally); 36 (fifth region – plus specify roughly where this is – also applies elsewhere with ‘5th’); 66 (electing); 62 (please refer to males rather than men if Trivers is the citation); use of terms ‘relational’ and ‘residue’ (versus ‘relationship status’ and ‘residuals’); 291 (‘tested to determine’); 326 (senses).

Thanks for the opportunity to review this interesting research.

**********

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PLoS One. 2020 Aug 31;15(8):e0237315. doi: 10.1371/journal.pone.0237315.r002

Author response to Decision Letter 0

13 May 2020

RESPONSE TO EDITOR AND REVIEWERS COMMENTS

Dear Dr Polo,

R: We totally agree about the continuous nature of human reproductive strategies. We change some parts in the introduction be clearer about this In addition, we also have addressed the issues about fluctuating asymmetry as a reliable signal of quality and attractiveness. More details about the changes performed can be found in the answers given to reviewer 1.

R: We have provided more details about the background and answered the reviewer 2 concerns about the procedure.

R: Thanks for your comments. We have followed your advice and have changed our analyses to avoid mediation. Our original proposal was that more symmetric individuals were expected to show higher short-term sociosexual orientation partially because they perceived themselves as highly attractive. It is reasonable to think that it exists a causal relationship between FA and self-perception of physical attractiveness, although it is true that the later can be affected by multiple factors in addition to FA. A similar reasoning was made for testosterone and self-perception of fighting abilities. In addition, previous studies (e.g. Lukaszewsky et al., 2014), proposed a mediation effects between variables of attractiveness and fighting ability on short-term sociosexual orientation. However, as you mention, mediation analysis with cross-sectional data produces estimators that are biased and this is problematic, especially with full mediation effects. Consequently, we decided to change our proposal and consider now that psychological variables are expected to moderate the effect of FA and testosterone. This decision has produced major changes in the manuscript, but we think that it has gained in methodological rigor.

1. Lukaszewski AW, Larson CM, Gildersleeve KA, Roney JR, Haselton MG. Condition-dependent calibration of men’s uncommitted mating orientation: evidence from multiple samples. Evol Hum Behav. 2014;35(4):319-26. doi: 10.1016/j.evolhumbehav.2014.03.002

5. Review Comments to the Author

R: We have clarified and justified the predictions proposed this study based on expectations from the SPH. In addition, based on a suggestion from the editor, we have modified the analytical approach since mediation analysis applied to cross-sectional data may be problematic. The predictions now are based on moderation effects rather than mediation effects. We agree that an alternative hypothesis would enrich the paper, but we did not include them in the paper for several reasons. First, from an evolutionary perspective, the Strategic Pluralism Hypothesis and the Sexual Strategies Theory are the two main theoretical frameworks that aim to explain the variability in human reproductive strategies. However, both proposals are not incompatible rather they focus in different aspects of human reproductive strategies. We selected the SPH because is focused more in within sex variation and flexibility in the behavioral manifestations and we were interested in these aspects in this paper. Second, the other theoretical framework that aim to explain the variability in human reproductive strategies is the Social Role Theory. Unfortunately, our research design was not developed to test the influence of cultural of social norms in the expression of these behaviors as to do so we had needed a sample with a significant variation in cultural background and in the gender roles. Moreover, our study is aimed in within sex variability, and Social Role Theory mainly focuses in between sex differences.

R: We totally agree that reproductive tactics are a spectrum. Indeed, we decided to employ the SOI questionnaire developed by Jackson & Kirkpatrick because, in our knowledge, it is the only one that considers the attitudes towards unrestricted (short-term) and restricted (long-term) sociosexuality as two independent factors. In this paper, we focused on short-term reproductive tactics and postulated that the traits considered are affecting the predisposition of individuals to invest in short-term partners. Accordingly, we expected a higher unrestricted sociosexuality for those individuals with higher expression of attractive and competitive traits, regardless of their restricted sociosexuality. We have rewritten some parts in the introduction to make clear this point and to add ecological and social features as additional selective forces that play a role in the mentioned trade-off.

