Female social response to male sexual harassment in poeciliid fish: a comparison of six species

Female preference counteracts negative frequency dependent selection for a stable polymorphism in a livebearing fish

The maintenance of variation within natural populations is key for natural selection to operate. Polymorphism is an intriguing form of variation that involves the persistence of multiple discrete phenotypes called morphs. Polymorphism is often explained by negative frequency dependent selection (NFDS), under which rare morphs have an advantage, such that no one morph takes over. We tested whether mating polymorphism is maintained by NFDS generated by female choice, in the livebearing poeciliid fish, Girardinus metallicus, whose males are either common, plain morphs or rare, black morphs. Females were treated with one morph for several weeks and tested with both, in mate choice and direct competition assays. Contrary to NFDS via female choice, females preferred the morph with which they were treated. This may disrupt the polymorphism given the rarity of black morphs in the wild, unless black morphs have other advantages: we found that black morphs tended to exhibit higher mating activity, and other studies have demonstrated that they are more aggressive. Interestingly, only black morphs display to females prior to copulation; however, there is little evidence for female preference for this morph or the mating display. These results suggest functions for the mating display of black morphs beyond courtship, including aggressive mate guarding, and prompt a discussion of what constitutes courtship behavior.

Effects of past mating behavior versus past ejaculation on male mate choice and male attractiveness

Past reproductive effort allows males to assess their ability to acquire mates, but it also consumes resources that can reduce their future competitive ability. Few studies have examined how a male's reproductive history affects his subsequent mate choice, and, to date, no study has determined the relative contribution of past mating behavior and past ejaculate production because these two forms of investment are naturally highly correlated. Here, we disentangled the relative effects of past mating behavior and past ejaculate production in mosquitofish (Gambusia holbrooki) by experimentally preventing some males from ejaculating when trying to mate. We assessed the effect of mating behavior on mate choice by comparing males that had previously been with or without access to females and male rivals for 8 and 16 weeks and assessed the effect of ejaculation on mate choice by comparing males that either could or could not ejaculate when they had access to females for 16 weeks. Reproductive treatment did not affect male attractiveness, but it did affect male mate choice. Somewhat surprisingly, in five of the six treatment-by-age at testing combinations, males preferred a female in the vicinity of a male rival over a solitary female. This preference was marginally stronger for males that had previously engaged in mating behavior but were unaffected by past ejaculate production. We discuss the potential benefits to males of associating with another male when seeking mates. This is the first study to quantify the relative influence of pre- and post-copulatory reproductive investment on male mate choice.

Complex sexual-social environments produce high boldness and low aggression behavioral syndromes

Introduction

Evidence of animal personality and behavioral syndromes is widespread across animals, yet the development of these traits remains poorly understood. Previous research has shown that exposure to predators, heterospecifics, and urbanized environments can influence personality and behavioral syndromes. Yet, to date, the influence of early social experiences with conspecifics on the development of adult behavioral traits is far less known. We use swordtail fish (Xiphophorus nigrensis), a species with three genetically-determined male mating strategies (courtship display, coercion, or mixed strategy) to assess how different early-life social experiences shape adult behavioral development.

Methods

We raised female swordtails from birth to adulthood in density-controlled sexual-social treatments that varied in the presence of the type of male mating tactics (coercers only, displayers only, coercers and displayers, and mixed-strategists only). At adulthood, we tested females’ boldness, shyness, aggression, sociality, and activity.

Results

We found that the number of different mating strategies females were raised with (social complexity) shaped behavioral development more than any individual mating strategy. Females reared in complex environments with two male mating tactics were bolder, less shy, and less aggressive than females reared with a single male mating tactic (either courtship only or coercion only). Complex sexual-social environments produced females with behavioral syndromes (correlations between aggression and activity, shyness and aggression, and social interaction and activity), whereas simple environments did not.

