The turn of the sword: length increases male swimming costs in swordtails

Sexual selection via female mate choice can result in the evolution of elaborate male traits that incur substantial costs for males. Despite increased interest in how female mating preferences contribute to the evolution of male traits, few studies have directly quantified the locomotor costs of such traits. A sexually selected trait that could affect movement costs is the sword exhibited by male swordtail fishes: while longer swords may increase male mating success, they could negatively affect the hydrodynamic aspects of swimming activities. Here, we examine the energetic costs of the sword in Xiphophorus montezumae by experimentally manipulating sword length and measuring male aerobic metabolism during two types of activity, routine swimming and courtship swimming. Direct measurements of oxygen consumption indicate that males with longer swords expend more energy than males with shortened swords during both types of swimming. In addition, the sword increases the cost of male courtship. Thus, while sexual selection via female choice favours long swords, males with longer swords experience higher metabolic costs during swimming, suggesting that sexual and natural selection have opposing effects on sword evolution. This study demonstrates a hydrodynamic cost of a sexually selected trait. In addition, this study discriminates between the cost of a sexually selected trait used in courtship and other courtship costs.     

Male Biases for Male Characteristics in Females in Priapella olmecae and Xiphophorus helleri (Family Poeciliidae)

Preexisting receiver biases can affect the evolution of sexually selected traits once traits favored by such biases arise. Female guayacón olmeca, Priapella olmecae , and green swordtail, Xiphophorus helleri , possess a bias favoring sworded conspecific males, despite phylogenetic evidence that the sexually selected sword expressed by male swordtails arose in Xiphophorus after the divergence of the two genera. In this study, we investigated the state of the bias favoring a sword in male Priapella and Xiphophorus to determine whether males also possess a bias that could operate in an intersexual selection context. Male P. olmecae preferred conspecific females with swords to those without swords. Thus, males and females in this unsworded species appear to share a preexisting bias favoring individuals of the opposite sex with swords. Male X. helleri , however, did not express a bias favoring sworded females; instead, in this species in which the sword is restricted to males, males discriminated against conspecific females with swords. Previous work suggests that female mate choice and male–male competition likely contribute to the maintenance of the sword in X. helleri . The sword may also play a role in sex recognition in swordtails. The absence of a sword preference in male green swordtails could reflect the current function of the sword.     

Covariation between predation risk, body size and fin elaboration in the green swordtail, Xiphophorus helleri

Natural and sexual selection can have either opposing or synergistic effects on the evolution of traits. In the green swordtail Xiphophorus helleri , sexual selection arising from female choice is known to favour larger males and males with longer swords. We examined variation in male and female size and fin morphology among 15 populations that varied in their predation environments. Males and females from populations in which piscivorous fishes were present had longer and deeper bodies than did males and females from populations in which piscivorous fishes were absent. Controlling for a positive effect of body size on sword length, males from populations in which piscivores were present had relatively shorter swords than did males from populations in which piscivores were absent. The associations between morphology and predation environment may be due to direct effects of predation, indirect effects of predation, other sources of selection that covary with predator presence, or other environmental effects on trait expression. These results suggest that while sexual selection favours longer swords, natural selection may have an opposing effect on sword length in populations with predators. Natural selection on body size, however, may act synergistically with sexual selection in populations with predators; both may favour the evolution of larger body size. The body size results for X. helleri contrast with related taxa that have become model systems for the study of life history evolution.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 83 , 87–100.     

Predator exposure alters female mate choice in the green swordtail

Female green swordtails, Xiphophorus helleri, show a matingpreference for males with brightly colored, elongated swords.This preference is thought to be due to a preexisting receiverbias favoring longer sworded males. In this study, we examinedvariation in the expression of this sword bias in females. Specifically,we tested the hypothesis that an increase in perceived predationrisk will decrease female response to males with longer swords.We used a video playback experiment to evaluate female choicebetween two recordings of a displaying male that differed onlyin sword length. We scored responses of females to these recordingsimmediately before and after they had been exposed to a videorecording of a predation event between a cichlid and a malepossessing a long sword. We found that prior to exposure tothis predation event, females preferred the male with the longersword. However, after exposure to the predator, females alteredtheir mating response, preferring the male with the sword removed.Exposure to the predator also caused an increase in the frequencywith which females moved from potential mating positions toa neutral zone. The results presented here suggest that thefemale preference for males with longer swords can be modulatedbased on the perceived risk of predation.     

