Male fish use prior knowledge about rivals to adjust their mate choice

Social competence improves the performance of biomimetic robots leading live fish

Collective motion is commonly modeled with static interaction rules between agents. Substantial empirical evidence indicates, however, that animals may adapt their interaction rules depending on a variety of factors and social contexts. Here, we hypothesized that leadership performance is linked to the leader's responsiveness to the follower's actions and we predicted that a leader is followed longer if it adapts to the follower's avoidance movements. We tested this prediction with live guppies that interacted with a biomimetic robotic fish programmed to act as a 'socially competent' leader. Fish that were avoiding the robot were approached more carefully in future approaches. In two separate experiments we then asked how the leadership performance of the socially competent robot leader differed to that of a robot leader that either approached all fish in the same, non-responsive, way or one that did change its approach behavior randomly, irrespective of the fish's actions. We found that (1) behavioral variability itself appears attractive and that socially competent robots are better leaders which (2) require fewer approach attempts to (3) elicit longer average following behavior than non-competent agents. This work provides evidence that social responsiveness to avoidance reactions plays a role in the social dynamics of guppies. We showcase how social responsiveness can be modeled and tested directly embedded in a living animal model using adaptive, interactive robots.

Live fish learn to anticipate the movement of a fish-like robot. BIOINSPIRATION & BIOMIMETICS 2022;17:065007. [PMID: 36044889 DOI: 10.1088/1748-3190/ac8e3e]

The ability of an individual to predict the outcome of the actions of others and to change their own behavior adaptively is called anticipation. There are many examples from mammalian species-including humans-that show anticipatory abilities in a social context, however, it is not clear to what extent fishes can anticipate the actions of their interaction partners or what the underlying mechanisms are for that anticipation. To answer these questions, we let live guppies (Poecilia reticulata) interact repeatedly with an open-loop (noninteractive) biomimetic robot that has previously been shown to be an accepted conspecific. The robot always performed the same zigzag trajectory in the experimental tank that ended in one of the corners, giving the live fish the opportunity to learn both the location of the final destination as well as the specific turning movement of the robot over three consecutive trials. The live fish's reactions were categorized into a global anticipation, which we defined as relative time to reach the robot's final corner, and a local anticipation which was the relative time and location of the live fish's turns relative to robofish turns. As a proxy for global anticipation, we found that live fish in the last trial reached the robot's destination corner significantly earlier than the robot. Overall, more than 50% of all fish arrived at the destination before the robot. This is more than a random walk model would predict and significantly more compared to all other equidistant, yet unvisited, corners. As a proxy for local anticipation, we found fish change their turning behavior in response to the robot over the course of the trials. Initially, the fish would turn after the robot, which was reversed in the end, as they began to turn slightly before the robot in the final trial. Our results indicate that live fish are able to anticipate predictably behaving social partners both in regard to final movement locations as well as movement dynamics. Given that fish have been found to exhibit consistent behavioral differences, anticipation in fish could have evolved as a mechanism to adapt to different social interaction partners.

Does the presence of conspecifics facilitate exploratory behaviour in a cichlid fish (Etroplus suratensis)?

Animals confronted with any kind of novelty show behavioural responses driven by avoidance and exploration. The expression of both tendencies is modulated by anxiety. Especially in group-living animals, the presence of conspecifics can reduce anxiety in novel situations and hence increase exploratory tendency. Such intensified behavioural responses triggered by the social environment of an individual are called social facilitation. Here, we tested for social facilitation on the exploratory tendency of group-living juvenile Green chromide cichlid fish (Etroplus suratensis) by assessing each individual twice in an open field test: once alone and once together with a conspecific. Contrary to our expectations, we found no difference in exploratory behaviour between the groups. However, our results suggest that changes in exploratory tendency across the two treatments were highly individual, both in extent and direction, and are likely driven by the presence of the conspecific and the focal individual’s own behavioural type.

