Make love not war: a common arthropod defence compound as sex pheromone in the forest cockchafer Melolontha hippocastani

Mate finding in the forest cockchafer, Melolontha hippocastani, occurs during a spectacular swarming period at dusk. The swarming flights are mainly performed by males whereas most of the females stay within the host trees and continue feeding. Males orientate towards damage-induced green leaf volatiles (GLV) allowing location of mechanically damaged foliage. In order to distinguish between unspecific leaf damage and damage caused by feeding females, male cockchafers orientate by a sex attractant. Here we show this compound to be 1,4-benzoquinone, which has been known for more than 40 years as a highly effective defence compound used by numerous arthropod species to repel enemies. 1,4-benzoquinone synergistically increased the number of males caught in GLV-baited funnel traps during the swarming period. Significantly more males landed on wire cages baited with a combination of 1,4-benzoquinone and GLV than on cages baited with only GLV. The results suggest that the sex pheromone of M. hippocastani might have evolved from a primary role as a defence compound.     

Mate finding in the forest cockchafer, Melolontha hippocastani, mediated by volatiles from plants and females

The response of the forest cockchafer, Melolontha hippocastani F. (Coleoptera, Scarabaeidae), towards volatiles emitted by different host plants and conspecifics was tested in field experiments during the flight period at dusk. Funnel traps containing artificially damaged leaves from the host plants Carpinus betulus L. and Quercus rubra L., as well as from the non‐host plant Prunus serotina Ehrh. caught significantly more beetles than empty control traps. On the other hand, traps baited with undamaged leaves from Q. rubra did not catch significantly more beetles than empty controls. Leaves from C. betulus damaged by beetle feeding did not attract more beetles than artificially damaged leaves. By use of gas chromatography coupled with electroantennographic detection (GC‐EAD) electrophysiological responses of males and females were shown for 18 typical plant volatiles. A synthetic mixture of selected typical green plant volatiles was also highly attractive in the field. A total of 9982 beetles was caught during the field experiments, among them only 33 females. This suggests that attraction to damaged foliage during flight period at dusk is male‐specific. Field experiments testing the attractiveness of female M. hippocastani towards conspecific males by employing caged beetles and beetle extracts indicated that males of M. hippocastani use a female‐derived sex pheromone for mate location. On wired cages containing either unmated feeding females, or unmated females without access to foliage, or feeding males in combination with extracts from unmated females, significantly more males landed during the flight period than on comparable control cages containing feeding males or male extracts. A possible scenario of mate location in M. hippocastani involving feeding‐induced plant volatiles and a female‐derived sex pheromone is discussed.     

Males,but not females,contribute to sexual isolation between two sympatric species of <Emphasis Type="Italic">Gambusia</Emphasis>

Reproductive isolation restricts genetic exchange between species. Various pre- and post-mating barriers, such as behavior, physiology and gametic incompatibility, have been shown to evolve in sympatry. In certain scenarios, isolation can be asymmetrical, where species differentially prefer conspecifics. We examined sexual isolation via conspecific mate preference between Gambusia affinis and G. geiseri in both sexes. To investigate male contribution to sexual isolation, we compared the number of mating attempts (gonopodial thrusts) directed at either a conspecific or a heterospecific female, in both species. We also examined sperm priming and expenditure in males in the presence of conspecific or heterospecific females. We then measured female preference for either a conspecific or heterospecific male, in both species. We found that males of both species preferred to mate with conspecific females, but showed no difference in sperm production or expenditure between conspecific and heterospecific females. Females of both species did not prefer conspecific over heterospecific males. Our results suggest that sexual isolation might be mediated by male mate choice in this system and not female choice, suggesting that there is asymmetrical reproductive isolation between the sexes in G. affinis and G. geiseri, but symmetrical species isolation.     

