How sexual and natural selection shape sexual size dimorphism: Evidence from multiple evolutionary scales

On the resolution of sexual conflict over shared traits

Anisogamy, different-sized male and female gametes, sits at the heart of sexual selection and conflict between the sexes. Sperm producers (males) and egg producers (females) of the same species generally share most, if not all, of the same genome, but selection frequently favours different trait values in each sex for traits common to both. The extent to which this conflict might be resolved, and the potential mechanisms by which this can occur, have been widely debated. Here, we summarize recent findings and emphasize that once the sexes evolve, sexual selection is ongoing, and therefore new conflict is always possible. In addition, sexual conflict is largely a multivariate problem, involving trait combinations underpinned by networks of interconnected genes. Although these complexities can hinder conflict resolution, they also provide multiple possible routes to decouple male and female phenotypes and permit sex-specific evolution. Finally, we highlight difficulty in the study of sexual conflict over shared traits and promising directions for future research.

The tale of the rattle: Using rattle size to understand growth and sexual dimorphism in an insular population of rattlesnakes (Crotalus oreganus caliginis)

Islands have played a key role in our understanding of rapid evolution. A large body of literature has examined morphological changes in response to insularity and isolation, which has yielded useful generalizations about how animals can adapt to live in very small geographic areas. However, understanding the evolution of morphological variation in insular populations often requires detailed data sets on longitudinal patterns of growth and development, and such studies typically necessitate long-term mark-recapture on a large sample of individuals. Rattlesnakes provide a unique opportunity to address some of these difficulties because the addition of rattle segments to the rattle string occurs with regular periodicity and their size directly correlates with the body size of the snake at the time of the ecdysis cycle generating the segment. Here, we used a large database of rattle segment sizes recorded from island (Isla Coronado Sur, Baja California, Mexico) and mainland (Camp Pendleton, California, United States) populations of Western Rattlesnakes (Crotalus oreganus and C. o. caliginis) that separated approximately 10,000 years ago to compare body sizes at different ecdysis cycles, which allowed us to assess differences in growth rates and patterns of sexual size dimorphism. Our results show that rattlesnakes on Isla Coronado Sur appear to be born smaller and grow more slowly than their mainland counterparts, resulting in a "dwarfed" island population. However, despite significant differences in body size, both populations exhibited the same degree of sexual dimorphism. Our study demonstrates the potential to use rattle characteristics to recover detailed estimates of fundamental demographic parameters.

Influence of microhabitat, fecundity, and parental care on the evolution of sexual size dimorphism in Caribbean Eleutherodactylus frogs

Rensch's rule suggests that sexual size dimorphism (SSD) increases with species size when males are the larger sex, whereas it decreases when females are the larger sex. However, the process responsible for this pattern remains obscure. SSD can result from sexual selection, such as intrasexual competition for access to mates, or from natural selection, due to resource partitioning or fecundity selection. We studied SSD in Caribbean Eleutherodactylus frogs using phylogenetic comparative methods to investigate the influence of microhabitat, fecundity, and parental care. Our results show that in Caribbean Eleutherodactylus females tend to be larger and, contrary to Rensch's rule, dimorphism increases with species size. SSD was not related to microhabitat use. However, SSD was positively correlated with fecundity, mediated by a greater increase in female size. SSD was also influenced by parental care, suggesting that male care promotes larger male size and reduces the female bias in SSD. As suggested for other anurans, female-biased SSD in Caribbean Eleutherodactylus results from fecundity selection, although the magnitude is countered by increased male size in species with paternal care. Our results highlight the importance of considering various selective forces that may act in concert to influence the evolution of SSD.

Phylogenetic and environmental patterns of sex differentiation in physiological traits across Drosophila species

Sex-based differences in physiological traits may be influenced by both evolutionary and environmental factors. Here we used male and female flies from >80 Drosophila species reared under common conditions to examine variance in a number of physiological traits including size, starvation, desiccation and thermal tolerance. Sex-based differences for desiccation and starvation resistance were comparable in magnitude to those for size, with females tending to be relatively more resistant than males. In contrast thermal resistance showed low divergence between the sexes. Phylogenetic signal was detected for measures of divergence between the sexes, such that species from the Sophophora clade showed larger differences between the sexes than species from the Drosophila clade. We also found that sex-based differences in desiccation resistance, body size and starvation resistance were weakly associated with climate (annual mean temperature/precipitation seasonality) but the direction and association with environment depended on phylogenetic position. The results suggest that divergence between the sexes can be linked to environmental factors, while an association with phylogeny suggests sex-based differences persist over long evolutionary time-frames.