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.     

Evolutionary change in a receiver bias: a comparison of female preference functions.

Female poeciliid fishes of the sister genera Xiphophorus and Priapella share a preference for males with swords, despite phylogenetic information suggesting that swords arose in Xiphophorus after the divergence of the two genera. This study examines the strength of sword and body-size preferences in a representative of both genera. A comparison of the preference functions reveals that the strength of the preference favouring a sword in P. olmecae is significantly stronger than that in X. helleri. This result demonstrates that the pre-existing bias is not evolutionarily fixed, and that there has been change in the bias favouring the sword, in either the Priapella lineage, or the Xiphophorus lineage, or in both. Although females in both species prefer conspecific males with swords, only X. helleri females also demonstrate a body-size preference. The preference functions for body size and sword length for X. helleri are not significantly different, whereas in P. olmecae the preference function for sword length is significantly stronger than for body size. These combined results indicate that an ancestral bias for body size cannot alone explain the pre-existing bias favouring a sword in P. olmecae.     

Male golden hamsters (<Emphasis Type="Italic">Mesocricetus auratus</Emphasis>) are more reactive than females to a visual predator cue

In most species of small mammals, males are exposed to higher levels of risk than females because they compete for mates, travel greater distances to find and procure mates, and/or defend a territory. This suggests that males and females might have different responses to risky situations, such as the presence of a predator. We tested responses to a visual predator cue (an owl silhouette) in male and female golden hamsters (Mesocricetus auratus). In a laboratory arena, there was no significant sex difference in the latency to enter the burrow or time spent in the burrow immediately after exposure to the owl silhouette. Males, however, were less likely to be active during the 3-min period following the animal’s exposure to the silhouette, indicating that male golden hamsters are more wary after exposure to an aerial predator cue than females. Most studies of responses to predators or predator cues have not considered sex differences, but our results show that males and females may have quite different responses to predator cues. Further work should be done to characterize and quantify sex differences in response to predators or predator cues.     

Hybrid origin of a swordtail species (Teleostei: Xiphophorus clemenciae) driven by sexual selection

The swordlike exaggerated caudal fin extensions of male swordtails are conspicuous traits that are selected for through female choice. Swords are one of only few examples where the hypothesis of a pre-existing bias is believed to apply for the evolution of a male trait. Previous laboratory experiments demonstrated that females prefer males with longer swords and even females from some swordless species show an affiliation for males of sworded species. Earlier phylogenetic studies based on maternally inherited mitochondrial DNA placed the sworded southern swordtail Xiphophorus clemenciae with swordless platies, contradicting its morphology-based evolutionary affinities. The analyses of new nuclear DNA markers now recover its traditional phylogenetic placement with other southern swordtails, suggesting that this species was formed by an ancient hybridization event. We propose that sexual selection through female choice was the likely process of hybrid speciation, by mating of platy females with males of an ancestral swordtail lineage. In artificial crosses of descendent species from the two potential ancestral lineages of X. clemenciae the hybrid and backcross males have swords of intermediate lengths. Additionally, mate choice experiments demonstrate that hybrid females prefer sworded males. These experimental lines of evidence make hybridization through xeno-specific sexual selection by female choice the likely mechanism of speciation.     