The scent of food and defence: green leaf volatiles and toluquinone as sex attractant mediate mate finding in the European cockchafer Melolontha melolontha

Mate finding in the European cockchafer, Melolontha melolontha , has been investigated in the field. Our data suggest that mainly males show flight activity at dusk. In a landing cage bioassay, male cockchafers preferred cages baited with females over cages baited with males. Gas chromatographic analysis of beetle extracts with electronantennographic detection revealed the presence of electrophysiologically active compounds, among them toluquinone. In funnel trap bioassays none of these compounds was attractive towards males per se . A mixture of green leaf volatiles (GLV) mimicking the bouquet of mechanically damaged leaves was highly attractive to M. melolontha males. The attractiveness of the same GLV mixture was synergistically enhanced when toluquinone was added to the lure. GLV attract males to damaged leaves. Toluquinone as a sex pheromone indicates the presence of conspecific females and synergistically enhances the attractiveness of GLV.     

Interspecific interactions and learning variability jointly drive geographic differences in mate preferences

Co‐occurrence of closely related species can cause behavioral interference in mating and increase hybridization risk. Theoretically, this could lead to the evolution of more species‐specific mate preferences and sexual signaling traits. Alternatively, females can learn to reject heterospecific males, to avoid male sexual interference from closely related species. Such learned mate discrimination could also affect conspecific mate preferences if females generalize from between species differences to prefer more species‐specific mating signals. Female damselflies of the banded demoiselle (Calopteryx splendens) learn to reject heterospecific males of the beautiful demoiselle (C. virgo) through direct premating interactions. These two species co‐occur in a geographic mosaic of sympatric and microallopatric populations. Whereas C. virgo males have fully melanized wings, male C. splendens wings are partly melanized. We show that C. splendens females in sympatry with C. virgo prefer smaller male wing patches in conspecific males after learning to reject heterospecific males. In contrast, allopatric C. splendens females with experimentally induced experience with C. virgo males did not discriminate against larger male wing patches. Wing patch size might indicate conspecific male quality in allopatry. Co‐occurrence with C. virgo therefore causes females to prefer conspecific male traits that are more species specific, contributing to population divergence and geographic variation in female mate preferences.     

Electrophysiological and behavioural responses of Melolontha melolontha to saturated and unsaturated aliphatic alcohols

Male Melolontha cockchafers are known to use green leaf volatiles induced by female feeding on host plants for their mate location. Earlier studies of the response of the European cockchafer, Melolontha melolontha L. (Coleoptera: Scarabaeidae), to different green leaf aldehydes, alcohols, and acetates revealed that only green leaf alcohols were attractive to males in the field. Females were not attracted at all by these volatiles. Here, we present a study that aimed to elucidate the structure–activity relationships of aliphatic alcohols. Both behavioural and physiological responses were studied in male and female M. melolontha by field tests and electroantennography. The compounds tested were saturated aliphatic alcohols with chain lengths between five and eight carbon atoms. Furthermore, the cockchafer's response to six‐carbon alcohols with (E)‐2‐, (E)‐3‐, (Z)‐2‐, (Z)‐3‐, and (Z)‐4‐configurated double bonds was tested. All compounds elicited dose‐dependent responses on the antennae of both sexes. In general, males showed a stronger normalized EAG response to the stimuli than females. However, only the naturally occurring six‐carbon alcohols, i.e., 1‐hexanol (E)‐2‐, (Z)‐3, and (E)‐3‐hexen‐1‐ol were attractive to M. melolontha males in the field. Females were not attracted to any of the tested compounds, confirming previous results on the olfactory orientation of Melolontha cockchafers.     