R: We employed Proclustes ANOVA analysis to decomposed variance in shape due to directional and fluctuating asymmetry. Therefore, directional asymmetry was detectable and took into account when calculating individual values of fluctuating asymmetry through this methodology. We have added more information about how we calculated individual values of fluctuating asymmetry in methods.

R: We agree with referee that there are three kinds of population asymmetries: fluctuating asymmetry, directional asymmetry, and antisymmetry. However, fluctuating asymmetry is the population-level measure of developmental instability, that accounts with the more robust theoretical support as a measure quality. In this sense, all individuals in population exhibit a determined level of fluctuating asymmetry, being the more symmetrical individuals, which possess the values near to zero. Therefore, the higher fitness males possess the lower values in fluctuating asymmetry. Directional asymmetry and antisymmetry usually have been considered as the result of strong genetic effect (e.g., 1). A better example for this is handedness in human being and their effect over biomechanical development of arm muscularity. As we stated before, we have followed the typical procedure of split fluctuating asymmetry form directional asymmetry, we knew that there is new proposal to integrate different kind of symmetry (2), however, we prefer to use the more validated methodology.

On the other hand, we agree with referee that there is evidence from transitions between fluctuating asymmetry to directional asymmetry in human being, for example for the orbital opening (3), but these analyses are centered in the study of symmetry from the effect of environmental or biological variables. We have not included these predictions in our theoretical model, because we are studying the effect over a determined behavior, and not the causes of fluctuating asymmetry. Therefore, we have not estimated transitions in symmetry, and have used the typical estimation.

Finally, despite that we consider fluctuating asymmetry as a measure of developmental instability, we mention in the new version of the manuscript that there is controversy about the importance of fluctuating asymmetry as a marker of this instability.

1. Özener, B. (2010). Fluctuating and directional asymmetry in young human males: effect of heavy working condition and socioeconomic status. American journal of physical anthropology, 143(1), 112-120.

2. Ekrami, O., Claes, P., White, J. D., Weinberg, S. M., Marazita, M. L., Walsh, S., ... & Dongen, S. V. (2020). A multivariate approach to determine the dimensionality of human facial asymmetry. Symmetry, 12(3), 348.

3. Tomaszewska, A., Kwiatkowska, B., & Jankauskas, R. (2015). Is the area of the orbital opening in humans related to climate?. American Journal of Human Biology, 27(6), 845-850.

R: Thanks for the comment. In the reviewed manuscript, we explicitly mention that we assume that fluctuating asymmetry is underlying individual differences in asymmetry. In addition, we clarify that it is facial symmetry rather than asymmetry the feature that is considered attractive.

R: In the new version of the manuscript, we acknowledge the controversial issue about the consideration of fluctuating asymmetry as an indicator of genetic quality. In addition, we have replaced the term genetic quality with a more general term of individual quality.

R: Following to our ethical protocol, we selected one picture of each individual and calculation of fluctuating asymmetry were performed on them. In former studies, two researchers placed the landmarks in each individual (39 landmarks) and we calculated error from this source. This time we decided to employ FACE++ to increase the number of landmarks to 106, but as the points were placed always in the same place for each picture, we lack measurement error.

The methodology has been clarified. Fluctuating asymmetry was calculated as the deviation of each individual’s asymmetry from the overall average asymmetry. To do this first, we calculated Proclustes distances that reflect both directional and fluctuating asymmetry. Then, we employed a Proclustes ANOVA to decompose variance in these two types of asymmetries. Accordingly, we obtained a single measure of fluctuating asymmetry for each individual with larger numbers indicating more fluctuating asymmetry.

R: We have clarified and justified the predictions proposed this study based on expectations from the SPH. We agree that alternative hypothesis would enrich the paper, but we did not include them in the paper for several reasons. First, from an evolutionary perspective, the Strategic Pluralism Hypothesis and the Sexual Strategies Theory are the two main theoretical frameworks that aim to explain the variability in human reproductive strategies. However, both proposals are not incompatible; rather, the focus is on different aspects of human reproductive strategies. We selected the SPH because it is focused more in within sex variation and flexibility in the behavioral manifestations, and we were just interested in this aspect in this paper. Second, the other theoretical framework that aims to explain the variability in human reproductive strategies is the Social Role Theory. Unfortunately, our research design was not developed to test the influence of cultural of social norms in the expression of these behaviors as to do so we had needed a sample with a significant variation in cultural background and in the gender roles. Moreover, our study is aimed in within sex variability, and Social Role Theory mainly focuses in between sex differences.