Discussion

Importantly, the characteristics of these socially-induced behavioral syndromes differ from those driven by predation, but converge on characteristics emerging from animals found in urban environments. Our findings suggest that complexity of the sexual-social environment shapes the development of personality and behavioral syndromes to facilitate social information gathering. Furthermore, our research highlights the previously overlooked influence of sexual selection as a significant contributing factor to diverse behavioral development.

Social partner cooperativeness influences brain oxytocin transcription in Trinidadian guppies (Poecilia reticulata)

For non-kin cooperation to be maintained, individuals need to respond adaptively to the cooperative behaviour of their social partners. Currently, however, little is known about the biological responses of individuals to experiencing cooperation. Here, we quantify the neuroregulatory response of Trinidadian guppies (Poecilia reticulata) experiencing cooperation or defection by examining the transcriptional response of the oxytocin gene (oxt; also known as isotocin), which has been implicated in cooperative decision-making. We exposed wild-caught females to social environments where partners either cooperated or defected during predator inspection, or to a control (non-predator inspection) context, and quantified the relative transcription of the oxt gene. We tested an experimental group, originating from a site where individuals are under high predation threat and have previous experience of large aquatic predators (HP), and a control group, where individuals are under low predation threat and naïve to large aquatic predators (LP). LP, but not HP, fish showed different behavioural responses to the behaviour of their social environment, cooperating with cooperative partners and defecting when paired with defecting ones. In HP, but not LP, fish brain mid-section oxt relative transcription varied depending on social partner behaviour. HP fish experiencing cooperation during predator inspection had lower oxt transcription than those experiencing defection. This effect was not present in the control population or in the control context, where the behaviour of social partners did not affect oxt transcription. Our findings provide insight into the neuromodulation underpinning behavioural responses to social experiences, and ultimately to the proximate mechanisms underlying social decision-making.

Male size and reproductive performance in three species of livebearing fishes (Gambusia spp.): A systematic review and meta-analysis

The genus Gambusia represents approximately 45 species of polyandrous livebearing fishes with reversed sexual size dimorphism (i.e. males smaller than females) and with copulation predominantly via male coercion. Male body size has been suggested as an important sexually selected trait, but despite abundant research, evidence for sexual selection on male body size in this genus is mixed. Studies have found that large males have an advantage in both male-male competition and female choice, but that small males perform sneaky copulations better and at higher frequency and thus may sire more offspring in this coercive mating system. Here, we synthesized this inconsistent body of evidence using pre-registered methods and hypotheses. We performed a systematic review and meta-analysis of summary and primary (raw) data combining both published (n = 19 studies, k = 106 effect sizes) and unpublished effect sizes (n = 17, k = 242) to test whether there is overall selection on male body size across studies in Gambusia. We also tested several specific hypotheses to understand the sources of heterogeneity across effects. Meta-analysis revealed an overall positive correlation between male size and reproductive performance (r = 0.23, 95% confidence interval: 0.10-0.35, n = 36, k = 348, 4,514 males, three Gambusia species). Despite high heterogeneity, the large-male advantage appeared robust across all measures studied (i.e. female choice, mating success, paternity, sperm quantity and quality), and was considerably larger for female choice (r = 0.43, 95% confidence interval: 0.28-0.59, n = 14, k = 43). Meta-regressions found several important factors explaining heterogeneity across effects, including type of sperm characteristic, male-to-female ratio, female reproductive status and environmental conditions. We found evidence of publication bias; however, its influence on our estimates was attenuated by including a substantial amount of unpublished effects, highlighting the importance of open primary data for more accurate meta-analytic estimates. In addition to positive selection on male size, our study suggests that we need to rethink the role and form of sexual selection in Gambusia and, more broadly, to consider the ecological factors that affect reproductive behaviour in livebearing fishes.

