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. 2025 Jan 21;106(1):e4530. doi: 10.1002/ecy.4530

Sexual dimorphic effects of a keystone predator on prey communities

Jon M Davenport 1,✉, Alan M Babineau 1, Reese K Sloan 1, Autumn Groesbeck 1, Ali J Montazeri 1, Maxwell Ramey 1
PMCID: PMC11749136  PMID: 39835940

Abstract

The importance of trait variation has long been recognized in ecological and evolutionary research. The divergence of sexually dimorphic traits (e.g., body size, morphology, behavior, etc.) is primarily attributed to sexual selection, and sexual dimorphism can have consequences for diets and habitat use. Recent evidence for one aquatic predator species (adult newts; Notophthalmus viridescens) suggests that trait differences and habitat partitioning between the sexes may be important in structuring zooplankton communities. However, newts are known to increase amphibian diversity within pond communities via keystone predation. Yet, no data are available on differentiating potentially sexually dimorphic effects of newts on larval amphibian communities. Thus, we conducted a series of mesocosm experiments to determine the effects of sexual dimorphism of adult newts on larval amphibian communities. Based on previous work with newts and zooplankton, we hypothesized that male and female newts would have differing effects on prey communities. We found that female newts consumed one prey species more than male newts did and no newt treatments. There were no differences between the sexes in prey consumption of another prey species. Size at metamorphosis was greater in the presence of newts (either male or female) for wood frogs and in the presence of female newts for spotted salamanders in comparison with no newt treatments. Our findings indicate that sexual dimorphism within a known keystone predator can have differential effects on prey. Indeed, our results indicate that while the effects of predators on one response (survival) can differ between sexes, the impacts on another response (prey fitness; measured as size at metamorphosis) were similar. Our research to understand the effects of sexual dimorphism is timely as sex ratios of predators may become skewed in nature due to anthropogenic change. If intraspecific differences exist via top‐down effects, then downstream impacts on prey communities may go unnoticed.

Keywords: adaptation, amphibian, pond, salamander

INTRODUCTION

The importance of trait variation has long been recognized in ecology and evolutionary biology (Bolnick et al., 2011; Darwin, 1859). Specifically, variation in phenotypic or other traits can determine mating success, foraging success, and differential survival to set the stage for natural selection (Bolnick et al., 2011; Fox, 1978; Schluter & Smith, 1986). Recent reviews have reinforced, however, that some intraspecific differences within an individual species can be greater than interspecific differences among species (Bolnick et al., 2011; Des Roches et al., 2018). Moreover, these intraspecific trait differences among individuals can have important consequences for communities (Des Roches et al., 2018). For example, intraspecific differences in alewife phenotypes (e.g., foraging traits and habitat use) can alter zooplankton biomass and species richness in divergent manners (Jones & Post, 2013; Post et al., 2008; Walsh & Post, 2011). This variation in traits can change the strength of species interactions and reverberate throughout food webs.

Sexual dimorphism is a form of intraspecific trait variation with potential effects between trophic levels in food webs (Fryxell et al., 2015; Start & De Lisle, 2018). Indeed, “ecological sex dimorphism” is a term coined to describe niche variation that can be attributed to the sex of a species (Shine, 1989, 1991). The divergence of sexually dimorphic traits (body size, morphology, behavior, etc.) is primarily attributed to sexual selection (Lande, 1980; Maan & Cummings, 2009; Moore, 1990), but variability can have consequences for diets and habitat use (Dayan & Simberloff, 1994; Schoener et al., 1977). Research has demonstrated that an ecological result of sexual selection can be either increased resource partitioning or resource competition (Clutton‐Brock, 2017; Fryxell et al., 2019; Svensson, 2019; West‐Eberhard, 1983). For example, initially, differences in nutrient consumption were nonexistent between Drosophila melanogaster sexes. After three generations of adaptation to different resource environments, males from high‐competition environments (low resources) consumed more sucrose while females consumed more yeast. Survival was also higher for high competition individuals in trials with resource restrictions suggesting differential survival due to adaptation. Thus, evolutionary history and divergence in sexually dimorphic traits can alter interactions between sexes but also has the potential to alter community composition of prey communities (Svensson, 2019).

