Towards an Integrated Concept of Personality in Human and Nonhuman Animals

Abstract

Every individual has an idiosyncratic way of feeling, thinking and behaving, which is relatively stable across time and situations. Usually known as Personality, today this phenomenon is recognized in many species, including arthropods, fish, avian or mammals. From an evolutionary perspective, research has shown that personality differences are manifest in distinctive forms of dealing with selective pressures, with consequences for fitness. Despite these facts, the study of personality in animals other than humans is relatively new. Only two decades ago, consistent behavioral individual differences were considered 'noise' around an optimal strategy for behavioral ecologists. Also, psychologists were not interested in animal personality as a consequence of the fear of anthropomorphization and the erroneous belief that humans are unique in nature. Fortunately, this misconception seems already overcome but there are still conceptual issues preventing a unified concept of personality. Throughout this review, we first explore the etymological origins of personality and other terminological issues. We further revise the historical course of the study of personality in humans and other animals, from the perspectives of Psychology and Behavioral Ecology, on the basis of the most used approach, the trait theory. We present the study of nonhuman primates as a paradigmatic example in between both frameworks. Finally, we discuss about the necessity of a unified science of personality.

Introduction

The fact that nonhuman animals possess behavioral idiosyncratic characteristics, which may be identified as ‘personality’, has never been a secret for pet owners, veterinarians or zookeepers (Gosling & Mehta, 2013). However, for many reasons, in the psychological and biological sciences this fact has been neglected. On the one hand, biologists, within the branch of behavioral ecology, were looking for environmental variables capable to predict regularities in behavior, ignoring individual variation. On the other hand, psychologist felt that classifying individual differences on other species by using the term ‘personality’, would fall into a risk of anthropomorphizing (but see, Kwan et al., 2008).

A large number of animal studies, particularly in the last two decades (Réale et al., 2010), have shown that, independently from other variables such as sex, age-class or dominance rank, individuals of the same species differ systematically in their behavior. Most of this variation is nonrandom and is consistent over time and/or across contexts (Réale et al., 2007), so it can be attributed to individual variation, it can be measurable, it shows significant cross-species generality for some dimensions (Gosling, 2008) and, subsequently, can be called ‘personality’.

Although anthropomorphism is a logical risk in comparative cross-species research, because of the extrapolation of methods from one area to another, we strongly believe that it is time to ‘animalize’ psychological research by the understanding that we are just another animal species.

Along this text, we will first explore the origins of the term and other terminological issues of personality research. Then, we will make a very brief historical review of the topic, on the basis of the most used approach, the trait theory, which is commonly applied in human and nonhuman animals’ personality research. Subsequently, we will compare results from different primate studies for, eventually, discuss if there is a common structure of primate personality.

What is Personality?

Etymological Origins

Personality is an every-day usage word that comes from the Latin word persona (i.e., person), which originally meant ‘mask of the actor’, the ‘theatrical mask’ of a character on a performance. Although its etymological origin is not well established, persona is possibly related to the Latin verb per-sonare (literally: sounding through), associated to the above-mentioned theatrical mask, which often included a small megaphone (Fig. 1). Other sources situate the origin of persona in the Etruscan word, ‘phersu’ (φersu, “mask”), derived from the Ancient Greek word πρόσωπον (prósōpon, literally: “in front of the face”). Through metonymy, that mask, the persona, became the technical term for referring a member of the theater, an actor, or to a social role, a citizen, in court. Then, in some moment during the XIV century, the term was popularized and began to be used as a synonymous of individual.

Fig. 1.

Examples of theatrical masks used on the ancient Greece. a) A terracotta comedy mask, 200–250 BCE. (Agora Museum, Athens); b) Bronze tragedy theatre mask, possibly by Silanion, fourth century BCE (Archaeological Museum of Piraeus). Photos by a) Giovanni Dall'Orto (2009) uploaded under the following license: Creative Commons ShareAlike 1.0 and b) Mark Cartwright (2015), uploaded under the following license: Creative Commons: Attribution-NonCommercial-ShareAlike

Personality can then be interpreted as the mask someone uses to interact with the world. There are yet, however, many issues around the definition of personality that must be overcome until we get to a consensus about this term. First, as many terms in the psychological sciences, the cosslloquial meaning of personality blurs the significance of scientific meaning (e.g., she has lot of personality; this wine has soft personality). In addition, even within the scientific community, there are multiple definitions for this term (see Table 1), depending on the scientific framework adopted (e.g., psychoanalysis, behavioral sciences, developmental psychology, personality psychology, affective neuroscience, evolutionary psychology or behavioral ecology). Finally, the etymological origins of the term personality denote a concept referring only to our species, but, as we will see below, this is not the case. In the first decade of the twenty-first century, with the incorporation of biologists and behavioral ecologists to the study of personality, with focus in animal profiles, efforts have been made in order to get to an unanimous definition (Réale et al., 2007).

Table 1.

An unsystematic list of definitions for Personality, Temperament, Coping Style and Behavioral Syndrome within some different theoretical approaches