Mate Choice by Both Sexes Maintains Reproductive Isolation in a Species Flock of Pupfish (Cyprinodon spp) in the Bahamas

Female and male mate choices can reinforce reproductive isolation after sympatric speciation. Using a binary choice design, we examine the importance of visual cues in female mate choice in all three sympatric species of pupfish on San Salvador Island. We also examine the importance of olfactory cues in female choice of the hard‐shelled invertebrate specialist (Cyprinodon brontotheroides). We examine male mate choice in two of the three species, the scale eater (C. desquamator) and the detritivore (C. variegatus). Females of all three species use visual cues and prefer conspecific males. C. brontotheroides females do not use olfactory cues to discriminate between conspecific and heterospecific males. Males of C. desquamator and C. variegatus also preferentially court conspecific females. Thus, mutual mate choice, where both females and males exhibit mate choice, acts as a strong behavioral pre‐mating isolation mechanism in these sympatrically speciated pupfish.     

Male mate preference for large size overrides species recognition in allopatric flat lizards (<Emphasis Type="Italic"> Platysaurus broadleyi</Emphasis>)

Species recognition and mate preference both influence mate choice but can be in conflict with each other. In such cases the relative importance of the two functions depends on the costs of mating with heterospecifics and the frequency of such interactions. We tested whether male flat lizards (Platysaurus broadleyi) are able to discriminate between conspecific females and females of its allopatric sister species P. capensis. Given a simultaneous choice between equally sized females of both species, males courted conspecific females in 85% of trials. We then tested whether mate preference for large female body size can override species recognition. When offered a choice between a larger heterospecific female and a smaller conspecific, males showed no preference for conspecifics and courted larger heterospecific females in 58% of trials. Comparison of the two sets of trials showed a significant effect of female body size on male mate preference, supporting the hypothesis that mate quality can override species recognition.Communicated by K. Kotrschal     

Unilateral Mate Choice Causes Bilateral Behavioral Isolation Between Two Closely Related Phytophagous Ladybird Beetles (Coleoptera: Coccinellidae: Epilachninae)

Behavioral isolation between two sympatric and closely related phytophagous ladybird beetles, Henosepilachna vigintioctomaculata and Henosepilachana pustulosa, was studied in laboratory. Mating behavior was investigated using the male‐choice test. Males of H. vigintioctomaculata preferred conspecific females to heterospecific females as mates, whereas males of H. pustulosa did not show such preference. Females of H. vigintioctomaculata appeared more fastidious at mating than females of H. pustulosa, irrespective of the species of males. Males of neither species showed tenaciousness following rejection by females. The choice of conspecifics by H. vigintioctomaculata males and a difference in the intensity of rejection between H. vigintioctomaculata females (strong) and H. pustulosa females (weak) result in positive behavioral isolation in both directions of heterospecific matings. Our results thus indicated that positive unilateral mate choice yields bilateral behavioral isolation between these two species.     

Plant volatiles in the sexual communication of Melolontha hippocastani: response towards time-dependent bouquets and novel function of (Z)-3-hexen-1-ol as a sexual kairomone

1. Swarming males of Melolontha hippocastani are known to locate females that stay feeding within the host trees by orienting towards damage‐induced plant volatiles (green leaf volatiles) and a sex pheromone. Thus, volatiles emitted by freshly damaged leaves might indicate to a male the presence of currently feeding females. 2. The hypothesis was studied that volatiles from freshly damaged leaves are more attractive to males than volatiles from old damaged leaves. The odour bouquets of damaged leaves from three plant species that have been shown to attract male M. hippocastani in the field were analysed 10 min (fresh damage) and 1.5 h (old damage) after damaging, using coupled gas chromatography–mass spectrometry. The results showed clear differences between the bouquets: the bouquet of freshly damaged leaves of all species consisted of typical leaf aldehydes, i.e. hexanal, (Z)‐3‐hexenal, (Z)‐2‐hexenal, (E)‐2‐hexenal, the leaf alcohol (Z)‐3‐hexen‐1‐ol, and the corresponding acetate. One and a half hours after damage, aldehydes had almost vanished and (Z)‐3‐hexen‐1‐ol and (Z)‐3‐hexenyl acetate predominated; however males of M. hippocastani were equally attracted to traps baited with volatiles from old and freshly damaged leaves in field experiments. When traps were baited with synthetic volatile mixtures mimicking the bouquets of old and freshly damaged leaves, M. hippocastani males even preferred the old damage mixture. 3. Experiments addressing the role of individual green leaf volatiles revealed that only (Z)‐3‐hexen‐1‐ol was highly attractive while the other compounds tested individually were behaviourally inactive, however all tested compounds elicited comparable electrophysiological responses on male M. hippocastani antennae. 4. In analogy to the term aggregation kairomone used for feeding‐induced plant volatiles that attract both sexes of an insect, the term sexual kairomone is suggested to describe the novel function of (Z)‐3‐hexen‐1‐ol in the sexual communication of M. hippocastani.     