Selection in the city: Rapid and fine-scale evolution of urban eastern water dragons

Oceanic archipelagos have long been treated as a Petri dish for studies of evolutionary and ecological processes. Like archipelagos, cities exhibit similar patterns and processes, such as the rapid phenotypic divergence of a species between urban and nonurban environments. However, on a local scale, cities can be highly heterogenous, where geographically close populations can experience dramatically different environmental conditions. Nevertheless, we are yet to understand the evolutionary and ecological implications for populations spread across a heterogenous cityscape. To address this, we compared neutral genetic divergence to quantitative trait divergence within three native riparian and four city park populations of an iconic urban adapter, the eastern water dragon. We demonstrated that selection is likely acting to drive divergence of snout-vent length and jaw width across native riparian populations that are geographically isolated and across city park populations that are geographically close yet isolated by urbanization. City park populations as close as 0.9 km exhibited signs of selection-driven divergence to the same extent as native riparian populations isolated by up to 114.5 km. These findings suggest that local adaptation may be occurring over exceptionally small geographic and temporal scales within a single metropolis, demonstrating that city parks can act as archipelagos for the study of rapid evolution.

Extreme natural size variation in both sexes of a sexually cannibalistic mantidfly

In sexually cannibalistic animals, the relative sizes of potential mates often predict the outcome of aggressive encounters. Mantidflies are spider egg predators as larvae and generalist predators as adults. Unlike most cannibalistic species, there is considerable individual variation in body size in both sexes. Using preserved collections of Dicromantispa sayi, we focused on three body size metrics that we found to be positively correlated and accurately measured across researchers. We found extreme size variation in both sexes: the largest 10% of females were 1.72× larger than the smallest 10%, and the largest 10% of males were 1.65× larger than the smallest 10%. On average, females were 7.94% larger than males. In exploring possible causes of this variation, we uncovered differences among populations. To explore the effect of spider egg sac size on adult mantidfly size, we reared mantidfly larvae on egg sacs from two jumping spider species with small or large egg sacs. Mantidfly larvae reared on small egg sacs were smaller than those reared on large egg sacs. This study provides the groundwork to design ecologically relevant experiments exploring the causes and consequences of extreme size variation in an understudied system with intriguing natural history.

How urbanization affects sexual communication

Urbanization is rapidly altering landscapes worldwide, changing environmental conditions, and creating novel selection pressures for many organisms. Local environmental conditions affect the expression and evolution of sexual signals and mating behaviors; changes in such traits have important evolutionary consequences because of their effect on reproduction. In this review, we synthesize research investigating how sexual communication is affected by the environmental changes associated with urbanization-including pollution from noise, light, and heavy metals, habitat fragmentation, impervious surfaces, urban heat islands, and changes in resources and predation. Urbanization often has negative effects on sexual communication through signal masking, altering condition-dependent signal expression, and weakening female preferences. Though there are documented instances of seemingly adaptive shifts in trait expression, the ultimate impact on fitness is rarely tested. The field of urban evolution is still relatively young, and most work has tested whether differences occur in response to various aspects of urbanization. There is limited information available about whether these responses represent phenotypic plasticity or genetic changes, and the extent to which observed shifts in sexual communication affect reproductive fitness. Our understanding of how sexual selection operates in novel, urbanized environments would be bolstered by more studies that perform common garden studies and reciprocal transplants, and that simultaneously evaluate multiple environmental factors to tease out causal drivers of observed phenotypic shifts. Urbanization provides a unique testing ground for evolutionary biologists to study the interplay between ecology and sexual selection, and we suggest that more researchers take advantage of these natural experiments. Furthermore, understanding how sexual communication and mating systems differ between cities and rural areas can offer insights on how to mitigate negative, and accentuate positive, consequences of urban expansion on the biota, and provide new opportunities to underscore the relevance of evolutionary biology in the Anthropocene.

Ecological sexual dimorphism is modulated by the spatial scale of intersexual resource competition

In Focus: Li, X-Y., & H. Kokko. (2021). Sexual dimorphism driven by intersexual resource competition: Why is it rare, and where to look for it? Journal of Animal Ecology, 00, 1-13. Ecological sexual dimorphism, that is differences between the sexes in traits that are naturally selected as opposed to sexually selected, is gaining increasing attention after having often been dismissed as the 'less-parsimonious' explanation for differences between sexes. One potential driver of ecological sexual dimorphism is intersexual resource competition, in a process analogous to ecological character displacement between species; yet, clear empirical examples are scarce. Li and Kokko present mathematical models that introduce novel pieces to the puzzle: the role of the scale of mating competition and the spatial variation in resource availability. They show that ecological sexual dimorphism evolves when local mating groups are small (e.g. monogamous pairs) and when different resources are homogeneously available across habitats. Counterintuitively, larger mating groups (e.g. polygyny), and consequently higher intralocus sexual conflict, lead to sexual monomorphism. Habitat heterogeneity also leads to overlapping niches, although it can sometimes drive polymorphism within sexes. This study highlights why the conditions for intrasexual resource competition to drive evolution of sexual dimorphism are stringent, even in the absence of genetic constraints or competing species. Crucially, it highlights the importance of considering the mating system and the spatial scale of resource competition for understanding the occurrence of ecological sexual dimorphism, showing a large potential for future work considering different aspects of species' life histories and spatial dynamics.