Theoretical aproach	Term/Definition
	PERSONALITY
PERSONALITY PSYCHOLOGY	The dynamic organization within the individual of those psychophysical systems that determine his unique adjustment to the environment	Allport, (1937)
	Psychological qualities that contribute to an individual’s enduring and distinctive patterns of feeling, thinking, and behaving	Cervone & Pervin, (2013)
	The relatively enduring styles of thinking, feeling and acting that characterize an individual	Costa et al., (1995)
	That which permits a prediction of what a person will do in a given situation	Cattell (1950) apud Ellis et al., (2009)
	The commonly observable, more or less stable differences among people’s behaviors	Strelau (1987)
PSYCHIATRY	Individual differences in the adaptive systems involved in the reception, processing, and storing of information about the environment defines 'personality'	Cloninger, (1987)
EVOLUTIONARY PSYCHOLOGY	Stable individual differences in cognitive, emotional and motivational aspects of mental states that result in stable behavioral action (especially emotional) tendencies of humans and other animals […] Most theorists, in the context of cognition, refer to personality factors being manifested in stable thinking patterns	Montag & Panksepp, (2017)
	Personality differences can be conceptualized as alternative strategies for solving recurrent adaptive problems	D. M. Buss, (2009)
BEHAVIORAL ECOLOGY	Between-individual differences in behavior that persist through time	Carter et al., (2013)
	Individual behavioral differences that are repeatable over time and/or across situations	Réale et al., (2007)
	TEMPERAMENT
DEVELOPMENTAL PSYCHOLOGY	A set of inherited personality traits that appear early in life […] that are genetic in origin, like other psychological dispositions that are inherited (intelligence, e.g.) […] and that appear in infancy-more specifically, during the first year of life	Buss & Plomin (1975) in Goldsmith et al., (1987)
	Individual differences in the probability of experiencing and expressing the primary emotions and arousal	Goldsmith & Campos (1982) in Goldsmith et al., (1987)
	The stylistic component of behavior, that is, the how of behavior as differentiated from motivation, the why of behavior, and abilities, the what of behavior […] People differ in their motor activity, their intensity and equality of mood expression, their ease of adaptability, their persistence, or their degree of distractibility in the process of functioning	Thomas & Chess, (1977)
	Temperament consists of the individual differences in emotion, motor activation and attentional reaction to stimuli	Rothbart et al., (2012)
COMPARATIVE PSYCHOLOGY	Behavioral styles or tendencies (rather than discrete behavioral acts) that show continuity over time and can be identified in early infancy. [..] such styles are reflected in the degree and nature (i.e., approach vs. avoidance) of responsivity to novel or stressful stimuli, and nearly all temperamental frameworks include a dimension of intensity, distress, or emotionality	Clarke and Boinski, (1995)
	Temperament is considered a construct closely related to personality, and has been defined as the inherited, early-appearing tendencies that continue throughout life and serve as the foundation for personality	Gosling & Mehta, (2013)
PERSONALITY PSYCHOLOGY	The characteristic phenomena of an individual’s emotional nature, including his susceptibility to emotional stimulation, his customary strength and speed of response, the quality of his prevailing mood, and all peculiarities of fluctuation and intensity in mood; these phenomena being regarded as dependent on constitutional makeup and therefore largely hereditary in origin	Allport, (1937)
	Biologically based emotional and behavioral tendencies that are evident in early childhood	Cervone & Pervin, (2013)
	Since antiquity, the notion of temperament has been ascribed to these relatively stable differences in human behavior, which might be explained in terms of biological mechanisms	Strelau, (1987)
BEHAVIORAL ECOLOGY	Individual behavioral differences that are repeatable over time and/or across situations	Réale et al., (2007)
	COPING STYLE
ANIMAL PHYSIOLOGY	A coherent set of behavioral and physiological stress responses which is consistent over time and which is characteristic to a certain group of individuals	Koolhaas et al., (1999)
	Is a response to aversive situations (escape, remove, search, wait), i.e. to situations that induce physiological stress reactions, with an activation of the sympathetic-adrenomedullary system, the pituitary-adrenocortical system, and other neuroendocrine systems	Wechsler, (1995)
	The individual response to a stressor by which normally harmful physiological effects of this stressor are reduced	Schouten & Wiepkema, (1991)
	The suppression or reduction of deleterious effects of stress although the mismatch is objectively still present	Ödberg, (1989)
	BEHAVIORAL SYNDROME
BEHAVIORAL ECOLOGY	Correlation between rank-order differences between individuals through time and/or across situations and is therefore a property of a population. In contrast, a ‘behavioral type’ refers to the particular configuration of behaviors that an individual expresses and is therefore a property of an individual	Bell, (2007)
	Behavioral syndromes exist when the average phenotypes of individuals in one context/situation are correlated with the average phenotypes of the same individuals in a different context/situation such that populations harbor consistent individual variation in suites of correlated behaviors	Dingemanse et al., (2012)
	Correlations between different behaviors in a population	Herczeg & Garamszegi, (2012)
	Correlations between two or more personality traits through time or across contexts	Carter et al., (2013)

This is not an exhaustive list of definitions

Terminological Issues

Defining the concept of personality is also challenging due to confounding related concepts such as, temperament, copying styles, or behavioral syndromes. These concepts are not exactly synonymous to personality, but they are frequently used as if they were.

Personality

Psychologists use the term personality to refer to human psychological qualities that contribute to an individual’s enduring and distinctive patterns of feeling, thinking, and behaving (Cervone & Pervin, 2013). However, still there are many other definitions (Table 1). Pioneer on personality study, Allport (1937), evaluated 49 different definitions of the term and concluded for “the dynamic organization within the individual of those psychophysical systems that determine his unique adjustment to the environment” (Table 1). Many other definitions have been proposed since, and there is no consensus yet. Nevertheless, there are some well-established agreements: personality has traditionally been used to describe individual differences in adulthood (John et al., 2008), personality is composed by traits or domains (McCrae & Costa, 1997), personality differences are the result of selective pressures (B. R. Smith & Blumstein, 2007).

Temperament

Temperament is the preferred term for developmental psychologists (Cloninger, 1993; Goldsmith et al., 1987; Kagan et al., 1988; Zentner & Bates, 2008), consisting in individual differences in emotion, motor activation and attentional reaction to stimuli (Rothbart et al., 2012). Within this perspective, the basic ingredients of temperament are: early ontogenetic onset, a moderate stability, and distinctive physiological correlates (A. H. Buss & Plomin, 1984). For developmental psychologists, temperament refers to the biological and physiological bases of personality (Goldsmith et al., 1987; Strelau, 1987), creating the emotional substrate of some later personality characteristics. For example, reactivity to environmental stimuli in four-month-old infants derived into two different profiles (i.e., inhibited and uninhibited children) that could predict further anxiety (Kagan et al., 1988), which is usually related to Neuroticism.

In behavioral ecology, Réale and collaborators’ influential work defined temperament as “individual behavioral differences repeatable over time and across situations” (2007, p. 291). The authors, however, decided to use personality and temperament interchangeably (p. 294). Today, these authors adhered to the main-stream animal personality studies, using only the term personality (Montiglio et al., 2013; Réale & Dingemanse, 2012; Réale et al., 2010).

Coping Strategies

Coping styles or coping strategies, although sometimes used as synonym of temperament or personality (e.g., Réale et al., 2007), have a more restricted meaning, referring to individual differences in the behavioral and physiological reactions to challenging or novel situations (Koolhaas et al., 1999; Wechsler, 1995). Thus, a coping style is generally linked to the idiosyncratic response to stress, and it has been proven to be related to health and illness in human (Olff et al., 1993) and primate research (Capitanio, 2011; Sullivan et al., 2011). Since stress response is easily studied using animal models, this term is very popular in animal welfare literature (Broom, 1991, 1997, 2001; Koolhaas et al., 2010). Moreover, in the recent years, a growing interest in animal coping strategies in response to Human Induced Rapid Environmental Change-HIREC—has begun, on the basis that coping styles may have been shaped by evolution, being general adaptive responses in reaction to challenges in natural habitats (Ferreira et al., 2016; Japyassú & Malange, 2014; Sih, 2013; Wingfield, 2013).