Attraction of forest cockchafer Melolontha hippocastani to (Z)-3-hexen-1-ol and 1,4-benzoquinone: application aspects

Abstract The application aspects of chemicals which attract the forest cockchafer, Melolontha hippocastani F., were investigated in field and laboratory experiments. Previous studies have shown that males of M. hippocastani are attracted by a synthetic mixture of green leaf volatiles (GLV) and the sex pheromone 1,4‐benzoquinone (BQ), that synergistically enhances the male response to GLV. In the present study, we demonstrated that BQ also synergised the male response to one single component of the GLV mixture, the leaf alcohol (Z)‐3‐hexen‐1‐ol (Z‐3‐ol). BQ enhanced the attractiveness of Z‐3‐ol at doses between 0.05 and 5 mg per trap, reaching a maximum at 5 × 10^?1 mg day^?1. The addition of an insecticide (cyhalothrin) to traps baited with BQ and Z‐3‐ol did not affect the lures’ attractiveness. However, when a conidiospore formulation of the entomopathogenic fungus Beauveria brongniartii (Saccardo) Petch was added, the attractiveness of baited traps was significantly reduced. Furthermore, two types of dispensers baited with a solution of BQ in Z‐3‐ol at 20 mg ml^?1 were tested over the entire 4‐week flight season. Both a membrane dispenser and a dispenser based on a porous polyethylene (PPE) absorbent disk attracted more males than controls over the entire 4 weeks. The membrane dispenser attracted as many males each week as a reference formulation that was renewed daily. Furthermore, the membrane dispenser attracted more males than the PPE dispenser in weeks 2–4, although laboratory experiments showed that the latter released even higher or at least equal amounts of the joined lure over the entire 4 weeks. However, estimation of the BQ/Z‐3‐ol ratios of the released material by solid phase microextraction (SPME) and coupled gas chromatography–mass spectrometry (GC‐MS) revealed that the membrane dispenser released a higher proportion of BQ than the PPE dispenser in weeks 2–4. Therefore, a higher BQ/Z‐3‐ol ratio might be responsible for the advantage of the membrane dispenser in the field.     

Body Size and the Maintenance of Reproductive Isolation in Stickleback,Genus Gasterosteus