The English is grammatically good, though somewhat wordy. In several places, I found the meaning of a phrase or sentence to be obscure. This is usually attributable to wordiness and passive voice.

R: Thanks for the comment. We have tried to avoid passive voice and wordiness in this new version.

In the figures, it would be more effective to spell out SOI, FFA, and SPA. I was finding myself having to look these up every time I came across them.

R: In this new version, we avoided the use of acronyms.

I have also included many comments on the manuscript itself.

R: Thanks, we followed almost all your comments and suggestions. The exception is L77 in which we mention that men with lower levels of fluctuating asymmetry tend to be more direct in approaching the opposite sex. We agree with your comment, but we do not mention the proximal causes of this behavior so we decided to keep the sentence unchanged.

Based on the information reported, I deem there no ‘fatal flaws’ in the work, but would suggest the work requires ‘major revision’ in light of the points below.

General method

R: The general procedure involved economic games that were aimed to measure cooperative and aggressive tendencies as a part of a wider project. These games were played after the participants answered the questionnaires. Consequently, answers in the questionnaires were unaffected by games. Basal levels of testosterone were measured before participants played the games. But, as you mention, we unknow whether the knowledge of the potential reward, that was mentioned in the ads, affected testosterone levels. We have added more information about the games in the section of method and discuss the potential effect of the reward in levels of testosterone as a limitation of our study.

Regarding the second point, that question is the only one that considers paired individuals in its original formulation. We employed the same question but during the explanation we indicated that individuals that were not paired could answer that question considering how often they fantasize about having sex with women or men (according to their sexual orientations).

Statistical models and their relation to hypotheses

R: We did not include fluctuating asymmetry in the model including competitive ability because we found that its effect was fully mediated by self-perception of physical attractiveness. In any case, following a suggestion from the editor, we have modified the analytical approach since mediation analysis applied to cross-sectional data may be problematic, especially with regards to full mediation. Then, we investigated moderation effects rather than mediation effects in this new version. In addition, we have reduced the models to two, one considering a reduced data set and including measures of basal testosterone, and another with the full data set and excluding measures of testosterone. Each model was fitted in three successive steps in order to investigate main effects and the predicted interactions.

We agree with your comment about that good fighters may be not show a competitive personality, but our point is that being a good fighter reduces costs involved in intrasexual competition because may be associated to greater chances to win a potential fight or to signals that intimidate rivals. We have tried to clarify this in the text.

Finally, we initially expected that traits related to attractiveness and competitive abilities would be positively associated to short-term sociosexual orientation in single and paired individuals, although global levels of short-term sociosexual orientation could be affected by relationship status. The reason is that individuals in a relationship may be actually in an uncommitted relationship and then, competing and looking for mates. Nevertheless, we explored the interactions between partnership status and the predicted effects and were not significant.

R: We have reduced the models and we hope that analyses are easier to follow now.

Discussion

R: Due to the analytical changes, discussion has suffered a large amount of modifications. We agree, the sentence in line 348 is confusing, we meant to say that we did not find any effect of testosterone conversely of we had expected. We deleted the last part of the sentence to avoid confusions.

Minor points.

• Second sentence of abstract (better wording?)

R: We have tried to improve clarity.

R: This sentence has been changed.

R: Thanks for the comment. We have included null findings and stressed that the effect depends on the context.

• Line 100 – ‘more interested’ rather than ‘highly interested’ (the latter may imply a strong effect).

R:Thanks, we replaced the word.

R: We have modified this paragraph in order to be clearer in our argument.

• Just as a general thought - it’s a shame self-rated attractiveness wasn’t anchored to an ‘average’ in the same way that the fighting scale was.

R: In this new version, we have centered all the predictor variables to their means.

• In the tables, I haven’t heard of the phrase ‘typical error’ – do you just mean ‘standard error’?

R: Sorry, it was a mistake. We meant to indicate standard error.

• First paragraph of discussion – a bit ‘wordy’ (please clarify/unpack).