Recent evidence for one aquatic predator species (adult Eastern newts; Notophthalmus viridescens) suggests that trait differences and habitat partitioning between the sexes may be important in structuring prey communities (De Lisle et al., 2018; Start & De Lisle, 2018). Prey consumption by newt populations dominated by males generated distinct zooplankton communities different in composition from newt populations dominated by females (Start & De Lisle, 2018). While this research provides some of the first evidence that newt predator effects may be partitioned between the sexes, the densities of newts (25/2600‐L mesocosm) were significantly higher than many pond communities' experience, and only zooplankton prey were considered. Adult newts are generalist predators on zooplankton, but they consume a diversity of other prey items (Petranka, 1998). Moreover, adult newts have been repeatedly documented as keystone predators on larval amphibian assemblages (Chalcraft & Resetarits, 2003; Fauth & Resetarits, 1991; Morin, 1983) Thus, to examine the effects of sexual dimorphism of adult newts on larval amphibians, we conducted a series of mesocosm experiments in 2021 and 2022. We deployed three predator treatments (no newt, one male newt, one female newt) with either larval spotted salamanders (2021 experiment) or wood frog tadpoles (2022 experiment). Based on previous work with newts and zooplankton (Start & De Lisle, 2018), we predicted that female newts would have stronger consumptive effects on pelagic amphibian prey than male newts. We did not predict any differences in prey growth in response to male and female newts.

METHODS

Study system

Eastern newts (N. viridescens) are a common species ranging across much of eastern North America (Petranka, 1998). Adult newts can be found in both fish and fishless ponds throughout most of the year (Petranka, 1998). In western North Carolina populations, newts reproduce in ponds in late spring with larvae metamorphosing by late summer or early fall (Chadwick, 1944). Adult newts are generalist predators that can also be cannibalistic (Harris, 1989) with female newts typically exhibiting larger heads than male newts. In western NC populations, female newts also appear to have wider and longer heads than male newts (Appendix S1: Figures S1 and S2). Because of their year‐round occupation of ponds across a wide geographic range and toxicity, adults are important top predators in pond communities (Davenport & Chalcraft, 2013; Morin, 1983; Wilbur, 1997). Common spring‐breeding larval amphibians that co‐occur with newts in western North Carolina are Rana sylvatica (wood frogs), Ambystoma maculatum (spotted salamander), and Pseudacris crucifer (spring peeper).

Field methods

Spotted salamander (A. maculatum) egg masses were collected from local ponds in Watauga County, NC, across several dates ranging from 12 February 2021 until 25 March 2021. To ensure enough similar‐sized A. maculatum for the experiment, earlier collected clutches were stored in a refrigerator at lower temperatures (4–5°C) to slow development. This cooling technique has previously been used in larval amphibian experiments with no issues (Anderson et al., 2020; Urban, 2008). Egg masses of A. maculatum were allowed to hatch in water baths in the lab. Twelve clutches of R. sylvatica eggs were collected in Watauga County, NC, on 25 February 2022 and 14 March 2022. Clutches hatched in the lab and then were subsequently mixed, with tadpoles and larvae randomly assigned to treatments to minimize any possible genetic biases. Adult eastern newts (N. viridescens) are present in all ponds that we collected each prey species from. Newts for the experiments were collected from local ponds in Watauga County, NC, on 19 April 2021 and 5 March 2022. All animals were housed in lab at Appalachian State University (ASU) until introduction to their respective mesocosm.

Experimental design

For both experiments, we filled 300‐L mesocosms (modified cattle water tanks) with city water treated with water conditioner (Kordon Amquel) to remove any chlorine and chloramine. Standardized inoculations (500 mL) of local wetland water containing concentrated zooplankton were added to create a food source for the larval salamanders. We added 0.5 kg of dry leaf litter to provide structural complexity and a nutrient source for larval amphibians and zooplankton. Mesocosms were covered with lids (60% shade cloth covering) to prevent colonization of the mesocosms from unwanted predators and to prevent animals undergoing metamorphosis from escaping prior to collection.

Both 2021 and 2022 experiments ran from March through August on the campus of ASU. We deployed three treatments for each experiment: (1) no newt predator, (2) one adult male newt, and (3) one adult female newt. This density of newts in a mesocosm was 2.9× less than reported by Start and De Lisle (2018). All treatments received amphibian prey. In 2021, prey were 14 larval spotted salamanders (A. maculatum). In 2022, prey were 30 R. sylvatica tadpoles. Each of these experimental food webs was replicated five times for a total of 15 mesocosms. All newts were randomly assigned to a mesocosm and added into their respective mesocosms first (26 April 2021, 7 March 2022). We selected male and female newts of similar size to ensure body size was not confounded with head width. There was no significant difference in mean body size of male and female newts added to the experiments in 2021 (mass, t = −1.475, df = 7.692, p = 0.179; snout‐vent‐length [SVL], t = −1.225, df = 6.917, p = 0.260) or 2022 (mass, t = −0.616, df = 7.687, p = 0.555; SVL, t = 1.23, df = 7.792, p = 0.251). A. maculatum hatchlings were added on 26 April 2021. R. sylvatica tadpoles were added on 6 March 2022. Densities of all predators and prey were within densities previously reported in the literature (Anderson & Whiteman 2015; Cortwright & Nelson, 1990; Davenport & Chalcraft, 2012) and observed in local surveys.