The blackspotted stickleback Gasterosteus wheatlandi and the widely studied threespine stickleback G. aculeatus are sympatric throughout the former’s range and share many aspects of life history and reproductive behaviour. These two species differ significantly in size, with G. wheatlandi of both sexes measured at approximately 60% of the standard length of their G. aculeatus counterparts. This study concentrated on G. wheatlandi courtship behaviour and investigated its role in the maintenance of reproductive isolation with G. aculeatus. Specifically, the roles that (1) female body size plays in influencing male courtship preferences and (2) male body size and behaviour play in female courtship preferences were investigated through dummy and live conspecific and heterospecific stimulus presentations. Male G. wheatlandi courtship preferences are consistent with previously described patterns for G. aculeatus. Males of both species preferentially approach and court the larger of two simultaneously presented live or dummy females. Thus, the smaller G. wheatlandi males are indiscriminate with respect to assortative mate choice; not only preferring to approach and court more fecund conspecific females but, more significantly, G. aculeatus‐sized females. In contrast, females of both species demonstrate strong assortative courtship preferences. When presented with pairs of flask‐enclosed males, females of both species preferentially orient and court the conspecific male over the heterospecific. Similarly, when presented with a conspecific male and a heterospecific male presented singly, females prefer to enter the nest of the conspecific. Systematic analysis of the interactions between these pairs of fish (one male, one female) demonstrates that the breakdown of courtship in heterospecific courtship occurs late in the courtship sequence when the widely differing forms of male leading behaviour results in drastically differing female responses. I suggest that, as previously described in G. aculeatus, the supernormality effect plays a significant role in mediating adaptive mate choice behaviour in G. wheatlandi. However, the added element of a larger sympatric species introduces a possible cost in time and energy devoted to courting heterospecific, and sympatric, females that the larger G. aculeatus do not likely incur. There is substantial evidence from many sympatric G. aculeatus species pairs that there is assortative mate choice based on size and/or courtship behaviour. Courtship trials suggest a more pervasive role for females in assortative mate choice. Whether it is male body size per se, or in combination with behaviour, morphology or other cues, is unresolved in the present study.     

Adult female hamsters require long and sustained exposures to heterospecific males to avoid interspecific mating

Interspecific mating normally decreases female fitness. In many species, females avoid heterospecific males innately or by imprinting on their parents. Alternatively, adult females could learn to discriminate against heterospecific males after exposure to such males. For example, Syrian hamster (Mesocricetus auratus) females learn to discriminate between conspecific males and Turkish hamster (M. brandti) males during adulthood by exposure to males of both species. Adult females not previously exposed to Turkish hamster males will mate similarly with conspecific and heterospecific males. However, in a previous study we showed that exposure to a heterospecific male and a conspecific male for 8 days led to mating avoidance and aggression towards the heterospecific male. Here we conducted two experiments to investigate how much exposure to the heterospecific male was required for females to avoid mating with the heterospecific male (Experiment 1) and how long that avoidance lasted in the absence of continuous exposure to heterospecific stimuli (Experiment 2). Fast and durable learning would indicate the evolution of an efficient avoidance response. In Experiment 1, females were exposed to a heterospecific male for 1, 4 h, 4 or 8 days and then paired with that male. We found more avoidance of interspecific mating after 4 or 8 days of exposure than after 1 or 4 h of exposure. In Experiment 2, females were exposed to a heterospecific male for 8 days and then paired with that male either 10 min later or 8 days later. We found that after an 8-day delay females were highly sexually receptive to the heterospecific male. Additionally, a comparison between the current experiments and a previous study indicates that female Syrian hamsters do not require concurrent exposure to a conspecific male and a heterospecific male to learn to avoid interspecific mating; exposure to a heterospecific male is sufficient.     

Lack of species discrimination based on chemical cues by male sailfin mollies, <Emphasis Type="Italic">Poecilia latipinna</Emphasis>

When making mating decisions, individuals may rely on multiple cues from either the same or multiple sensory modalities. Although the use of visual cues in sexual selection is well studied, fewer studies have examined the role of chemical cues in mate choice. In addition, few studies have examined how visual and/or chemical cues affect male mating decisions. Male mate choice is important in systems where males must avoid mating with heterospecific females, as is found in a mating complex of Poecilia. Male sailfin mollies, Poecilia latipinna, are sexually parasitized by gynogenetic Amazon mollies, P. formosa. Little is known about the mechanism by which male sailfin mollies base their mating decisions. Here we tested the hypothesis that male sailfin mollies from an allopatric and a sympatric population with Amazon mollies use multiple cues to distinguish between conspecific and heterospecific females. We found that male sailfin mollies recognized the chemical cues of conspecific females, but we found no support for the hypothesis that chemical cues are by themselves sufficient for species discrimination. Lack of discrimination based on chemical cues alone may be due to the close evolutionary history between P. latipinna and P. formosa. Males from populations sympatric with Amazon mollies did not differentially associate with females of either of the two species when given access to both visual and chemical cues of the females, yet males from the allopatric population did associate more with conspecific females than with heterospecific females in the presence of both chemical and visual cues. The lack of discrimination by males from the sympatric population between conspecific and heterospecific females based on both chemical and visual cues suggests that these males require more complex combinations of cues to distinguish species, possibly due to the close relatedness of these species.     