Do male meadow voles (Microtus pennsylvanicus) always prefer and signal to 'popular' females? The role of social information context

For males, the behaviour of rival conspecifics can provide vital information about finding reproductively available females, and which have high-quality phenotypes. While valuable, this information represents sperm competition risk and even the potential for rejection by females. Thus, males should show flexibility in signalling behaviours towards females based on social information acquired with emphasis on the context of that information. We examined male meadow vole scent-marking behaviours and preferences for females based on previously obtained social information across different contexts. Social information context resulted in increased scent marking depending on the social odour pairing. Males scent marked more near a female whose odour had associated with three older rivals than one younger rival. But males marked equally toward a female whose odour had associated with three younger rivals and a female whose odour had associated with one older rival. This demonstrates that social odours are not of all equal value and that males can distinguish differences. Males then use these differences to tailor their signalling towards potentially high-quality phenotype females thereby maximizing their reproductive opportunities.

The effect of social odour context on the amount of time male meadow voles, Microtus pennsylvanicus, self-groom

Communicational behaviours by individuals provide information for not only the intended target(s) of the signal but any non-target individual(s) that may be nearby. For terrestrial mammals a major form of communication and social information is through odours via scent marking and self-grooming. Self-grooming is a ubiquitous behaviour in mammals with the function thought to primarily be centred on personal care. But it has been found in rodents that self-grooming will occur in the presence of social odours thus potentially serving a communicative role. For example, male meadow voles (Microtus pennsylvanicus) self-groom in the presence of a female conspecific odour but not a male conspecific odour. Most studies examining self-grooming as a form of olfactory communication have used single odour donors but in a natural environment individuals will come across complex social odour situations. Therefore, we examined how male meadow voles respond to complex social odours with regards to their self-grooming behaviour. We tested the hypothesis that self-grooming can act as a form of olfactory communication and that male meadow voles will control this behaviour measured by differences in self-grooming rates based on social contexts. Male meadow voles did not show differences in the amount of time spent self-grooming to social odours that contained a female and varying number of rival males (0, 1, 3, or 5) or if the social odour contained an acquainted or novel male. Male meadow voles did self-groom more to a social odour that contained a female and a younger male compared to when the social odour contained a female and older male. Male meadow voles appear to adjust their self-grooming behaviour based on the context of the social information. This may be a strategy that can maximize that individual’s fitness by adjusting how much information is provided to potential rivals and mates.

Shifts in Reproductive Investment in Response to Competitors Lower Male Reproductive Success

AbstractIn many species, males exhibit phenotypic plasticity in sexually selected traits when exposed to social cues about the intensity of sexual competition. To date, however, few studies have tested how this plasticity affects male reproductive success. We initially tested whether male mosquitofish, Gambusia holbrooki (Poeciliidae), change their investment in traits under pre- and postcopulatory sexual selection depending on the social environment. For a full spermatogenesis cycle, focal males were exposed to visual and chemical cues of rivals that were either present (competitive treatment) or absent (control). Males from the competitive treatment had significantly slower-swimming sperm but did not differ in sperm count from control males. When two males competed for a female, competitive treatment males also made significantly fewer copulation attempts and courtship displays than control males. Further, paternity analysis of 708 offspring from 148 potential sires, testing whether these changes in reproductive traits affected male reproductive success, showed that males previously exposed to cues about the presence of rivals sired significantly fewer offspring when competing with a control male. We discuss several possible explanations for these unusual findings.

Smaller rival males do not affect male mate choice or cortisol but do affect 11-ketotestosterone in a unisexual-bisexual mating complex of fish

Male mate discrimination may be affected by the social environment (presence or absence of rival males or mates), which can also affect stress and sex hormones (e.g., cortisol and 11-ketotestosterone (11-KT)). The Amazon molly, Poecilia formosa, is an all-female fish species dependent on sperm from mating with male P. latipinna. We investigated male mate choice in P. latipinna between conspecific females and P. formosa with a rival male present and no rival male present. We measured cortisol and 11-KT release rates from all fish. The presence of a rival male had no effect on male mate choice for conspecific females nor overall mating effort. Male 11-KT decreased on the second day after exposure to a rival male on the first day. Focal male 11-KT is positively correlated with the size of the rival male. Both conspecific and heterospecific females released more 11-KT when in the rival male treatment than when not. Neither male nor female cortisol was affected by the presence or absence of the rival male. We did not find an effect of rival males on male mate choice in contrast to our prediction. Instead, our findings may indicate a hormonal response to social competition.