Mesocosms were monitored daily for individuals undergoing metamorphosis. Metamorphosis was considered complete after gill absorption for salamanders and tail absorption for R. sylvatica tadpoles. Survival was calculated as the total number of animals recovered (metamorphs plus any remaining larvae at breakdown) by mesocosm. All metamorphosed animals were measured for SVL using digital calipers in the lab. Size at metamorphosis is correlated with future reproductive success in amphibians (Berven, 1990; Scott, 1990; Semlitsch et al., 1988). Mesocosms were drained (17 August 2021 and 5 July 2022) and thoroughly searched to recover any remaining animals. All animals were either released back at collection sites or euthanized in accordance with IACUC protocols (#19‐13 and #20‐14). Any specimens were deposited in the ASU collection.

Statistical analysis

We analyzed survival and SVL for both A. maculatum and R. sylvatica separately by year. Survival for both species was analyzed as counts of survivors by mesocosm with a generalized linear model and a Poisson distribution. Treatment was modeled as a fixed effect. Pairwise comparisons of prey survival were conducted using the emmeans package in R. Mean SVL by mesocosm was analyzed using a linear model with treatment as a fixed effect. Tukey's test was deployed to test for differences in treatment means of SVL for both prey species. We deployed a regression analysis post hoc to investigate potential thinning of prey populations with mean prey survival as a predictor of mean prey size at metamorphosis by mesocosm. All statistical analyses were conducted using lme4 and tidyverse packages in R version 4.3.1 (Team, 2023).

RESULTS

2021 experiment

A. maculatum survival was highest in the absence of newt predators and lowest with female newts (Figure 1). A. maculatum survival in no newt predator treatments was significantly different from treatments with female newts (est. = −0.59, z = 2.64, p = 0.02, Figure 1) but not male newts (est. = −0.24, z = 1.19, p = 0.46, Figure 1). Treatments with male newts did not differ in survival from treatments with female newts (est. = −0.35, z = 1.49, p = 0.29, Figure 1). The SVL of surviving A. maculatum was significantly different across treatments (F 2,12 = 5.59, p = 0.02, Figure 2). Post hoc comparisons indicate that A. maculatum SVL was greater in the presence of female newts (p = 0.02) in comparison with no newt treatments. There was no difference in male newt treatments compared with no newt treatments (p = 0.12) and male newt treatments compared with female newt treatments (p = 0.49).

FIGURE 1.

FIGURE 1

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Mean spotted salamander (Ambystoma maculatum) survival (±1 SE) by newt treatment (female newt = presence of female newt, male newt = presence of male newt, no newt). Letters indicate statistical differences among treatments.

FIGURE 2.

FIGURE 2

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Box plot of snout‐vent length (SVL, in millimeters) of spotted salamander (Ambystoma maculatum) by newt treatment (female newt = presence of female newt, male newt = presence of male newt, no newt). Points are jittered means by mesocosm. Letters indicate statistical differences among treatments.

2022 experiment

R. sylvatica survival was also highest in the absence of newt predators and lowest with female newts (Figure 3). R. sylvatica survival in no newt predator treatments was significantly higher than treatments with female newts (est. = 3.39, z = 11.81, p < 0.01, Figure 3) and treatments with male newts (est. = 2.59, z = −8.99, p < 0.01, Figure 3). There was weak evidence of differences in R. sylvatica survival between male and female newt treatments (est. = 0.81, z = 2.43, p = 0.06, Figure 3). The SVL at metamorphosis of R. sylvatica survivors did differ across treatments (F 2,17 = 84.11, p < 0.01, Figure 4). Post hoc comparisons found that surviving R. sylvatica SVL was significantly greater in the presence of male (p < 0.01) or female newts (p < 0.01) in comparison with no newt treatments. There was weak evidence of differences in R. sylvatica SVL between male and female newt treatments (p = 0.06).

FIGURE 3.

FIGURE 3

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Mean wood frog (Rana sylvatica) survival (±1 SE) by newt treatment (female newt = presence of female newt, male newt = presence of male newt, no newt). Letters indicate statistical differences among treatments.