Widespread reliance on olfactory sex and species identification by lyssomanine and spartaeine jumping spiders

Jumping spiders (Salticidae) are renowned for their exceptional vision, but this does not preclude use of other senses. Here we provide evidence that olfactory pheromones are widespread in the Spartaeinae and Lyssomaninae, two subfamilies regarded as basal clades within the Salticidae. Pheromone use by salticids was tested in a series of experiments: males were tested with the odour of conspecific females, heterospecific females, and conspecific males, and females were tested with the odour of conspecific males. With seven of the 29 species tested, we also tested males using the draglines of conspecific females (spider absent) as the odour source. Males of all species tested were attracted to the odour of conspecific females and to the odour of the draglines of conspecific females. There was no evidence of males responding to the odour of heterospecific females or conspecific males, or of females responding to the odour of conspecific males. These findings suggest that it is primarily males that respond to olfactory sex pheromones, consistent with the apparent trend within spiders of males more actively searching for females and females placing greater emphasis on mate‐choice decisions. Compared with most salticid groups, lyssomanines and spartaeines are unusually sedentary and this lifestyle may favour olfactory mate searching. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 107 , 664–677.     

Incomplete behavioural isolation and asymmetric female preference in darter sister species (Percidae: Etheostoma)

In this study, female preferences and behavioural isolation were estimated in a pair of allopatric sister species, Etheostoma duryi and Etheostoma flavum. Dichotomous mate preference trials were conducted to determine whether females prefer to associate with conspecific over heterospecific males and free‐spawning assays were conducted to determine whether those preferences translated into behavioural isolation. Dichotomous mate choice trials revealed asymmetric female preference, as female E. flavum preferred conspecific males, whereas female E. duryi showed no preference. Free‐spawning assays indicated that behavioural isolation remains incomplete between E. duryi and E. flavum (I_B = 0·19). In addition to female mating preferences, male behaviour also appeared to influence mating outcomes as male E. flavum consistently courted conspecific females more often in free‐spawning assays whereas male E. duryi did not. The data therefore suggest that despite marked divergence in male nuptial colour, divergence in female preferences between these species may not be sufficient to maintain species boundaries upon secondary contact. These results contrast with similar work in a sympatric darter species pair and may be explained by considering the contributions of reinforcement and differences in colour pattern as well as colour value.     

Plasticity of the mate choice mind: courtship evokes choice‐like brain responses in females from a coercive mating system

Female mate choice is fundamental to sexual selection, and determining molecular underpinnings of female preference variation is important for understanding mating character evolution. Previously it was shown that whole‐brain expression of a synaptic plasticity marker, neuroserpin, positively correlates with mating bias in the female choice poeciliid, Xiphophorus nigrensis, when exposed to conspecific courting males, whereas this relationship is reversed in Gambusia affinis, a mate coercive poeciliid with no courting males. Here we explore whether species‐level differences in female behavioral and brain molecular responses represent ‘canalized’ or ‘plastic’ traits. We expose female G. affinis to conspecific males and females, as well as coercive and courting male Poecilia latipinna, for preference assays followed by whole‐brain gene expression analyses of neuroserpin, egr‐1 and early B. We find positive correlations between gene expression and female preference strength during exposure to courting heterospecific males, but a reversed pattern following exposure to coercive heterospecific males. This suggests that the neuromolecular processes associated with female preference behavior are plastic and responsive to different male phenotypes (courting or coercive) rather than a canalized response linked to mating system. Further, we propose that female behavioral plasticity may involve learning because female association patterns shifted with experience. Compared to younger females, we found larger, more experienced females spend less time near coercive males but associate more with males in the presence of courters. We thus suggest a conserved learning‐based neuromolecular process underlying the diversity of female mate preference across the mate choice and coercion‐driven mating systems.     