Trade-offs between reproduction and behavioural resistance against ectoparasite infection

Reproduction is a key determinant of organismal fitness, but organisms almost always face the threat of parasite infection. Thus, potential trade-offs between mating and parasite resistance may have substantial impacts on the ecology and evolution of host species. Although trade-offs between microbial resistance and mating in arthropods are well-documented, there is a paucity of evidence that mating compromises host resistance to the ubiquitous threat posed by ectoparasites. Despite the centrality of reproduction to host fitness and the widespread risk of parasites, there is a dearth of experiments showing a trade-off between mating/reproduction and anti-parasite behaviours. In this study, we test if mating increases the susceptibility of female flies to mite infection. We also investigated a potential underlying mechanism for the trade-off: that mating reduces overall endurance and hence anti-parasitic defenses among female flies. We experimentally mated female Drosophila nigrospiracula, with or without a chance to recover from male harassment, and challenged them with a natural ectoparasite, the mite Macrocheles subbadius. Mated females, regardless of time for recovery from male harassment, acquired more infections than unmated females. Furthermore, mated females had lower endurance in negative geotaxis assays, suggesting the increased susceptibility is due to reduced endurance. Our research shows a trade-off between reproduction and parasite resistance in a host-macroparasite system and suggests that trade-off theory is a fruitful direction for understanding these associations.

Male Sexual Preference for Female Swimming Activity in the Guppy (Poecilia reticulata)

Mate choice that is based on behavioural traits is a common feature in the animal kingdom. Using the Trinidadian guppy, a species with mutual mate choice, we investigated whether males use female swimming activity-a behavioural trait known to differ consistently among individuals in many species-as a trait relevant for their mate choice. In the first experiment, we assessed male and female activity in an open field test alone (two repeated measures) and afterwards in heterosexual pairs (two repeated measures). In these pairs, we simultaneously assessed males' mating efforts by counting the number of sexual behaviours (courtship displays and copulations). Male and female guppies showed consistent individual differences in their swimming activity when tested both alone and in a pair, and these differences were maintained across both test situations. When controlling for male swimming behaviour and both male and female body size, males performed more courtship displays towards females with higher swimming activity. In a second experiment, we tested for a directional male preference for swimming activity by presenting males video animations of low- and high-active females in a dichotomous choice test. In congruence with experiment 1, we found males to spend significantly more time in association with the high-active female stimulus. Both experiments thus point towards a directional male preference for higher activity levels in females. We discuss the adaptive significance of this preference as activity patterns might indicate individual female quality, health or reproductive state while, mechanistically, females that are more active might be more detectable to males as well.

Females of a solitary bee reject males to collect food for offspring

The time dedicated to courtship and copulation is the most general cost of mating for females. However, quantitative estimates of this cost and the consequences for female mating behavior have been investigated for only a few model organisms and mostly under laboratory conditions. We determined the costs of copulations and persistent courtship by males in terms of time for females of the solitary bee Anthrenoides micans. We estimated the rate and duration of male mating behaviors and the consequences for sexual interactions for females with respect to the loss of foraging opportunity in the wild. Males invested most of their time searching for mates and intercepted foraging females every 3 min. Copulas lasted, on average, 10 times longer than the time females took to resist male mating attempts. Despite the high frequency of these rejections (82%), females spent 3-fold more time copulating than rejecting males. Considering the rate of encounters with males and the mean duration of flower visits by females, we estimated that females would perform 64% fewer flower visits per hour if they accepted all copulation attempts. The loss of time is especially significant in the natural habitat of the species, where host cacti blossom for extraordinary short periods of time and females compete with other cacti-specialized bees and conspecifics. Because the offspring production of a female solitary bee depends on its pollen collection capacity, reduced foraging performance directly influences female reproductive success.

Sex differences in cognitive performance and style across domains in mosquitofish (Gambusia affinis)