Environmental drivers of sexual dimorphism in a lizard with alternative mating strategies

Understanding the relative importance of sexual and natural selection in shaping morphological traits is a long-standing goal of evolutionary ecology. Male-biased sexual size dimorphism (SSD) is typically associated with male-male competition. Similarly, male polymorphisms are considered a consequence of competitive social interactions. This classic paradigm overlooks the fact that environmental factors mediate social interactions and can lead to ecological adaptations. Common side-blotched lizards, Uta stansburiana, are a model system for this paradigm due to well-known rock-paper-scissors social dynamics between male morphs. SSD in this species has been considered primarily a consequence of social interactions, with male size resulting from the number of morphs in each population and female size being constrained through fecundity benefits. We test if the environment explains intraspecific variation in SSD and number of male morphs in U. stansburiana. By compiling data from 49 populations, we show that environmental variables are stronger predictors of SSD than the number of male morphs. Similarly, we show that the environment mediates SSD and potentially contributes to morph loss in colder environments. We propose that the environment favours smaller males in areas of high seasonality. Our results demonstrate the importance of the environment as a mediator of SSD.

Contingency and determinism in the evolution of bird song sound frequency

Sexual signals are archetypes of contingent evolution: hyper-diverse across species, often evolving fast and in unpredictable directions. It is unclear to which extent their evolutionary unpredictability weakens deterministic evolution, or takes place bounded by deterministic patterns of trait evolution. We compared the evolution of sound frequency in sexual signals (advertisement songs) and non-sexual social signals (calls) across > 500 genera of the crown songbird families. Contrary to the acoustic adaptation hypothesis, we found no evidence that forest species used lower sound frequencies in songs or calls. Consistent with contingent evolution in song, we found lower phylogenetic signal for the sound frequency of songs than calls, which suggests faster and less predictable evolution, and found unpredictable direction of evolution in lineages with longer songs, which presumably experience stronger sexual selection on song. Nonetheless, the most important deterministic pattern of sound frequency evolution—its negative association with body size—was stronger in songs than calls. This can be explained by songs being longer-range signals than most calls, and thus using sound frequencies that animals of a given size produce best at high amplitude. Results indicate that sexual selection can increase aspects of evolutionary contingency while strengthening, rather than weakening, deterministic patterns of evolution.

The comparative method is not macroevolution: across-species evidence for within-species process

It is common for studies that employ the comparative method for the study of adaptation, i.e. documentation of potentially adaptive across-species patterns of trait-environment or trait-trait correlation, to be designated as "macroevolutionary." Authors are justified in using "macroevolution" in this way by appeal to definitions such as "evolution above the species level." I argue that regarding the comparative method as "macroevolutionary" is harmful because it hides in serious ways the true causal content of hypotheses tested with the comparative method. The comparative method is a means of testing hypotheses of adaptation and their alternatives. Adaptation is a population level phenomenon, involving heritable interindividual variation that is associated with fitness differences. For example, given heritable intrapopulational variation, more streamlined individuals in populations of fast-moving aquatic animals have higher locomotory efficiency and thus better survivorship and more resources directed to reproduction than less streamlined ones. Direct evidence consistent with this population-level scenario includes the observation that many unrelated species of fast-moving aquatic animals have similar streamlined shapes, an example of the comparative method. Crucial to note in this example is that although the data are observed across species, the comparative method for studying adaptation tests hypotheses regarding standard population-level natural selection with no content that can be construed as "macro." Even less "macro," individual-level developmental dynamics can limit or bias the range of variants available for selection. Calling any of these studies "macroevolutionary" implies that some additional process is at work, shrouding the need to test adaptation hypotheses and study the range of variants that can be produced in development.