FIGURE 4.

FIGURE 4

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Box plot of snout‐vent length (SVL, in millimeters) of wood frog (Rana sylvatica) by newt treatment (female newt = presence of female newt, male newt = presence of male newt, no newt). Points are jittered means by mesocosm. Letters indicate statistical differences among treatments.

DISCUSSION

Our findings indicate that sexual dimorphism within a known keystone predator can have differential effects on prey. Female newts consumed more prey than male newts in one of two experiments in experimental pond communities (Figures 1 and 3). The consumptive effects of female newts on survival were greater with one prey (R. sylvatica tadpoles) than another (larval A. maculatum). The implications for future reproductive fitness (higher likelihood of survival to 1st reproduction) were similar with prey surviving in treatments with newts always metamorphosing larger than prey in treatments with no predators (Figures 2 and 4). Therefore, our results indicate that while the effects of predators on one response (survival) can differ within sexes, the impacts on another response (prey fitness; measured as size at metamorphosis) are the same.

One mechanism that may promote divergence between sexes is the within‐species ecological character displacement (ECD) hypothesis. The ECD hypothesis posits that resource partitioning reduces competition between the sexes and likely leads to differing effects on prey (De Lisle, 2023; De Lisle & Rowe, 2015). While a previous study reported differences in habitat use by newts in Canada, our haphazard scan sampling could not address differences in foraging habitat in our study. Our study was conducted in mesocosms, like the Start and De Lisle (2018) research, which could conflate artificial conditions with limited habitat for foraging relative to natural pond communities. Nonetheless, our field data from local ponds suggest there may be a seasonal component to different habitat use by newt sexes (ranging from a 6:1 male‐dominated population in some pond pelagic zones during the late winter to 1:1 ratio in late spring/early summer). Thus, there is considerable overlap in habitat use by the sexes during spring and early summer when most amphibian prey breed at our field sites.

Body size differences between the sexes have also been implicated as a potential mechanism for intraspecific trait variation within a species (Des Roches et al., 2018). We measured all newts before deployment into our experiment. We purposely selected newts of similar body size to remove potentially confounding differences in foraging based on size. Newt SVL and mass were not significantly different between the sexes for either experiment. However, we did not measure head width or head length of adult newts in our experiments. Head morphology of amphibians can have effects on prey resource use (Michimae & Hangui, 2008; Reilly et al., 1992; Van Buskirk & Schmidt, 2000; Walls et al., 1993). Larger heads increase the spread of prey sizes that can be consumed and also can prevent size refuge of many prey (Kishida et al., 2006; Reilly et al., 1992). Interestingly, the published data available for eastern newts suggest that females generally have larger heads than males in most populations (De Lisle & Rowe, 2015; De Lisle & Rowe, 2017; Harris, 1989). This suggests that females should have larger effects (i.e., longer window of foraging on larger‐sized prey as they grow) in comparison with male newts. This is exactly what we found in our study that is one of the first to explicitly test this hypothesis (Figures 1 and 3). Previous researchers have found indirect evidence that female newts benefit more (e.g., gain more mass) in the presence of anuran prey than male newts (Wilbur & Fauth, 1990). This benefit may be due to larger gapes and the ability to consume a more diverse pool of amphibian prey species of different sizes for longer windows of time. Indeed, the documented pattern in our NC populations of females with wider heads supports the hypothesis that wider heads of newt predators could have stronger top‐down effects. Our findings have implications beyond our study and suggest that this pattern of sexually dimorphic effects of predatory newts may be widespread and could have implications on their keystone roles within pond communities. Future research investigating the top‐down effects of intraspecific trait variation on more diverse prey communities should measure a full spectrum of potential traits to reinforce the current literature.

One alternative hypothesis for our documented effects between newt sexes could be the result of differential energetic demand. All newts in our experiment were introduced into mesocosms in the spring season. This is also true of most other experimental conditions with newts as top predators (Chalcraft & Resetarits, 2003; Fauth & Resetarits, 1991; Morin, 1983). Newt courtship occurs from late fall to early spring (Petranka, 1998; Sever, 2006). This courtship leads to delayed egg oviposition in the late spring for most populations. Indeed, we found larval newts in several replicates during 2021 and 2022 as we terminated the experiments. Ultimately, the period before egg deposition (e.g., spring) is vital for female newts to garner sufficient energy via prey consumption to ensure successful reproduction. Moreover, the energetic demand for female salamanders is likely significantly greater than the demand for male salamanders (Fitzpatrick, 1973; Hom, 1988). The increased foraging of female newts in early spring during our wood frog experiment may explain the higher mortality of wood frog tadpoles. The higher consumption of prey by female newts in both of our experiments suggest support for this hypothesis and highlights the importance of between‐sex differences among predators during the prereproductive period. Future studies could examine foraging efficiency and energetic requirements between the sexes on a common set of prey to fully address this hypothesis.