Behavioral Syndromes

Behavioral syndrome is the term preferred by evolutionary ecologists, to whom the word ‘syndrome’ refers to a set of correlated characters (e.g., dispersal syndromes). Thus, a behavioral syndrome corresponds to a set of behaviors that are usually correlated over time and across situations (Sih & Bell, 2008; Sih et al., 2004a, b). The most studied behavioral syndrome shows that bold individuals are usually the most belligerent (e.g., Huntingford, 1976), resulting in the bold-aggressive behavioral syndrome. Behavioral syndromes explain the persistence in population of maladaptive behaviors, probably resulting from genetic constrains, that make behavior less flexible, according to ecological and evolutionary implications. Although considered “analogous to personality or temperament” (Bell, 2007), behavioral syndromes refer to the correlation among rank-order differences between individuals through time and/or across situations, so it is a populational not an individual property (in contrast to behavioral style).

Thus, temperament, personality, copying styles and behavioral syndromes present similarities, but also subtle differences that make these concepts not exchangeable. Depending on the target of an investigation and the scientific area in which it is included, researchers may use one of these terms, but it is an important issue to understand their differences.

The Science of Personality

First Descriptions

When this interest on human variation began? The idea that every individual manifest an idiosyncratic behavioral style, from an emotional nature, was already present in antiquity. The first recognized attempt of a personality theory formulation in Western cultures, occurred in the Ancient Greece. Around the fourth century BCE1 the Greek physician Hippocrates (ca. 460- ca. 370 BCE), who is also known as “the father of medicine”, formulated the humoral theory. According to this physician, the humors were vital bodily fluids that required to be balanced for pursuing a healthy body. Later, in the Roman period, another Greek physician, Galen (129–200 CE), based upon Hippocrates’ texts, postulated that the features of people’s temperament were related to the physical humors of the body: blood, black bile, yellow bile, and phlegm, corresponding to sanguine, melancholic, choleric, and phlegmatic temperaments, respectively. Dyscrasia (= bad mixture), or imbalance of humors, was thought to be a direct cause of sickness, while health, or eucrasia (= best mixture possible), correspond to equilibrated humors. Galen also recognized the influence of environmental factors, such as weather or diet, on temperament imbalance. However, since he was a physician, interested in health, he put emphasis on the relation between humors, illness and physical constitution (Stelmack & Stalikas, 1991). Today, some evidence that personality traits may moderate important health outcomes (e.g., cancer, heart disease, smoking) has been found (Booth‐Kewley & Vickers, 1994; Eysenck, 1985; T. W. Smith, 2006; Terracciano & Costa, 2004). Indeed, the COVID-19 pandemic-related challenges have revealed how personality traits can mediate individual responses and mental health (e.g., Osimo et al., 2021).

In the present day, Galen’s ancient formulation is considered pure folklore, however, two ideas remain alive in current theories of personality and temperament: (1) inherited biological factors are seen to underlie observable characteristics of individuals, and (2) emotions are seen as core and defining features of temperament (Clark & Watson, 2008).

A Science of Human Personality

The scientific study of personality begins by the end of the nineteenth century with the pioneering works of Francis Galton, (1822–1911), and William Stern, (1871–1938). Individual Psychology, as it was known along this time, emerged in opposition to General Psychology, which studied the general properties of psychical processes, common to all individuals (Sharp, 1899). Knowing later as Differential Psychology, this area of research developed taxonomies of the psychological characteristics of individuals, such as personality and intelligence, for understanding why and how people differ (Stern, 1910).

In the British tradition, Galton was the first to apply statistical methods to the study of individual differences. He was known for using questionnaires and collecting data on anthropometric features (i.e., weight, height, color of the eyes), for his research about the inheritance of intelligence. In the study of personality, Galton, (1884) proposed the "fundamental lexical hypothesis”, according to which, personality categories are cultural universals, defined in all languages, usually by a single word. The "lexical hypothesis" is the basis of the structural theories (i.e., personality is structured in traits) of human personality, developed after Galton by the most important personality psychologists, such as Allport, (1927), Cattell, (1943), and Eysenck, (1967). Traits tradition represent the predominant model of empirical research in human psychology, but we cannot ignore that there are other approaches (e.g., psychoanalysis, behaviorism, cognitive approaches), including those that nearly neglect personality (e.g., situational psychology: Mischel, 1969).

Personality Traits

Allport (1927, 1931) considered a trait as the basic unit of personality, consisting in “a general and habitual mode of adjustment which exerts a directive effect upon the specific response” (Allport, 1927, p. 290). Like Allport, Cattell (1943) also relied in trait as the basic unit of personality, the result of correlations among variables that were further disentangled through factor analysis. Along the historical consolidation of the trait theory, different minimum number of basic traits have been proposed, from three to sixteen (Cattell, 1943; Costa & McCrae, 1992a; Eysenck, 1991, 1992; Goldberg, 1990). Currently, the most widespread taxonomies are the “Big Five” (Goldberg, 1990), which relies in a pure lexical methodology, and the "Five-Factor Model" (FFM) (Costa & McCrae, 1992a), which employs self-rating scales. Besides the methodological differences, driving to subtle divergences in the facets inside each model (e.g., warmth is a facet of Extraversion in the FFM, but it belongs to Agreeableness in Goldberg’s Big Five), both models mostly coincide in facets within each trait, and so, the scientific community considers both taxonomies equivalent (John et al., 2008) calling them indifferently Big Five (and we do so along this text).

The basic factors of the Big Five model are Neuroticism, Extraversion, Openness to experience, Agreeableness, and Conscientiousness (remembered by the acronym OCEAN, coined by John, 1990, p. 96). Since they are the result of a factor analysis, they are hierarchically sorted by the amount of variance explained, from maximum (Neuroticism) to minimum (Conscientiousness). For a better understanding of each trait is convenient examining facets assessed by the scale (Table 2). Neuroticism assesses adjustment versus emotional instability; identifies individuals prone to psychological distress, unrealistic ideas, excessive cravings or urges, and maladaptive coping responses. Some authors (Goldberg, 1990) preferred this trait reversed and called it Emotional stability. Extraversion assesses quantity and intensity of interpersonal interaction; activity level; need for stimulation; and capacity for joy. Openness to experience assesses proactive seeking and appreciation of experience for its own sake; toleration for and exploration of the unfamiliar. Agreeableness assesses the quality of one’s interpersonal orientation along a continuum from compassion to antagonism in thoughts, feelings, and actions. Conscientiousness assesses the individual’s degree of organization, persistence, and motivation in goal-directed behavior (McCrae & Costa, 2006, pp. 46–47).