R: We tried to make clear this paragraph.

General proofreading

R: Thank you for the proofreading

Thanks for the opportunity to review this interesting research.

Attachment

Submitted filename: Response to Reviewers.docx

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PLoS One. doi: 10.1371/journal.pone.0237315.r003

Decision Letter 1

Alex Jones

5 Jun 2020

PONE-D-19-27652R1

Testing strategic pluralism: The roles of attractiveness and competitive abilities to understand conditionality in men’s short-term reproductive strategies

PLOS ONE

Dear Dr. Polo,

==============================

Dear Dr Polo,

Thank you for resubmitting the manuscript, and addressing the comments of the reviewers. Both reviewers have now responded, and there are a small set of outstanding changes to be made regarding some discrepancies with reported statistics and results discussed in previous reviews. These are only minor changes and I feel confident you can address them easily.

Very best wishes, and I hope you are all safe.

Alex

==============================

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Alex Jones

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PLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: (No Response)

Reviewer #2: (No Response)

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

Reviewer #1: Yes

Reviewer #2: Yes

**********

6. Review Comments to the Author

Reviewer #1: The manuscript is much improved over the original.

I have to respectfully disagree with this statement: “However, fluctuating asymmetry is the population-level measure of developmental instability, that accounts with the more robust theoretical support as a measure quality. In this sense, all individuals in population exhibit a determined level of fluctuating asymmetry, being the more symmetrical individuals, which possess the values near to zero. Therefore, the higher fitness males possess the lower values in fluctuating asymmetry. Directional asymmetry and antisymmetry usually have been considered as the result of strong genetic effect (e.g., 1).”

Imagine two populations: high-fitness individuals (blue) and low-fitness individuals (orange). The two curves differ in their variances. But in both cases, the modal individuals are still perfectly symmetrical. There are just fewer perfectly symmetrical individuals among the low-fitness population. Now imagine a transition from FA (blue curve) to antisymmetry (bimodal distribution) in the low-fitness individuals. If the low-quality individuals have an antisymmetric distribution of individual asymmetries, then most individuals are asymmetrical. There is abundant evidence for such transitions, even in the very first paper on fluctuating asymmetry by Kenneth Mather. When Mather selected for increased asymmetry in Drosophila, the population transitioned from FA to antisymmetry. And the heritability of antisymmetry and directional asymmetry is only slightly greater than that of fluctuating asymmetry, so the statement that such asymmetries are “the result of strong genetic effect” doesn’t hold up to scrutiny. See the very detailed papers by Larry Leamy. In some of my own (unpublished) research I’ve been able to document a transition from FA to antisymmetry by knocking down (RNAi) the activity of a single gene. These were in inbred lines with little genetic variation.

[See the figure in the attached file.]

Regardless, the authors need to document that the statistical distributions they are dealing with are symmetrical, unimodal, and with a mean of zero. Does a normal distribution fit the data? One just cannot wish away these other forms of asymmetry, and they may be important.

Reviewer #2: I am happy that the authors have addressed my concerns, except for the following relatively minor points:

• For completeness, please could you include brief critical details on the photography procedure, term the payment as ‘reimbursement’ rather than ‘incentive’, and provide (in-text) the age characteristics of the smaller sample who had testosterone measured.

• The manuscript reads well. One more proofread would be beneficial, e.g., for typos (e.g., ‘physical’ misspelt) wording that is ‘hyperbolic’ (e.g. describing something as having a ‘great’ influence when the effect may be small/moderate), and three overly-complex passages:

“according to the expression of features dependent on the individual's condition, such as a selective response to the reproductive trade-off”

“but there are variables linked to attractiveness (such as self-perception) that have a greater effect in terms of intersexual selection for the case of unrestricted strategies”

I also couldn’t understand the final concluding paragraph (except for its first sentence) – which also refers to mediation, which you no longer do.

Hypothesis 1 also seems oddly worded: “fluctuating asymmetry should be negative associated with short-term reproductive strategies, especially in individuals with high levels of self-perceived physical attractiveness…” – by that I mean it would read better if it referred to ‘facial symmetry’ – as I think you’re trying to get across here that this POSITIVE relationship (with symmetry) would be stronger in individuals who think of themselves as attractive (as they are better able to offset any costs of engaging in short-term mating competition)?