Factors that affect non-independent mate choice

Mate choice is generally regarded as an independent event, but a growing body of evidence indicates that it can be influenced by social information provided by conspecifics. This is known as non-independent mate choice. Individuals use information gathered by observing interactions between conspecifics to copy or not copy the mate choice of these conspecifics. In this review, we examine the factors that affect non-independent mate choice and mate choice copying and how it is influenced by social and environmental information that is available to the subject or focal individual. Specifically, we discuss how non-independent mate choice and whether individuals copy the choices of conspecifics can be influenced by factors such as habitat and differences in ecology, mating system and parental care. We focus on the social information provided to the focal animal, the model and the audience. Nearly all studies of non-independent mate choice and mate copying have focused on individuals in species that use visual cues as the source of social information. Nevertheless, we highlight studies that indicate that individuals in some species may use chemical cues and signals as sources of social information that may affect non-independent mate choice and mate copying.

The role of familiarity in signaller-receiver interactions

In animal communication, individuals of species exhibiting individual recognition of conspecifics with whom they have repeated interactions, receive signals not only from unfamiliar conspecifics, but also from individuals with whom they have prior experience. Empirical evidence suggests that familiarity with a specific signaller aids receivers in interpreting that signaller's signals, but there has been little theoretical work on this effect. Here, we develop a Bayesian decision-making model and apply it to the well-studied systems of primate ovulation signals. We compare the siring probability of learner males versus non-learner males, based on variation in their assessment of the best time to mate and mate-guard females. We compare males of different dominance ranks, and vary the number of females, and their cycle synchrony. We find strong fitness advantages for learners, which manifest very quickly. Receivers do not have to see the full range of a signaller's signals in order to start gaining familiarity benefits. Reproductive asynchrony and increasing the number of females both enhance learning advantages. We provide theoretical evidence for a strong advantage to specific learning of a signaller's range of signals in signalling systems. Our results have broad implications, not only for understanding communication, but in elucidating additional fitness benefits to group-living, the evolution of individual recognition, and other characteristics of animal behavioural biology.

Test of the Deception Hypothesis in Atlantic Mollies Poecilia mexicana-Does the Audience Copy a Pretended Mate Choice of Others?

Animals often use public information for mate-choice decisions by observing conspecifics as they choose their mates and then copying this witnessed decision. When the copier, however, is detected by the choosing individual, the latter often alters its behavior and spends more time with the previously non-preferred mate. This behavioral change is called the audience effect. The deception hypothesis states that the choosing individual changes its behavior to distract the audience from the preferred mate. The deception hypothesis, however, only applies if the audience indeed copies the pretended mate choice of the observed individual. So far, this necessary prerequisite has never been tested. We investigated in Atlantic molly males and females whether, first, focal fish show an audience effect, i.e., alter their mate choices in the presence of an audience fish, and second, whether audience fish copy the mate choice of the focal fish they had just witnessed. We found evidence that male and female Atlantic mollies copy the pretended mate choice of same-sex focal fish. Therefore, a necessary requirement of the deception hypothesis is fulfilled. Our results show that public information use in the context of mate choice can be costly.