Table 2.

Personality and Temperament Taxonomies including traits, definitions and facets, within different theoretical approximations

BIG FIVE/FFM		HEXACO			AFFECTIVE NEUROSCIENCE		TEMPERAMENT		BEHAVIORAL ECOLOGY
Questionnaire (NEO-PI-R)		Questionnaire (HEXACO-PI)			Questionnaire (ANPS)		Questionnaire (CBQ)		Behavioral tests
Costa & McCrae, 1992a, b		Lee & Ashton, 2004			Davis & Panksepp, 2003		Rothbart et al., 2001		Reale et al., 2007
NEUROTICISM (N)		Emotionality (E)			FEAR/SADNESS		Negative affectivity		Aggressiveness
Worrying, nervous, emotional, insecure, inadequate, hypochondriacal	Anxiety Hostility Depression Self-consciousness Impulsiveness Vulnerability	Defined by anxiety, vulnerability, sentimentality, lack of bravery, and lack of toughness, but not anger or ill temper		Fearfulness Anxiety Dependece Sentimentallity	FEAR was defined as having feelings of anxiety, feeling tense, worrying, struggling with decisions, ruminating about past decisions and statements, losing sleep, and not typically being courageous	Anxiety Worried Indecisive	Neuroticism	Frustration/Anger Fear Discomfort Sadness Low Soothability	an individual’s agonistic reaction towards conspecifics	Aggressive
					SADNESS was conceptualized as feeling lonely, crying frequently, thinking about loved ones and past relationships, and feeling distress when not with loved ones	Lonelyness Distressed Crying
EXTRAVERSION (E)		Extraversion (X)			PLAYFULNESS		Extraversion/surgency		Activity
Sociable, active, talkative, person-oriented, optimistic, fun-loving, affectionate	Gregariousness Activity Level Assertiveness Excitement Seeking Positive Emotions Warmth	Defined by talkativeness, energy and cheerfulness (though not by bravery and toughness)		Expressiveness Social Boldness Sociability Liveliness	PLAY was conceptualized as having fun vs. being serious, playing games with physical contact, humor, and laughter, and being generally happy and joyful	Fun Happy Contact seeker Social Joy	Positive emotion	Activity Low Shyness High-Intensity Pleasure Smiling & Laughter Impulsivity Positive Anticipation	the general level of activity of an individual	Active vs Inactive
OPENNESS (O)		Openness to Experience (O)			SEEKING		Effortful control		Boldness
Curious, broad interests, creative, original, imaginative, untraditional	Fantasy Aesthetics Feelings Ideas Actions Values	Former known as Intellect/Imagination factor. Is one of the most controversial because of Intellect. Imagination aspect, subsuming traits such as originality and creativity, appears to be a robust common element		Aesthetic Appreciation Inquisitiveness Creativity Unconventionality	SEEK was defined as feeling curious, feeling like exploring, striving for solutions to problems and puzzles, positively anticipating new experiences, and a sense of being able to accomplish almost anything	Enthusiastic Explorative Anticipator	Conscientousness	Attention Control Inhibitory Control Perceptual Sensitivity Low-Intensity Pleasure	an individual’s reaction to any risky situation, but not new situations	Bold Docile Tame Unfearful vs Shy, Untamed, Fearful
AGREEABLENESS (A)		Agreeableness (A)			CARING ( +) /ANGER (-)				Sociability
Soft-hearted, good-natured, trusting, helpful, forgiving, gullible, straightforward	Straightforwardness Trust Altruism Modesty Tendermindedness Compliance	Defined by good-naturedness, tolerance, and agreeableness, but also including anger and ill temper at its negative pole (different to Big Five)	Forgiveness Gentleness Flexibility Patience		CARING was defined as nurturing, being drawn to young children and pets, feeling softhearted toward animals and people in need, feeling empathy, liking to care for the sick, feeling affection for and liking to care for others, as well as liking to be needed by others	Empathetic Afective Social bonding			an individual’s reaction to the presence or absence of conspecifics (excluding aggressive behaviour)	Social Sociable vs Asocial
					ANGER was defined as feeling hotheaded, being easily irritated and frustrated, experiencing frustration leading to anger, expressing anger verbally or physically, and remaining angry for long periods	Irritated Frustrated Backfired
CONSCIENTIOUSNESS (C)		Conscientiousness (C)							Exploration
Organized, reliable, hard- working, self-disciplined, punctual, scrupulous, neat, ambitious, persevering	Self-discipline Dutifulness Competence Order Deliberation Achievement striving	Defined by such content as organization, hard work, carefulness, and thoroughness (rather than moral conscience). Therefore, this factor is almost identical to the Big Five Conscientiousness dimension		Organization Diligence Perfectionism Prudence					an individual’s reaction to a new situation	Exploratory Neophilic vs Unexploratory Neophobic
		Honesty-Humility (H)
		Defined by morality, sincerity or integrity		Sincerity Fairness Greed Avoidance Modesty

This classification is not exhaustive, we just show most representative examples

Both approaches, Big Five and FFM, proposed measurement tools that have been cross-validated in different cultures (NEO-PI: Costa & McCrae, 1992b; IPIP: Goldberg, 1999; NEO-PI-R: McCrae & Costa, 2004; McCrae & Terracciano, 2005). Recently, a six-factor taxonomy (HEXACO: Ashton & Lee, 2001) has been described in lexical studies of personality upon seven different languages (Ashton et al., 2004), and also developing questionnaires in a short and a long version (HEXACO-60: Ashton & Lee, 2009; HEXACO-PI-R: Lee & Ashton, 2004). In practice, it includes the same Big Five taxonomy, but adds a sixth factor, Honesty-humility (Table 2), related to the facet ‘straightforwardness’ inside Agreeableness factor (Ashton et al., 2000).