Given that the sexes often differ in their ecological and sexual selection pressures, sex differences in cognitive properties are likely. While research on sexually dimorphic cognition often focuses on performance, it commonly overlooks how sexes diverge across cognitive domains and in behaviors exhibited during a cognitive task (cognitive style). We tested male and female western mosquitofish (Gambusia affinis) in three cognitive tasks: associative learning (numerical discrimination), cognitive flexibility (detour task), and spatio-temporal learning (shuttlebox). We characterized statistical relationships between cognitive performances and cognitive style during the associative learning task with measures of anxiety, boldness, exploration, reaction time, and activity. We found sex differences in performance, cognitive style, and the relationships between cognitive domains. Females outperformed males in the spatio-temporal learning task, while the sexes performed equally in associate learning and cognitive flexibility assays. Females (but not males) exhibited a 'fast-exploratory' cognitive style during associative learning trials. Meanwhile, only males showed a significant positive relationship between domains (associative learning and cognitive flexibility). We propose that these sexually dimorphic cognitive traits result from strong sexual conflict in this taxon; and emphasize the need to explore suites of sex-specific cognitive traits and broader comparative work examining sexual selection and cognition.

Isotocin increases female avoidance of males in a coercive mating system: Assessing the social salience hypothesis of oxytocin in a fish species

The nonapeptide oxytocin (and its fish homolog isotocin (IT)) is an evolutionarily-conserved hormone associated with social behaviors across most vertebrate taxa. Oxytocin has traditionally been regarded as a prosocial hormone, but studies on social cognition in mammalian models suggest it may play a more nuanced role in modulating social discrimination based on social salience and stimulus valence. Here we test IT and its role in regulating female social decision-making and anxiety behaviors in a live-bearing fish with a male coercive mating system. Gambusia affinis males engage in a forced mating strategy, with frequent harassment and attempted copulatory thrusts directed towards unwilling females. Exogenous IT produced anxiolytic responses in female G. affinis that altered exploration (time in center of tank) but not time in dark vs. light regions of the tank. Exogenous IT also produced context-specific changes in social tendency: IT-treated G. affinis females spent less time associating with males while association time with conspecific females was not altered. Further, while overall activity levels were not changed by IT treatment, the amount of social behaviors IT-treated females directed towards males, but not females, was reduced. Our results support the social salience hypothesis of oxytocin action in a teleost and suggest that oxytocin's critical input into social cognitive processing is conserved across vertebrate taxa.

Sex biased expression of anti-Mullerian hormone (amh) gene in a live bearing fish, Gambusia holbrooki: Evolutionary implications and potential role in sex differentiation

The amh, a member of transforming growth factor-β (TGF-β) family, is known to play a critical role in vertebrate male sex differentiation, with its paralogue/s evolving to determine sex in few heterogametic (XX/XY) teleosts. However, it remains relatively unexplored in the reproductively unique live bearing teleosts. Therefore, this study comparatively examined the structure and content of G. holbrooki amh as well as characterised its expression. A paralogous Y-specific amh (amhy) was not detected, suggesting an unlikely role in sex determination. Two transcripts (1.4 and 1.5 kb) were detected in adults: the larger (1.5 kb) retaining intron 5, coding for a truncated AMH-N and no TGF-β domain. The small (1.4 kb) transcript, had both domains intact and clustered with members of Poeciliidae. In contrast to other vertebrates, a higher conservation between the N- rather than the C- terminus of amh in Poeciliidae was observed, suggesting an adaptation that may be unique to live bearing teleosts. The amh expression was 6 times higher in brain of both sexes and testis compared with ovaries (p = .001). Intriguingly, female splenic tissues showed 10 times higher expression (p = .006) and such female bias splenic expression has not been reported in any teleosts. Ontogenic expression was 25 times higher in male embryos at gastrulation stage (p = .001), much earlier than those reported in egg-laying teleosts. Such heightened expression in male embryos suggests a repressive role associated with proliferation and migration of primordial germ cells (PGCs) that are known to occur earlier at blastulation in teleosts-potentially influencing gonadal fate.