Female Choice Undermines the Emergence of Strong Sexual Isolation between Locally Adapted Populations of Atlantic Mollies (Poecilia mexicana)

Divergent selection between ecologically dissimilar habitats promotes local adaptation, which can lead to reproductive isolation (RI). Populations in the Poecilia mexicana species complex have independently adapted to toxic hydrogen sulfide and show varying degrees of RI. Here, we examined the variation in the mate choice component of prezygotic RI. Mate choice tests across drainages (with stimulus males from another drainage) suggest that specific features of the males coupled with a general female preference for yellow color patterns explain the observed variation. Analyses of male body coloration identified the intensity of yellow fin coloration as a strong candidate to explain this pattern, and common-garden rearing suggested heritable population differences. Male sexual ornamentation apparently evolved differently across sulfide-adapted populations, for example because of differences in natural counterselection via predation. The ubiquitous preference for yellow color ornaments in poeciliid females likely undermines the emergence of strong RI, as female discrimination in favor of own males becomes weaker when yellow fin coloration in the respective sulfide ecotype increases. Our study illustrates the complexity of the (partly non-parallel) pathways to divergence among replicated ecological gradients. We suggest that future work should identify the genomic loci involved in the pattern reported here, making use of the increasing genomic and transcriptomic datasets available for our study system.

Mate-choice copying, social information processing, and the roles of oxytocin

Social and sexual behaviors, including that of mate choice, are dependent on social information. Mate choice can be modified by prior and ongoing social factors and experience. The mate choice decisions of one individual can be influenced by either the actual or potential mate choice of another female or male. Such non-independent mate choice, where individuals gain social information and socially learn about and recognizes potential mates by observing the choices of another female or male, has been termed "mate-choice copying". Here we first briefly review how, why, and under what circumstances individuals engage in mate-choice copying. Secondly, we review the neurobiological mechanisms underlying mate-choice copying. In particular, we consider the roles of the nonapeptide, oxytocin, in the processing of social information and the expression of mate-choice copying.

A conceptual review of mate choice: stochastic demography, within-sex phenotypic plasticity, and individual flexibility

Mate choice hypotheses usually focus on trait variation of chosen individuals. Recently, mate choice studies have increasingly attended to the environmental circumstances affecting variation in choosers' behavior and choosers' traits. We reviewed the literature on phenotypic plasticity in mate choice with the goal of exploring whether phenotypic plasticity can be interpreted as individual flexibility in the context of the switch point theorem, SPT (Gowaty and Hubbell 2009). We found >3000 studies; 198 were empirical studies of within‐sex phenotypic plasticity, and sixteen showed no evidence of mate choice plasticity. Most studies reported changes from choosy to indiscriminate behavior of subjects. Investigators attributed changes to one or more causes including operational sex ratio, adult sex ratio, potential reproductive rate, predation risk, disease risk, chooser's mating experience, chooser's age, chooser's condition, or chooser's resources. The studies together indicate that “choosiness” of potential mates is environmentally and socially labile, that is, induced – not fixed – in “the choosy sex” with results consistent with choosers' intrinsic characteristics or their ecological circumstances mattering more to mate choice than the traits of potential mates. We show that plasticity‐associated variables factor into the simpler SPT variables. We propose that it is time to complete the move from questions about within‐sex plasticity in the choosy sex to between‐ and within‐individual flexibility in reproductive decision‐making of both sexes simultaneously. Currently, unanswered empirical questions are about the force of alternative constraints and opportunities as inducers of individual flexibility in reproductive decision‐making, and the ecological, social, and developmental sources of similarities and differences between individuals. To make progress, we need studies (1) of simultaneous and symmetric attention to individual mate preferences and subsequent behavior in both sexes, (2) controlled for within‐individual variation in choice behavior as demography changes, and which (3) report effects on fitness from movement of individual's switch points.

Sex-Specific Audience Effect in the Context of Mate Choice in Zebra Finches

Animals observing conspecifics during mate choice can gain additional information about potential mates. However, the presence of an observer, if detected by the observed individuals, can influence the nature of the behavior of the observed individuals, called audience effect. In zebra finches (Taeniopygia guttata castanotis), domesticated males show an audience effect during mate choice. However, whether male and female descendants of the wild form show an audience effect during mate choice is unknown. Therefore, we conducted an experiment where male and female focal birds could choose between two distinctive phenotypes of the opposite sex, an artificially adorned stimulus bird with a red feather on the forehead and an unadorned stimulus bird, two times consecutively, once without an audience and once with an audience bird (same sex as test bird). Males showed an audience effect when an audience male was present and spent more time with adorned and less time with unadorned females compared to when there was no audience present. The change in time spent with the respective stimulus females was positively correlated with the time that the audience male spent in front of its cage close to the focal male. Females showed no change in mate choice when an audience female was present, but their motivation to associate with both stimulus males decreased. In a control for mate-choice consistency there was no audience in either test. Here, both focal females and focal males chose consistently without a change in choosing motivation. Our results showed that there is an audience effect on mate choice in zebra finches and that the response to a same-sex audience was sex-specific.