Finally, there is one approach, Affective Neuroscience (Panksepp, 1998) that successfully analyzed how affective states are related to the variability that constitutes human personality. Within this perspective, the FFM / Big Five personality traits are related to the basic emotional system inside the human brain (Davis et al., 2003). These authors argue that emotional parts of personality are the oldest ones, from the evolutionary point of view, and act within human behavior in a “bottom-up fashion” (Davis & Panksepp, 2011), meaning they act from the deeply subcortical substrate to the upper-mid brain regions along the affective developmental-learning trajectory (Montag & Panksepp, 2017). Today, primarily by means of brain stimulation (i.e., DBS: deep brain stimulation), seven primal emotions have been identified (Panksepp, 1998, 2011) containing four emotional circuitries for positive emotions (SEEKING, LUST, CARE, PLAY) and three emotional circuitries for negative emotions (FEAR, RAGE/ANGER and SADNESS/PANIC) (see Table 2). In this approach individual emotional differences are assessed by means of the Affective Neuroscience Personality Scales (ANPS: Davis et al., 2003). The ANPS assesses, via self-report, individual differences in positive and negative emotions (except LUST), being based on cross-mammalian theoretical Panksepp’s background of Affective Neuroscience. Some strong correlations have been found between the ANPS primal emotions and the Big Five traits. For instance, CARE and ANGER are correlated (positively and negatively, respectively) to higher Agreeableness; SEEK is strongly related to Openness to Experience and PLAY goes along with Extraversion (Davis et al., 2003; Davis & Panksepp, 2011, for details see Table 2).

In spite of some limitations (such as focusing in just one specific model of human emotions), the Affective Neurosciences approach provides structural continuity among humans and other animals by highlighting the common shared mammalian brain pathways and substrates for the variation in personality traits.

Temperamental Traits

As mentioned above (item Terminological issues), developmental psychologists habitually use the term temperament to refer to personality in childhood. This is not just a preference for a term but a conceptual issue. While personality psychologists are interested in getting to a reliable and broad taxonomy of the real structure of personality (i.e., a descriptive task), developmental psychologists are focused on discovering the underlying biological bases of these structures, its genetic and environmental influences and its change or stability in time (Goldsmith et al., 1987). Some authors point out that temperament manifests itself throughout development as the response to external stimuli (Thomas & Chess, 1977), mediating the influence of the environment on behavior, while others consider temperament a more static concept and focus on its inherited factors (A. H. Buss & Plomin, 1984).

Several models for temperamental traits have been proposed, most on the basis of factor analysis (see review in Rothbart & Bates, 2006). However, temperamental studies lack a common taxonomy such as the Big Five model in personality. One of the most accepted model of temperament recognizes three broad ‘superfactors’, that are closely related to the Big Five taxonomy: “Neuroticism/negative emotionality” (N/NE), “extraversion/positive emotionality” (E/PE), and “disinhibition versus constraint” (DvC) (Clark & Watson, 2008). Briefly, individuals high in N/NE perceive the world as threatening, problematic, and distressing. Individuals high on E/PE are willing to engage the environment, enjoy the company of others, and approach life with enthusiasm. Finally, DvC superfactor reflects individual differences in the tendency to behave in an undercontrolled versus an overcontrolled manner (Clark & Watson, 2008, p. 269). Rothbart, (2007) got to a very similar broad dimensions taxonomy– Effortful control, Negative affectivity and Extraversion/surgency- (Table 2) in a comparative study of early childhood (i.e., six to seven years old) in populations from United States and China.

Similar to our perceiving senses (i.e., the sensory nervous system: vision, hearing, touch, taste and smell) that get improved and refined along early development, temperament changes across the developmental processes, so human infants are not born with their temperamental characteristics completely established. Consequently, a variety of temperamental dimensions are detected across ontogeny (Rothbart et al., 2012). However, some regularities can predict further behavior in children, such as babies initial reaction to unfamiliar events (i.e., inhibition) predicted children’s ‘shyness’ at different ages (Kagan et al., 1988), but such consistency is not easily distinguished: for instance ‘effortful control’ is only noticeable in infants after 30 months (Rothbart & Ellis, 2003).

A Science of Nonhuman Personality

We can trace back the origin of animal (nonhuman) personality research to the comparative psychology of the twentieth century. The Russian physiologist, Ivan Pavlov (1849–1936) observed significant individual differences in behavior in the dogs he used in his conditioning experiments (Pavlov & Petrova, 1934). Instead of ignore these differences, he decided to take them into account, calling them temperaments, in reference to Galen’s proposal, and relate them to different kinds of nervous systems (i.e., choleric, sanguine, phlegmatic and melancholic dogs) (Whitham & Washburn, 2017).

Other pioneers in comparative psychology, such as Yerkes (1876–1956) and Crawford (1910–2002) were interested in our most related species, nonhuman primates. As a psychobiology professor, Yerkes founded in 1924 the Yale Laboratories of Primate Biology, and explored there among other topics, some aspects of chimpanzee personalities (Yerkes, 1939). Years later, working in the same prolific laboratories, Crawford (1938) proposed the first rating scale of chimpanzee personality, and Donald Hebb (1904–1985) established a method for testing behavioral consistency (Hebb, 1949). Despite these, and some others scattered pioneering contributions (rats’ timidity: Billingslea, 1941; free-ranging baboons’ emotions: Buirski et al., 1973, chimpanzees’ emotions and personality: 1978; captive animals’ curiosity: Glickman & Sroges, 1966; stikelbacks’ boldness: Huntingford, 1976; rhesus macaques’ temperament: Stevenson-Hinde & Zunz, 1978), the field of animal personality did not flourish until the last decades of the twentieth century.

In behavioral ecology Wilson et al. (1994) marked a milestone in the study of personality, being the first ones to connect some personality traits (i.e., shyness-boldness axis) to evolutionary explanations (sensu Tinbergen, 1963). After the publication of that seminal paper, studies in practically every taxa emerged (for an early review see, Gosling, 2001), with a striking increase in the last decade (Whitham & Washburn, 2017).

In behavioral ecology trait is defined as “a specific aspect of a behavioral repertoire that can be quantified and that shows between-individual variation and within-individual consistency” (Carter et al., 2013, p. 467). However, the discussion about the basic units has never occurred (but see Réale et al., 2007), although behavioral ecologists rely in psychology to justified the analyzed traits (e.g., D. S. Wilson et al., 1993). In this tradition, researchers usually investigate one unique trait (e.g., shy-bold) within one species and its evolutionary consequences (e.g., Coleman & Wilson, 1998). Each trait is linked to ecological valid contexts, where measurements also differ according to the investigation’s goals. Thus, if a researcher is interested in ontogeny, measures around behavioral developmental changes will be taken. Alternatively, if the interest is focused in mechanisms, physiological measurements will be taken (Réale et al., 2007). Behavioral ecologists usually rely in behavioral codification in experimental situations (e.g., open field test) or natural environments for assessing the personality traits.