Social plasticity in the fish brain: Neuroscientific and ethological aspects

Social plasticity, defined as the ability to adaptively change the expression of social behavior according to previous experience and to social context, is a key ecological performance trait that should be viewed as crucial for Darwinian fitness. The neural mechanisms for social plasticity are poorly understood, in part due to skewed reliance on rodent models. Fish model organisms are relevant in the field of social plasticity for at least two reasons: first, the diversity of social organization among fish species is staggering, increasing the breadth of evolutionary relevant questions that can be asked. Second, that diversity also suggests translational relevance, since it is more likely that "core" mechanisms of social plasticity are discovered by analyzing a wider variety of social arrangements than relying on a single species. We analyze examples of social plasticity across fish species with different social organizations, concluding that a "core" mechanism is the initiation of behavioral shifts through the modulation of a conserved "social decision-making network", along with other relevant brain regions, by monoamines, neuropeptides, and steroid hormones. The consolidation of these shifts may be mediated via neurogenomic adjustments and regulation of the expression of plasticity-related molecules (transcription factors, cell cycle regulators, and plasticity products).

Sexual selection on the multicomponent display of black morph male Girardinus metallicus (Pisces: Poeciliidae)

Sexually selected displays often include suites of integrated traits. Black morph males of the poeciliid fish Girardinus metallicus perform courtship and aggressive displays that exhibit their conspicuous yellow and black coloration. Body size, gonopodium size and ventral black area are correlated with intermale aggression, which is key for access to mates. A previous study showed that females may prefer dominant males prior to watching them fight; however, that result was obtained in trials that allowed for male-male interactions across partitions, and to date no study has uncovered the traits important in female choice. We performed a more comprehensive investigation of the multicomponent sexual display including measures of male yellow hue, saturation and brightness. We examined the behavior of size-matched males paired to maximize the difference in yellow saturation, and measured female choice exclusive of male-male interactions and chemical cues. We found no female preference for any traits in the multicomponent sexual display. Males with brighter and more saturated yellow coloration were more likely to be dominant, and dominant males courted and attempted copulations more. Our results suggest that yellow coloration is sexually selected; however, the courtship display requires further investigation because we did not identify targets of female preference, and we discuss possible explanations for this finding.

Structure of sexual networks determines the operation of sexual selection

Sexual selection is a fundamental evolutionary process but remains debated, particularly in the complexity of polyandrous populations where females mate with multiple males. This lack of resolution is partly because studies have largely ignored the structure of the sexual network, that is, the pattern of mate sharing. Here, we quantify what we call mating assortment with network analysis to specify explicitly the indirect as well as direct relationships between partners. We first review empirical studies, showing that mating assortment varies considerably in nature, due largely to basic properties of the sexual network (size and density) and partly to nonrandom patterns of mate sharing. We then use simulations to show how variation in mating assortment interacts with population-level polyandry to determine the strength of sexual selection on males. Controlling for average polyandry, positive mating assortment, arising when more polygynous males tend to mate with more polyandrous females, drastically decreases the intensity of precopulatory sexual selection on male mating success (Bateman gradient) and the covariance between male mating success and postcopulatory paternity share. Average polyandry independently weakened some measures of sexual selection and crucially also impacted sexual selection indirectly by constraining mating assortment through the saturation of the mating network. Mating assortment therefore represents a key-albeit overlooked-modulator of the strength of sexual selection. Our results show that jointly considering sexual network structure and average polyandry more precisely describes the strength of sexual selection.

Positive relationship between risk-taking behaviour and aggression in subordinate but not dominant males of a Cuban poeciliid fish

Studies of integrated phenotypes sometimes reveal correlations between mating effort, favoured by sexual selection, and risk-taking, favoured by survival selection. We used Girardinus metallicus to examine the relationship between rank order of mating effort and risk-taking. We measured risk-taking in a novel environment containing a predator. We then paired males, using aggression to assign dominant or subordinate status, and examined mating behaviour. Dominant males showed higher mating effort, but did not exhibit any relationship between risk-taking and mating effort. Subordinate males exhibited a cross-context correlation, as males were either more willing to take risks and aggressive or more hesitant to take risks and nonaggressive. Less risk-averse, aggressive subordinate males may gain fitness advantages in a more realistic dominance hierarchy, despite being outranked by the rival with which they were paired in our study. Results highlight intraspecific variation in behavioural correlations and the importance of social environment in shaping integrated phenotypes.