• You explained in the response that single individuals could answer one of the items differently on the SOI, which seems OK. I don’t think you report this in the manuscript, though. Related to this (and my earlier point on partnership status), please carefully do a final proofing check on analyses in light of the major changes made to the manuscript. For example, you’ve said in the response document that there were no effects of relationship status, but there does appear to be an effect in Table 3.

• I leave this as the Editor’s decision, but personally I thought the in-text results could be even more concise – as you seem to be reporting everything in tables, perhaps the text could simply refer to what was/was not significant, cross-reference to the tables, and save page space by not reporting sets of statistical values twice. Please could you also format (italicize) all statistical values.

• Apologies if I’ve missed this, but do you refer anywhere to the scale end points for facial fluctuating asymmetry (and what high/low scores mean), just to get a general sense when reading it of how variable the sample were (variability seems quite low according to the descriptive statistics).

• It might be useful to mention around limitations, briefly, that any null effects of basal testosterone in the current study don’t necessarily rule out relationships between T changes and sexual/competitive behaviours (e.g. if elicited experimentally via a confederate or other manipulation). Please also very briefly mention, in light of my prior comments, that the later competitive tasks might represent a small limitation introducing noise into the current study (i.e., they know they are attending at some point to engage in a competitive task and are being reimbursed a reasonable amount to do so, albeit these tasks are after the measures taken for the current study).

**********

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Reviewer #1: No

Reviewer #2: No

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

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PLoS One. 2020 Aug 31;15(8):e0237315. doi: 10.1371/journal.pone.0237315.r004

Author response to Decision Letter 1

20 Jul 2020

RESPONSE TO REVIEWERS

PONE-D-19-27652R1

Testing strategic pluralism: The roles of attractiveness and competitive abilities to understand conditionality in men’s short-term reproductive strategies

6. Review Comments to the Author

Reviewer #1: The manuscript is much improved over the original.

[See the figure in the attached file.]

R: Thanks for your comment. We have analyzed the distribution of differences between the original and the reflected landmarks from each individual from the coordinates in the asymmetry component provided by the MorphoJ after the Procrustes fit. For the x’s coordinates, we found that 44 out of 48 distributions were normally distributed and for the y’s coordinates, 45 out of 48 distributions were normally distributed. We mentioned these distributions in the methods section. Most of the distributions were not around 0 (40 out 48 for the x-axis and 35 out 48 for the y-axis). This result is in accordance with the significant effect of “side” in the Procrustes ANOVA analysis denoting the existence of directional asymmetry; however, our measure of fluctuating asymmetry took into account the directional asymmetry as we describe in the method section. Normality tests, descriptives and t-tests are reported in the attached file. We appreciate your critical feedback and now the manuscript reflects more accurately the controversy about the significance of fluctuating asymmetry as a reliable indicator of quality. In addition, it is relevant to establish that we have not found articles were antisymmetry was linked to behaviour in human beings. However, there is evidence that supports the presence of a certain degree of antisymmetry from skulls; therefore, we agree with the decision to include these analyses in the manuscript.

Reviewer #2: I am happy that the authors have addressed my concerns, except for the following relatively minor points:

R: Thanks for your suggestions. We have added information about the focal length, speed, aperture and light conditions in which the pictures were taken. We have changed the word incentive and provide more information about age in the reduced data set.

R: A general proofread has been made.

“according to the expression of features dependent on the individual's condition, such as a selective response to the reproductive trade-off”

R: We have deleted the last part of the sentence. Our intention was to signal that facultative calibration of reproductive strategies can be understood as an adaptative response to the trade-off, but it is not critical information and deleting it reduced the complexity of the sentence.

“but there are variables linked to attractiveness (such as self-perception) that have a greater effect in terms of intersexual selection for the case of unrestricted strategies”

R: We have amended the last part of this paragraph since some of the argumentation was related to former results.

I also couldn’t understand the final concluding paragraph (except for its first sentence) – which also refers to mediation, which you no longer do.