Behavioral ecologists are not interested in defining the whole personality structure of a given animal species (e.g., describing cockroach’s personality). Instead, they try to respond ultimate and proximate questions about traits (e.g., have bold cockroaches more mating success?). Thus, trait definitions differ from one species to another. This lack of common definitions may seem confusing, hindering cross-species comparisons, but, on the other hand, it can be argued that, being bold is not the same for a prey than for a predator species. Gosling & John, (1999), before the “explosion” of personality research in behavioral ecology, already perceived that personality assessments in animals “came in different languages; used a variety of scales, methods and notations: and varied in their scope and reliability” (p. 69).

Nevertheless, as in human personality, in behavioral ecology there has been a proposal of agreement about the number and definition of traits. This proposal (Réale et al., 2007) also refers to a five-trait model, including Boldness, Exploration, Activity, Sociability and Aggressiveness (Table 2). However, Réale et al.’s (2007) approach is not a theoretical framework describing a precise personality structure common for all animals. Instead, the authors suggested these traits, but leave to the researcher’s choice, which one would be more appropriated for the studied species and for answering the research question. Perhaps for that reason, studies using Réale et al.’s (2007) approach do not investigate exhaustively all five traits, usually only accounting for a single trait or a few traits (e.g., exploration in Japanese macaques: Arnaud et al., 2017; boldness and docility in yellow-bellied marmots: Petelle et al., 2013), or for suits of correlated traits, in the sense of behavioral syndromes (e.g., zebrafish: Moretz et al., 2007).

Another characteristic of personality studies in behavioral ecology is that researchers frequently determine the relevant traits of the targeted species based upon its typical observed behavior. For instance, great tits differ consistently in exploratory and sociosexual behavior (Carere et al., 2005), so artificially rearing extreme lines from wild populations in several generations may led to different responses in behavioral tests (Groothuis & Carere, 2005). To asses these species-specific individual differences (that they call personality traits) ‘standard’ behavioral tests (e.g., open-field test, novel object test) are often used. Sometimes, behavioral codification, and rarely, subjective personality ratings are also employed (for a discussion about methods in animal personality research see, Vazire et al., 2007). Standard behavioral tests are tools for measuring same traits in different taxa, but at least three common mistakes involving these tests have been pointed out (Carter et al., 2013): 1) a test for one species might not be appropriate for another (again, it is not the same being bold for a prey or predator species); 2) definitions are not yet uniform [e.g., Réale et al. (2007) exclude neophilia from boldness while Coleman & Wilson, (1998) include it]; 3) one standard test could be measuring two or more different traits (e.g., open-field test can be a measure of boldness but also of activity level). As a consequence, these authors suggested that “the indiscriminate use of ‘standard’ behavioral tests within animal personality studies may lead to the spurious labelling of personality traits” (Carter et al., 2013, p. 472).

Personality Traits in Primates

Within the young science of nonhuman personality, primates’ personality research has followed a parallel path, apart from other animal species, more closely related to the tradition of studies in human personality. As mentioned above, human personality psychologists were more concerned with getting into an exact description of personality structure rather that in answering questions about the adaptive value of individual differences. Therefore, studying closely related species is a way of solving this lack of interest, since comparing structures can give light into the selective forces that gave rise to personality traits.

We can situate the very beginning of the interest in primate’s personality after Stevenson-Hinde and Zunz’s (1978) work with rhesus monkeys. Their work (Stevenson-Hinde & Zunz, 1978; Stevenson-Hinde 1980a, b) is salient for explicitly proposing a rating scale, specific for primate temperamental patterns, analyzed through principal component analysis (PCA), that was already being used in social behavior studies (Chamove et al., 1972; van Hooff, 1970) but not in (nonhuman) animal personality research. Although Buirski et al., (1973, 1978) were using rating scales (along with behavioral codification), they did not apply component reduction techniques. Instead, they applied Emotions Profile Index (EPI) and assumed its structure “derived from a theory of personality which stresses the adaptive significance of emotions at all evolutionary levels” (Buirski et al., 1978, p. 123).

Along the 1980–90’s, most primate personality research was conducted with captive animals, probably because there were many colonies maintained for medical research purposes. The main aim was to characterize individual differences in nonhuman primates and to test the reliability of the scales by comparing them with behavioral assessments. Furthermore, during this decade some studies evaluated the correlation of personality traits with other variables such as dominance, age and sex (e.g., Bolig et al., 1992; McGuire et al., 1994), attachment style and physiological profiles (e.g., Clarke et al., 1988), and reaction to rearing conditions such as social deprivation or environmental enrichment (e.g., Schneider et al., 1991).

As a consequence of the success of the Big Five (Costa & McCrae, 1992b; Goldberg, 1990) and its cross-cultural validation (McCrae & Costa, 1997; McCrae & Terracciano, 2005) in humans, a renewed interest on cross-species validation appeared in the middle 90’s. It was suggested that a “basic” personality factor must be identifiable among nonhuman species (Zuckerman, 1992). Inspired by this criterion, a new personality rating scale was proposed, composed by 43 adjectives taken from Goldberg’s (1990) adjectives list. The such called “Five-Factor Model plus dominance” (FFM + D: King & Figueredo, 1997) of chimpanzee personality was surprisingly similar to the structure found in humans. The rating scale proposed has been modified several times (King et al., 2008; Weiss et al., 2006, 2007) until its actual form -Hominoid Personality Questionnaire- (HPQ: Weiss et al., 2009; Weiss, 2017b), being applied successfully to many primate species. Orangutans (Adams et al., 2012; Morton et al., 2013; Weiss et al., 2006), langurs (Koneĉná et al., 2008), chimpanzees (Morton et al., 2013; Rawlings et al., 2020; Weiss et al., 2009, 2012; V. A. D. Wilson et al., 2020), bonobos (Garai et al., 2016; Weiss et al., 2015), gorillas (Eckardt et al., 2015), macaques (Adams et al., 2015; Konečná et al., 2012; Morton et al., 2013; Robinson et al., 2018, 2021; Simpson et al., 2019), baboons (Pritchard & Palombit, 2022), capuchin monkeys (Fernández-Bolaños et al., 2020; Morton et al., 2013; Robinson et al., 2016), marmosets (Koski et al., 2017; Šlipogor et al., 2020, 2021), tamarins (Masilkova et al., 2018) and squirrel monkeys (V. A. D. Wilson et al., 2018) have been rated by HPQ assessments. Although there have been other questionnaire proposals (Freeman et al., 2013; Manson & Perry, 2013), definitely, this one is the most widely spread.