Egg-laying site, fecundity and degree of sexual size dimorphism in frogs

Female fecundity is an important selective force leading to female-biased sexual size dimorphism (SSD) in frogs. Because anurans exhibit diverse reproductive modes, we investigated whether variation in SSD and fecundity are related with oviposition site. We asked whether arboreal breeding species show pronounced female-biased SSD and if, paradoxically, females have lower fecundity because of the costs of carrying oocytes and amplectant males. Conversely, we tested whether species that deposit eggs in concealed sites show less pronounced SSD, because females do not carry males and space limitation may reduce female size and fecundity. Our results showed that, in general, males were approximately 20% smaller than females. However, for species with hidden oviposition sites, males and females exhibited more similar body sizes and arboreal hylids showed more pronounced female-biased SSD. Overall, fecundity was higher in aquatic breeders, as expected, but in hylids, fecundity was smaller in arboreal breeders, which suggests that arboreality may impose restrictions on fecundity. By analysing SSD in a broader and more specific lineage (Hylidae), we found that reproductive microhabitat may also influence female size and fecundity, playing an important role in the evolution of SSD in frogs at different evolutionary scales.

Interspecific allometry for sexual shape dimorphism: Macroevolution of multivariate sexual phenotypes with application to Rensch's rule

Allometric trends in the degree of sexual dimorphism with body size have long fascinated evolutionary biologists. Many male-biased clades display more prominent sexual dimorphism in larger taxa (Rensch's rule), with most examples documenting this pattern for body size dimorphism. Although sexual dimorphism in traits other than body size is equally functionally relevant, characterizing allometric patterns of sexual dimorphism in such traits is hampered by lack of an analytical framework that can accommodate multivariate phenotypes. In this article, we derive a multivariate equivalency for investigating trends in sexual dimorphism-relative to overall body size-across taxa and provide a generalized test to determine whether such allometric patterns correspond with Rensch's rule. For univariate linear traits such as body size, our approach yields equivalent results to those from standard procedures, but our test is also capable of detecting trends in multivariate datasets such as shape. Computer simulations reveal that the method displays appropriate statistical properties, and an empirical example in Mediterranean lizards provides the first demonstration of Rensch's rule in a multivariate phenotype (head shape). Our generalized procedure substantially extends the analytical toolkit for investigating macroevolutionary patterns of sexual dimorphism and seeking a better understanding of the processes that underlie them.

Reproductive phenotype predicts adult bite-force performance in sex-reversed dragons (Pogona vitticeps)

Sex-related differences in morphology and behavior are well documented, but the relative contributions of genes and environment to these traits are less well understood. Species that undergo sex reversal, such as the central bearded dragon (Pogona vitticeps), offer an opportunity to better understand sexually dimorphic traits because sexual phenotypes can exist on different chromosomal backgrounds. Reproductively female dragons with a discordant sex chromosome complement (sex reversed), at least as juveniles, exhibit traits in common with males (e.g., longer tails and greater boldness). However, the impact of sex reversal on sexually dimorphic traits in adult dragons is unknown. Here, we investigate the effect of sex reversal on bite-force performance, which may be important in resource acquisition (e.g., mates and/or food). We measured body size, head size, and bite force of the three sexual phenotypes in a colony of captive animals. Among adults, we found that males (ZZm) bite more forcefully than either chromosomally concordant females (ZWf) or sex-reversed females (ZZf), and this difference is associated with having relatively larger head dimensions. Therefore, adult sex-reversed females, despite apparently exhibiting male traits as juveniles, do not develop the larger head and enhanced bite force of adult male bearded dragons. This pattern is further illustrated in the full sample by a lack of positive allometry of bite force in sex-reversed females that is observed in males. The results reveal a close association between reproductive phenotype and bite force performance, regardless of sex chromosome complement.

Presence of kin-biased social associations in a lizard with no parental care: the eastern water dragon (Intellagama lesueurii)

Numerous studies have observed kin-biased social associations in a variety of species. Many of these studies have focused on species exhibiting parental care, which may facilitate the transmission of the social environment from parents to offspring. This becomes problematic when disentangling whether kin-biased associations are driven by kin recognition, or are a product of transmission of the social environment during ontogeny, or a combination of both. Studying kin-biased associations in systems that lack parental care may aid in addressing this issue. Furthermore, when studying kin-biased social associations, it is important to differentiate whether these originate from preferential choice or occur randomly as a result of habitat use or limited dispersal. Here, we combined high-resolution single-nucleotide polymorphism data with a long-term behavioral data set of a reptile with no parental care to demonstrate that eastern water dragons (Intellagama lesueurii) bias their nonrandom social associations toward their kin. In particular, we found that although the overall social network was not linked to genetic relatedness, individuals associated with kin more than expected given availability in space and also biased social preferences toward kin. This result opens important opportunities for the study of kinship-driven associations without the confounding effect of vertical transmission of social environments. Furthermore, we present a robust multiple-step approach for determining whether kin-biased social associations are a result of active social decisions or random encounters resulting from habitat use and dispersal patterns.