In our study, the differences in prey survival were significantly different based on the sex of the predator (Figures 1 and 3). The observed differences in prey survival with newt predators, lower survival with female newts for wood frog tadpoles versus male newts in comparison with no differences in the sexes for larval spotted salamanders, was interesting. We did not identify the mechanism for this difference among prey but data from similar experiments suggest that both prey will exhibit phenotypic responses to predators (e.g., changes in activity levels and larger tail fins in treatments with predators) (Rack, 2016; Relyea, 2002; Urban, 2007). The observed survival differences in prey species may be due to a shorter window of predation risk for larval spotted salamanders. For example, adult newt gapes may limit the consumption of larval spotted salamanders to the 1st few weeks of the larval period (Urban, 2008). It is possible consumptive rates of adults may only diverge with prey that can be consumed for longer periods. Future experiments will have to explicitly test for prey trait differences when exposed to male and female newts. Yet another measure of fitness for both amphibian prey was larger size for survivors with predators, regardless of predator sex. Organisms with complex life cycles that metamorphose larger are more likely to survive to 1st reproduction and successfully mate and can also lay more eggs (Berven, 1990; Scott, 1990; Semlitsch et al., 1988). Post hoc regression analysis of prey survival as a predictor of prey size at metamorphosis indicated that less survival in a mesocosm for each species led to larger metamorphs (wood frogs: est. = −2.59, t = 7.52, p < 0.01, spotted salamanders: est. = −1.23, t = 4.44, p < 0.01; Appendix S1: Figures S3 and S4). This finding indicates that thinning occurred with predators, regardless of sex, which led to larger individuals at metamorphosis that are more likely to survive and reproduce (Semlitsch et al., 1988). This thinning effect has been previously documented with predators on larval amphibians (Anderson et al., 2021; Anderson & Semlitsch, 2014; Davenport & Chalcraft, 2012). Ultimately, our results indicate that the intraspecific effects between predator sexes (e.g., “ecological sex dimorphism”) can change, along with inferences, based on which response variable is measured in prey.

The community‐wide effects of sexual dimorphism are only beginning to be recognized and provide a pivotal link between evolutionary and ecological processes (Fryxell et al., 2015, 2019; Start & De Lisle, 2018). Indeed, the implications of sex‐specific effects of predators on prey communities are likely underestimated in explaining patterns of biodiversity. Given our findings, more research is pivotal to understanding the influence of sex‐specific effects of predators in structuring ecological communities.

AUTHOR CONTRIBUTIONS

Jon M. Davenport conceived the project. Jon M. Davenport, Alan M. Babineau, Reese K. Sloan, Autumn Groesbeck, Ali J. Montazeri, and Maxwell Ramey collected and analyzed the data. Jon M. Davenport wrote the manuscript. Alan M. Babineau, Reese K. Sloan, Autumn Groesbeck, and Ali J. Montazeri provided edits on the manuscript. Jon M. Davenport obtained the funding and permits.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

Supporting information

Appendix S1.

ECY-106-e4530-s001.pdf (249.9KB, pdf)

ACKNOWLEDGMENTS

We thank the Office of Student Research at Appalachian State University for funding. All work was done under North Carolina Wildlife Permit #21‐SC01312. We thank L. Geurs for help with fieldwork. We also thank S. De Lisle and an anonymous reviewer for helpful comments and input on a previous draft.

Davenport, Jon M. , Babineau Alan M., Sloan Reese K., Groesbeck Autumn, Montazeri Ali J., and Ramey Maxwell. 2025. “Sexual Dimorphic Effects of a Keystone Predator on Prey Communities.” Ecology 106(1): e4530. 10.1002/ecy.4530

Handling Editor: Mark C. Urban

DATA AVAILABILITY STATEMENT

Data (Davenport, 2024) are available in Zenodo at https://doi.org/10.5281/zenodo.10459588.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Appendix S1.

ECY-106-e4530-s001.pdf (249.9KB, pdf)

Data Availability Statement

Data (Davenport, 2024) are available in Zenodo at https://doi.org/10.5281/zenodo.10459588.

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