Using the same tool for assessing the personality structure of several primate species is an etic2 approach that has the advantage of making possible cross-species comparison (e.g., Morton et al., 2013). Moreover, comparing closely related primate species might help to answer ultimate questions about the evolution of personality traits, suggesting ‘when’ one trait appeared during phylogeny (e.g., squirrel monkeys: V. A. D. Wilson et al., 2018) or if ecological or social variables can affect the prevalence of traits (e.g., macaques’ social style: Adams et al., 2015). Finally, given that HPQ is based in human Big Five taxonomy, it can answer questions about the evolution of human personality traits.

Primate personality research is most commonly based on rating scales, similar to those applied in human personality research, whereas in other animal species it is more typical assessing traits by using behavioral codification. It has been pointed out that several methods must be combined for assessing personality in a reliable manner (Freeman et al., 2011; Manson & Perry, 2013), in fact, today, the majority of primate personality research relies in both, ratings and codings, to attain a validity criterion.

Uher (2018; Uher & Visalberghi, 2016) strongly criticizes the reliability of questionnaires considering that they are “retrospective memory-based judgments […] containing stereotypical biases”. The author proposed an alternative method, called “behavioral repertoire X behavioral situation approach” (Uher, 2008), fully based in behavioral codification, that she applied to some primate species (Uher, 2013; Uher & Asendorpf, 2008; Uher et al., 2013). In fact, subjective assessment in personality research has always have been surrounded by the long shadow of rater bias (i.e., expectation, projections, etc.), however, multivariate techniques ensure several criteria for avoiding these sources of error: (a) inter-rater reliability, (b) inter-item consistency, (c) cross-situational consistency, and (d) cross-temporal stability are usually controlled in such personality studies (Figueredo et al., 1995). Nevertheless, several authors defend the reliability of questionnaires and maintain that both methods should lead to similar results (Gosling & Vazire, 2002; Vazire & Gosling, 2003; Vazire et al., 2007).

Using a common approach and same measurement tools for assessing primate personality is allowing comparative psychologist to obtain personality structures for several species. Further comparative research is needed in order to determine how these structures are related.

Discussion

Along this text, we made a non-exhaustive review on the most remarkable problematic in personality research, both in humans and nonhuman animals. We began discussing the difficulties derived from a variety of definitions that are not exactly synonymous, thus creating confusion within the studied area. We then succinctly summarized the most important historical milestones in human and other animals’ personality research, and finally we explored the case of nonhuman primates’ personality research, as an example of a prolific field.

Temperament or Personality?

The terminological issues are not completely resolved nowadays. The terms imported from other areas outside the psychological sciences (i.e., coping style or behavioral syndrome), despite have been considered equivalent to personality, are today broadly accepted in behavioral ecology as useful, more specific constructs, having their own entity apart from personality traits. The conceptual differences between personality and temperament, the first referring to a constructed result of dispositions and the second to the inherited physiological bases that develop into personality (i.e., late-emerging or basic traits), are progressively disappearing. Personality psychologists have focused for a long time only in getting to an appropriate description of the taxonomy of adult traits. Today, the most widely spread taxonomy, the Big Five, is broadly accepted and cross-cultural validated so an interest for the ultimate questions have begun. In fact, the Affective Neuroscience approach has provided data on the correlations among the Big Five traits and basic emotions, in humans and in other mammal species (Montag & Panksepp, 2017). Heritability of personality traits, for a long time overviewed, has been discussed and demonstrated (Bouchard, 1994). Studies with humans twins (comparing monozygotic and dizygotic twins) showed consistent differences in heritability of personality traits (Bouchard & Loehlin, 2001; Power & Pluess, 2015), suggesting that “broad personality dimensions may show less variation in their genetic architecture than narrower ones do” (Loehlin, 2012, p. 505). The correspondence between personality and temperament traits have been debated and empirically tested (e.g., Angleitner & Ostendorf, 1994; Kohnstamm et al., 1998) with better results for some traits than for others (Table 2). Moreover, both kinds of traits are today recognized as not being immune from experience, so “even if identical factors were found in infants and adults, it would not imply that infant temperament is a good predictor of adult personality” (McCrae et al., 2000, p. 183). In that sense, empirical research in humans has shown that children and adults do show rank-order continuity over time, but significant change occurs as well (Roberts & DelVecchio, 2000). This fact must be taken into account when designing ontogenetic studies of personality (e.g., Delval et al., 2020).

Finally, there is an unresolved debate about the more appropriate term for referring to consistent individual differences in children and animals. Whereas comparative psychologist studying animal personality advocate for the use of personality (see arguments in Weinstein et al., 2008), developmental psychologists consider more appropriate for children and animals the term temperament (Rothbart & Bates, 2006). Perhaps, one principal reason for this distinction are the fundamentally different ways in which personality is assessed in human and animal (or children) studies. While human personality is studied almost exclusively by self or informant verbal reports, animal studies prefer behavioral tests. This contrast is expected because humans are typically living in an environment, which is over saturated with languages and other symbolic systems.

However, temperament and personality are not exactly the same, since the first is referring to innate dispositions that further develop into personality traits. Thus, we consider that temperament must be used when studying the development of behavioral traits, in children or infant animals, and personality when researching in adult behavioral traits, be these in human or nonhuman animals.

Is Trait Theory Enough for Explaining Personality?

Along the historical overview of personality in this text, we have only referred to the structural theories of personality. Of course, there are other relevant approaches (i.e., psychoanalysis, phenomenology, behaviorism, person/situation debate, etc.) that we have avoided here, in order to establish limits to our review. The trait perspective has been criticized for its reductionist approach, because it seems to be opposite of humanistic psychology, that consider persons as “unified, free, conscious beings seeking self-fulfillment” (Ellis et al., 2009, p. 219). This holistic view of the human being is very relevant in idiographic sciences, where approaches are qualitative in order to understand a relevant case of study. However, the scientific method needs to generalize and create categories for establishing laws or rules, and that is the case of personality trait psychology. The most critical approach to the trait theory is the situational personality theory (Mischel, 1969), that considers that contexts might have more predictable value on individual behavior than personality traits. Instead, psychological personality traits have the advantage of being operationally defined and measured, allowing to establish comparisons between individuals, between populations, between cultures, between species… The trait theory, contrary to person-situation debate, attributes behavioral observable differences to underling stable personality traits, both in humans and nonhuman animals. The Big Five trait-model of human personality has allowed to make cross-cultural research (Goldberg, 1993; McCrae & Costa, 1997; McCrae & Terracciano, 2005) showing that this model is consistent with an evolutionary theory. If personality traits are adaptive, they must have been selected by natural selection and they may not be exclusive of our species (D. M. Buss, 1991; D. M. Buss & Greiling, 1999; D. M. Buss & Hawley, 2011; Dingemanse & Réale, 2005). In fact, animal individual differences in personality have been shown to have consequences for evolutionarily relevant components of fitness, such as dispersal (Malange et al., 2016), metabolic rate (Careau et al., 2008), mating success (Both et al., 2005), status (Perry et al., 2017; Seyfarth et al., 2012), offspring production and parenting (Dingemanse et al., 2004; Reddon, 2012) or cognitive performance (Carere & Locurto, 2011). In this sense, behavioral ecology- defining personality as consistent behavioral individual differences across time or/and contexts- has also embraced trait theory recognizing the predictive power of traits over contexts (since personality must be stable across contexts). The evolutionary perspective of behavioral ecology have broadened the view of traits in human personality psychology, showing the intricate relationships between personality and plasticity (Dingemanse et al., 2010; Dochtermann & Dingemanse, 2013). Correlations among behaviors leading to personality traits might constraint evolutionary responses to environments limiting infinite plasticity (Dochtermann & Dingemanse, 2013).