Drosophila melanogaster males respond differently at the behavioural and genome‐wide levels to Drosophila melanogaster and Drosophila simulans females

Drosophila melanogaster are found in sympatry with Drosophila simulans, and matings between the species produce nonfertile hybrid offspring at low frequency. Evolutionary theory predicts that females choose mates, so males should alter their behaviour in response to female cues. We show that D. melanogaster males quickly decrease courtship towards D. simulans females. Courtship levels are reduced within 5 min of exposure to a heterospecific female, and overall courtship is significantly lower than courtship towards conspecific females. To understand changes at the molecular level during mate choice, we performed microarray analysis on D. melanogaster males that courted heterospecific D. simulans females and found nine genes have altered expression compared with controls. In contrast, males that court conspecific females alter expression of at least 35 loci. The changes elicited by conspecific courtship likely modulate nervous system function to reinforce positive conspecific signals and dampen the response to heterospecific signals.     

SEX DIFFERENCES IN MATE RECOGNITION AND CONSPECIFIC PREFERENCE IN SPECIES WITH MUTUAL MATE CHOICE

Sexual isolation is often assumed to arise because choosy females recognize and reject heterospecific males as mates. Yet in taxa in which both males and females are choosy, males might also recognize and reject heterospecific females. Here, we asked about the relative contribution of the sexes to the strong sexual isolation found in limnetic–benthic species pairs of threespine sticklebacks, which show mutual mate choice. We asked whether males and females of the two species recognize conspecifics and also prefer to mate with them. We found evidence for mate recognition by both sexes but only females prefer conspecifics. The nature of male courtship depended on which species of female they were courting, indicating that males recognized conspecific females and differentiated them from heterospecifics. However, males courted both species of females with equal vigor and changed courtship in a manner that would increase the chance of mating with heterospecifics. Females both recognized conspecifics and strongly preferred them. They responded very little to heterospecific male courtship and almost never mated with them. Therefore, males are likely to undermine sexual isolation, but females uphold it. Despite mutual mate choice and mate recognition in both sexes, females are primarily responsible for sexual isolation in these taxa.     

Predation Risk Reduces a Female Preference for Heterospecific Males in the Green Swordtail

The presence of a predator can result in the alteration, loss or reversal of a mating preference. Under predation risk, females often change their initial preference for conspicuous males, favouring less flashy males to reduce the risk of being detected by predators. Previous studies on predator‐induced plasticity in mate preferences have given females a choice between more and less conspicuous conspecific males. However, in species that naturally hybridize, it is also possible that females might choose an inconspicuous heterospecific male over a conspicuous conspecific male under predation risk. Our study addresses this question using the green swordtail (Xiphophorus helleri) and the southern platyfish (Xiphophorus maculatus), which are sympatric in the wild. We hypothesized that X. helleri females would prefer the sworded conspecific males in the absence of a predator but favour the less conspicuous, swordless, heterospecific males in the presence of a predator. Contrary to our expectation, females associated more with the heterospecific male than the conspecific male in the control (no predator) treatment, and they were non‐choosy in the predator treatment. This might reflect that females were attracted to the novel male phenotype when there was no risk of predation but became more neophobic after predator exposure. Regardless of the underlying mechanism, our results suggest that predation pressure may affect female preferences for conspecific versus heterospecific males. We also found striking within‐population, between‐individual variation in behavioural plasticity: females differed in the strength and direction of their preferences, as well as in the extent to which they altered their preferences in response to changes in perceived predation risk. Such variation in female preferences for heterospecific males could potentially lead to temporal and spatial variation in hybridization rates in the wild.