The effect of observers on behavior and the brain during aggressive encounters

What effect does an audience have on an animal's behavior and where is this influence registered in the brain? To answer these questions, we analyzed male cichlid fish fighting in the presence of audiences of various compositions and measured expression of immediate early genes in the brain as a proxy for neural activity. We hypothesized their behavior would change depending on who was watching them. We measured behavioral responses from both the "watchers" and the "watched" during aggressive encounters and found that males fighting in the presence of an audience were more aggressive than males fighting without an audience. Depending on the nature of the audience, immediate early gene expression in key brain nuclei was differentially influenced. Both when an audience of larger males watched fighting males, and when they were watching larger males fighting, nuclei in the brain considered homologous with mammalian nuclei known to be associated with anxiety showed increased activity. When males were in the presence of any audience or when males saw any other males fighting, nuclei in the brain known to be involved in reproduction and aggression were differentially activated relative to control animals. In all cases, there was a close relationship between patterns of brain gene expression between fighters and observers. This suggests that the network of brain regions known as the social behavior network, common across vertebrates, are activated not only in association with the expression of social behavior but also by the reception of social information.

Social cognition in fishes

Brain evolution has often been correlated with the cognitive demands of social life. Further progress depends on our ability to link cognitive processes to corresponding brain part sizes and structures, and, ultimately, to demonstrate causality. Recent research suggests that fishes are suitable to test general hypotheses about vertebrate social cognition and its evolution: brain structure and physiology are rather conserved among vertebrates, and fish are able to perform complex decisions in social context. Here, we outline the opportunities for experimentation and comparative studies using fish as model systems, as well as some current shortcomings in fish social cognition research.

Social recognition is context dependent in single male prairie voles

Single males might benefit from knowing the identity of neighbouring males when establishing and defending boundaries. Similarly, males should discriminate between individual females if this leads to more reproductive opportunities. Contextual social cues may alter the value of learning identity. Knowing the identity of competitors that intrude into an animal's territory may be more salient than knowing the identity of individuals on whose territory an animal is trespassing. Hence, social and environmental context could affect social recognition in many ways. Here we test social recognition of socially monogamous single male prairie voles, Microtus ochrogaster. In experiment 1 we tested recognition of male or female conspecifics and found that males discriminated between different males but not between different females. In experiment 2 we asked whether recognition of males is influenced when males are tested in their own cage (familiar), in a clean cage (neutral) or in the home cage of another male (unfamiliar). Although focal males discriminated between male conspecifics in all three contexts, individual variation in recognition was lower when males were tested in their home cage (in the presence of familiar social cues) compared to when the context lacked social cues (neutral). Experiment 1 indicates that selective pressures may have operated to enhance male territorial behaviour and indiscriminate mate selection. Experiment 2 suggests that the presence of a conspecific cue heightens social recognition and that home-field advantages might extend to social cognition. Taken together, our results indicate social recognition depends on the social and possibly territorial context.