The case of primates’ personality research is an especial one within animal personality research. In fact, in this review, we have chosen primates not as a random example, but because it follows a more similar path to that of human personality psychology, while taking into account some of the improvements that behavioral ecology brought (Weiss, 2017a). It employs trait-rating methodology, but primatologist have sometimes compared ratings with behavioral codification for validating traits (e.g., Manson & Perry, 2013). Unfortunately, the results of these comparisons are difficult to interpret since behavioral observations often revealed a different structure that the one obtained with ratings (e.g., Koneĉná et al., 2008). Behavioral observation seems to be free of biases because it relies in behavior as a unit of analysis. However, there are several ‘subjectivities’ that quantification techniques must confront. For instance, when quantifying in a hypothetical species foraging behavior, researchers can choose counting every food intake, or divide it into manipulation, chew and swallow, or measure the time spent foraging, or both. These decisions are sometimes arbitrary and can lead to very different frequencies of the counted behaviors, which will further modify the PCA results obtained from behavioral codings. This does not mean, of course, that behavioral coding is always biased and must be left apart. In the same way that questionnaires used in ratings must be carefully designed, tested and validated, behavioral codifications must follow a pattern, at least when measuring different populations of the same species. The fear of trait-rating methods by behavioral ecologists is unjustified and must be overcome: if the rater biases were omnipresent, inter-rater reliability would be difficult to obtain. Also, for rating an animal, the mean rating score of every item is obtained and, then, for revealing the structure of the species, mean ratings are submitted to component reduction techniques (i.e., factor or principal components analysis), so particular rater nonsystematic deviations from the mean are eliminated (Weiss et al., 2012). The HPQ has been properly validated (Weiss et al., 2012), and has been applied to more than 16 different primate species, but it has rarely been exhaustively compared with coding (but see Fernández-Bolaños et al., 2020; Morton et al., 2013). This pending task must be resolved with new research.

Trait tradition represents a successful example of a research framework that has been able to solve fundamental questions about animal and human personality, by means of the comparative method, incorporating recently to their agendas ultimate and proximate questions (see review in, Weiss, 2018). Thus, we consider it a fruitful and promisor framework that must be keep, even though if it coexists with other paradigms.

Is a Unified Science Possible?

Although personality research in human psychology and in behavioral ecology seem to differ substantially, mostly in research methods, they also have points in common and they have a very strong potential of becoming a unified science (Nettle & Penke, 2010). The first step for getting into a common definition of personality, useful for human and nonhuman research, has been achieved, at least inside the trait tradition. Human personality psychologists have understood that it is time to look further, overcoming their obsession with the taxonomy of personality, for answering proximate and ultimate questions (inspired from behavioral ecologists) and getting to a more complete and mature science of personality.

Conclusions

• Personality is a construct that can be identified in humans and other animals.

• Trait theory has been shown useful in the explanation of personality.

• Personality research improves our knowledge about the causes of the behavior of a given species (i.e., the evolutionary causes).

• Within-species personality research gives light about the evolutionary pathways of those traits. It also can answer questions about environmental pressures modelling personality traits.

• Primates studies have shown that we share most traits with closely related species. However, environmental pressures also seem to act in personality, showing little between species’ differences (e.g., dominance trait in chimpanzees, but not in humans or bonobos).

• Although personality has an inherited component, in humans it is shaped across development along the first years of life. This is also expected in any animal species in order to adjust genes x environment correlations.

Biographies

Irene Delval

is a Spanish Psychologist and Anthropologist living in Brazil. She obtained her PhD in Experimental Psychology at the Institute of Psychology of the University of São Paulo (IPUSP), conducting field studies on capuchin monkeys. She has experience in Psychology, focusing on Experimental Psychology, with special interest in Comparative Psychology, Sexual development, Same-sex sexual behavior, Personality, and naturalistic studies on primates of the genus Sapajus. She is currently a postdoctoral fellow at IPUSP.

Marcelo Fernández-Bolaños

holds a degree in Social and Cultural Anthropology - Universidad Nacional de Educación a Distancia (2010), a degree in Psychology from the Autonomous University of Madrid (2006), a MSc in Primatology from the Universitat de Barcelona (2007), a MSc (2014) and a PhD (2019) in Experimental Psychology from the University of São Paulo. He has experience in the field of Psychology, with an emphasis on Primatology.

Patrícia Izar

has a Bachelor degree in Biological Sciences from the University of São Paulo (1990), Master’s degree (1994) and PhD (1999) in Experimental Psychology from the University of São Paulo. Dr. Izar is currently a research fellow of the National Council for Scientific and Technological Development and an Associate Professor at the University of São Paulo. She has experience in Psychology, focusing on Naturalistic Studies of Animal Behavior, investigating on the following subjects: socioecology of Neotropical primates, development, social network analysis, evolutionary psychology, primate cognition. She is currently the Vice President for Education at the International Primatological Society, elected in 2016 and re-elected in 2020, a Member at Large of the Cultural Evolution Society, elected in 2019, and the 2020-2022 Director Secretary of the National Association of Psychology Research and Graduation. She is currently the President of the Brazilian Society of Primatology in the 2023-2024 board.

Author Contributions

The authors confirm contribution to the paper as follows: study conception, review of literature and design: I.D. The first draft of the manuscript was written by I.D., and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Declarations

Conflict of Interest

The authors have no conflicts of interest to declare. All co-authors agree with the contents of the manuscript and there is no financial interest to report. We certify that the submission is original work and is not under review at any other publication .