R: I am confused about your comment here. First, because in the first sentence of the last paragraph, we do not mention mediation. We mention mediation in the last sentence of the paragraph (maybe you meant to say last instead of first?), but here our point is to signal that designs that are more suitable to study mediation effects may contribute to understand the role of fluctuating asymmetry in future studies. Anyway, we have tried to be clearer in this paragraph.

R: We agree that it could be less confusing the use of symmetry instead of fluctuating asymmetry, however, we prefer to keep the term fluctuating asymmetry in the predictions because there are two more sources of asymmetry (directional and antisymmetry) and consequently, fluctuating asymmetry and facial symmetry are not 100% equivalent. In the introduction, we mention facial symmetry when talking about the relationship between symmetry and attractiveness, which includes fluctuating asymmetry.

R: Regarding the question about sociosexual desire “How often do you fantasize about having sex with someone other than your current dating partner?” I agree with you that seems to be intended to paired individuals (indeed I was imprecise in my first response), but the main aim of this item is to assess sexual attraction that is specifically targeted at potential mates to whom no committed romantic relationship exists. Since many single participants asked us that if they had to answer the question, we included in the general explanation a particular clarification on this point. We did not mention that because we only considered the short-term factor of the sociosexual attitude dimension due to the aim of this study.

Regarding relationship status, sorry if my previous response was not clear, but we meant to indicate that we explore potential interactions between relationship status and predictor variables (fluctuating asymmetry, self-perceived psychical attractiveness, basal levels of testosterone and self-perceived fighting ability) and we did not find any interaction effect. The result in Table 3 is related to the main effect of relationship status. In any case, we have carefully checked our analysis, and no inconsistencies in the results were found.

R: Thanks, we have amended the format of statistical values.

R: Facial fluctuating asymmetry was measured as the deviation of each individual’s asymmetry from the overall average asymmetry. This deviation represents a distance (unitless) in Euclidean space (Procrustes distance), therefore the minimum value is 0 (no deviation from the overall average asymmetry) and the value increases with the deviation from the overall average asymmetry. We have clarified the scale for facial fluctuating asymmetry in the manuscript.

R: We have included the suggested limitation in our discussion.

We appreciate all your critical comments and suggestions.

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PLoS One. doi: 10.1371/journal.pone.0237315.r005

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Alex Jones

24 Jul 2020

Testing strategic pluralism: The roles of attractiveness and competitive abilities to understand conditionality in men’s short-term reproductive strategies

PONE-D-19-27652R2

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PLoS One. doi: 10.1371/journal.pone.0237315.r006

Acceptance letter

Alex Jones

14 Aug 2020

PONE-D-19-27652R2

Testing strategic pluralism: The roles of attractiveness and competitive abilities to understand conditionality in men’s short-term reproductive strategies

Dear Dr. Polo:

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PERMALINK

Testing strategic pluralism: The roles of attractiveness and competitive abilities to understand conditionality in men’s short-term reproductive strategies

Oriana Figueroa

Jose Antonio Muñoz-Reyes

Carlos Rodriguez-Sickert

Nohelia Valenzuela

Paula Pavez

Oriana Ramírez-Herrera

Miguel Pita

David Diaz

Ana Belén Fernández-Martínez

Pablo Polo

Roles

Abstract

Introduction

Materials and methods

Ethics statement

Participants

Psychological measurements

Sociosexual Orientation Scale (SOI)

Self-perceived fighting ability questionnaire

Self-perception of attractiveness

Anthropometric and physiological measurements

Fluctuating facial asymmetry

Baseline testosterone

Statistical analysis

Results

Table 1. Descriptive statistics for the variables employed in this study.

Table 2. General linear model for short-term sociosexual orientation considering the reduced data set (N = 106).

Table 3. General linear model for short-term sociosexual orientation considering the full data set (N = 242).

Fig 1. Relationship between self-perceived physical attractiveness and short-term sociosexual orientation.

Discussion

Supporting information

Acknowledgments

Data Availability

Funding Statement

References

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Alex Jones

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Author response to Decision Letter 0

Decision Letter 1

Alex Jones

Roles

Author response to Decision Letter 1

Decision Letter 2

Alex Jones

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Acceptance letter

Alex Jones

Roles

Associated Data

Supplementary Materials

Data Availability Statement

ACTIONS

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