Knowing your audience affects male-male interactions in Siamese fighting fish (Betta splendens)

Aggressive interactions between animals often occur in the presence of third parties. By observing aggressive signalling interactions, bystanders may eavesdrop and gain relevant information about conspecifics without the costs of interacting. On the other hand, interactants may also adjust their behaviour when an audience is present. This study aimed to test how knowledge about fighting ability of an audience affects aggressive interactions in male Siamese fighting fish. Subjects were positioned between two dyads of non-interacting males and allowed to observe both dyads shortly before the view to one of the dyads was blocked, and the dyads were allowed to interact. Subjects were subsequently exposed to an unknown opponent in the presence of either the winner or the loser of the seen or unseen interaction. The results suggest a complex role of the characteristic of an audience in the agonistic behaviours of a subject engaged in an interaction. The presence of a seen audience elicited more aggressive displays towards the opponent if the audience was a loser. This response was different in the presence of an unseen audience. Subjects then directed a higher aggressiveness against their opponent if the audience was a winner. These results also suggest a potentially more complex and interesting process allowing individuals to gain information about the quality and threat level of an unknown audience while it is interacting with a third party. The importance of information acquisition for an individual to adapt its behaviour and the role of communication networks in shaping social interactions are discussed.

Casanovas are liars: behavioral syndromes, sperm competition risk, and the evolution of deceptive male mating behavior in live-bearing fishes

Male reproductive biology can by characterized through competition over mates as well as mate choice. Multiple mating and male mate choice copying, especially in internally fertilizing species, set the stage for increased sperm competition, i.e., sperm of two or more males can compete for fertilization of the female's ova. In the internally fertilizing fish Poecilia mexicana, males respond to the presence of rivals with reduced expression of mating preferences (audience effect), thereby lowering the risk of by-standing rivals copying their mate choice. Also, males interact initially more with a non-preferred female when observed by a rival, which has been interpreted in previous studies as a strategy to mislead rivals, again reducing sperm competition risk (SCR). Nevertheless, species might differ consistently in their expression of aggressive and reproductive behaviors, possibly due to varying levels of SCR. In the current study, we present a unique data set comprising ten poeciliid species (in two cases including multiple populations) and ask whether species can be characterized through consistent differences in the expression of aggression, sexual activity and changes in mate choice under increased SCR. We found consistent species-specific differences in aggressive behavior, sexual activity as well as in the level of misleading behavior, while decreased preference expression under increased SCR was a general feature of all but one species examined. Furthermore, mean sexual activity correlated positively with the occurrence of potentially misleading behavior. An alternative explanation for audience effects would be that males attempt to avoid aggressive encounters, which would predict stronger audience effects in more aggressive species. We demonstrate a positive correlation between mean aggressiveness and sexual activity (suggesting a hormonal link as a mechanistic explanation), but did not detect a correlation between aggressiveness and audience effects. Suites of correlated behavioral tendencies are termed behavioral syndromes, and our present study provides correlational evidence for the evolutionary significance of SCR in shaping a behavioral syndrome at the species level across poeciliid taxa.

Casanovas are liars: behavioral syndromes, sperm competition risk, and the evolution of deceptive male mating behavior in live-bearing fishes

Male reproductive biology can by characterized through competition over mates as well as mate choice. Multiple mating and male mate choice copying, especially in internally fertilizing species, set the stage for increased sperm competition, i.e., sperm of two or more males can compete for fertilization of the female’s ova. In the internally fertilizing fish Poecilia mexicana, males respond to the presence of rivals with reduced expression of mating preferences (audience effect), thereby lowering the risk of by-standing rivals copying their mate choice. Also, males interact initially more with a non-preferred female when observed by a rival, which has been interpreted in previous studies as a strategy to mislead rivals, again reducing sperm competition risk (SCR). Nevertheless, species might differ consistently in their expression of aggressive and reproductive behaviors, possibly due to varying levels of SCR. In the current study, we present a unique data set comprising ten poeciliid species (in two cases including multiple populations) and ask whether species can be characterized through consistent differences in the expression of aggression, sexual activity and changes in mate choice under increased SCR. We found consistent species-specific differences in aggressive behavior, sexual activity as well as in the level of misleading behavior, while decreased preference expression under increased SCR was a general feature of all but one species examined. Furthermore, mean sexual activity correlated positively with the occurrence of potentially misleading behavior. An alternative explanation for audience effects would be that males attempt to avoid aggressive encounters, which would predict stronger audience effects in more aggressive species. We demonstrate a positive correlation between mean aggressiveness and sexual activity (suggesting a hormonal link as a mechanistic explanation), but did not detect a correlation between aggressiveness and audience effects. Suites of correlated behavioral tendencies are termed behavioral syndromes, and our present study provides correlational evidence for the evolutionary significance of SCR in shaping a behavioral syndrome at the species level across poeciliid taxa.

It's all in your head: the role of quantity estimation in sperm competition

The study of animal cognition has provided valuable data throughout the years, yet its reliance on laboratory work leaves some open questions. The main question is whether animals employ cognition in daily decision-making. The following discussion uses sperm competition (SC) as a test case for demonstrating the effect of cognition on routine choices, in this case, sexual selection. Cognition is manifested here by males' ability to represent the number of rivals competing with them. I claim that response to SC is driven by quantity estimation and the ability to assess competition magnitude cognitively. Hence, cognition can determine males' response to SC, and consequentially it can be selected within this context. This supports the argument that cognition constitutes an integral part of an individual's toolbox in solving real-life problems, and shows that physical and behavioural phenomena can expose cognition to selection and facilitate its evolution.

Sex and the public: Social eavesdropping, sperm competition risk and male mate choice

Mate choice can be sensitive to social cues from neighboring individuals, e.g., animals can copy mate choice decisions. Males that are at risk of being copied by others may respond to this with reduced preference expression ("audience effects"). We review the various pathways by which sperm competition risk affects (1) male mate copying behavior and (2) audience effects. For example, a recent study suggests that males gather complex social information on rivals' sexual competitiveness (sexual activity and attractiveness to females) and respond with reduced expression of mating preferences only "when it matters," i.e., when a sexually competitive rival is present.

Divergent evolution of male aggressive behaviour: another reproductive isolation barrier in extremophile poeciliid fishes?

Reproductive isolation among locally adapted populations may arise when immigrants from foreign habitats are selected against via natural or (inter-)sexual selection (female mate choice). We asked whether also intrasexual selection through male-male competition could promote reproductive isolation among populations of poeciliid fishes that are locally adapted to extreme environmental conditions [i.e., darkness in caves and/or toxic hydrogen sulphide (H(2)S)]. We found strongly reduced aggressiveness in extremophile P. oecilia mexicana, and darkness was the best predictor for the evolutionary reduction of aggressiveness, especially when combined with presence of H(2)S. We demonstrate that reduced aggression directly translates into migrant males being inferior when paired with males from non-sulphidic surface habitats. By contrast, the phylogenetically old sulphur endemic P. sulphuraria from another sulphide spring area showed no overall reduced aggressiveness, possibly indicating evolved mechanisms to better cope with H(2)S.

Predator-induced changes of female mating preferences: innate and experiential effects

Background

In many species males face a higher predation risk than females because males display elaborate traits that evolved under sexual selection, which may attract not only females but also predators. Females are, therefore, predicted to avoid such conspicuous males under predation risk. The present study was designed to investigate predator-induced changes of female mating preferences in Atlantic mollies (Poecilia mexicana). Males of this species show a pronounced polymorphism in body size and coloration, and females prefer large, colorful males in the absence of predators.

Results

In dichotomous choice tests predator-naïve (lab-reared) females altered their initial preference for larger males in the presence of the cichlid Cichlasoma salvini, a natural predator of P. mexicana, and preferred small males instead. This effect was considerably weaker when females were confronted visually with the non-piscivorous cichlid Vieja bifasciata or the introduced non-piscivorous Nile tilapia (Oreochromis niloticus). In contrast, predator experienced (wild-caught) females did not respond to the same extent to the presence of a predator, most likely due to a learned ability to evaluate their predators' motivation to prey.

Conclusions

Our study highlights that (a) predatory fish can have a profound influence on the expression of mating preferences of their prey (thus potentially affecting the strength of sexual selection), and females may alter their mate choice behavior strategically to reduce their own exposure to predators. (b) Prey species can evolve visual predator recognition mechanisms and alter their mate choice only when a natural predator is present. (c) Finally, experiential effects can play an important role, and prey species may learn to evaluate the motivational state of their predators.