An evaluation of video playback using Xiphophorus helleri

Video playback is being increasingly used as a technique for behavioural research. The importance of critically evaluating the effectiveness of video playback is clear, as available video technology is not designed for nonhuman visual systems. We discuss several aspects concerning the perception of video images that could lead to inconclusive or erroneous results. Researchers should verify that behaviour observed in response to video playback is comparable to behaviour observed in response to live animals. We conducted such a verification using live and video playback methods to measure female response to swords of varying lengths in the green swordtail, Xiphophorus helleri. Using both methods, female response appeared to be an increasing function of male sword length. Females did not differ in their response to live and video versions of noncourting, noninteractive males, however, females tended to prefer video playbacks of males with longer swords, a result that has also been found in experiments using live males. These results suggest that females express the same qualitative mating preference, but not necessarily the same quantitative preference, for sword length when viewing video stimuli. Several methodological factors that may contribute to an apparent difference in the strength of the preference are discussed. Despite these differences, both methods produced comparable results; female response to sworded males tended to increase as sword length increased. These experiments demonstrate that video playback is an effective method to measure female preferences accurately in X. helleri and provide an example of how video playback can be evaluated in other species. Copyright 2000 The Association for the Study of Animal Behaviour.     

Secondary reduction of preference for the sword ornament in the pygmy swordtail Xiphophorus nigrensis (Pisces: Poeciliidae)

Female mating preferences can be secondarily lost for a number of reasons. We examined the preference of female pygmy swordtails, Xiphophorus nigrensis, for the sword, a conspicuous extension of the caudal fin in some males. Females failed to show a preference for conspecific males with swords when presented with live males naturally varying in sword length, with live males of manipulated sword length, and with synthetic animations of males expressing natural variation in sword length. Females showed a significant bias against swords when presented with synthetic animations bearing supernormal sword characteristics. The reduced preference for swords, relative to closely related fish, may result from an increase in the cost of choice due to predation risk, selection against mating with heterospecifics, or changes in the spatial and contrast properties of the conspecific signal.     

A test of sensory exploitation in the swordtail characin (Corynopoma riisei) based on colour matching between female prey and a male ornament

The sensory exploitation hypothesis states that pre-existing biases in female sensory systems may generate strong selection on male signals to match such biases. As environmental conditions differ between populations, sexual preferences resulting from natural selection are expected to vary as well. The swordtail characin (Corynopoma riisei) is a species in which males carry a flag-like ornament growing from the operculum that has been proposed to function as a prey mimic to attract females. Here, we investigated if female plasticity in feeding preferences is associated with plasticity in preference for an artificial male ornament in this species. Females were trained for 10 days by offering them differently coloured food items and were then tested for changes in preferences for differently coloured artificial male ornaments according to foraging experience. We found a rapid and pronounced change in female preference for the colouration of the artificial ornament according to food training. Thus our results support the possibility that sensory exploitation may act as a driving force for female preferences for male ornaments in this species.     

Environment contingent preferences: Exposure to visual cues of direct male–male competition and wealth increase women's preferences for masculinity in male faces

Previous studies show that parasite prevalence and mortality/health are related to cultural variation in women's preferences for attractive and masculine traits in men. Other studies have suggested that both male–male competition and wealth may also be important correlates of cross-cultural variation in women's masculinity preferences. Here we examined whether exposure to cues of direct male–male competition, violence, or wealth influenced women's face preferences. We showed women slideshows of images with cues of low and high direct male–male competition/violence or wealth and measured their visual preferences for masculine face traits. Recent visual experience changed women's preferences for facial masculinity, with women preferring more masculine male faces after exposure to images of men engaged in direct physical competition, images of weapons, or images depicting items of high monetary value. Recent visual experience had no significant effects on preferences for masculinity in same-sex faces. Given that high levels of direct physical competition and violence among males may increase the importance of direct intra-sexual competition, it may be adaptive for women to shift visual preferences in favor of males with face cues indicating physical strength and dominance over investment in such environments. Similarly, in wealthy environments investment may be less important than other aspects of quality and so it may be adaptive for women to shift visual preferences in favor of males with face cues indicating other aspects of quality over investment. Overall, our data demonstrate that preferences can be strategically flexible according to recent visual experience and support the notion of environment contingent preferences.     

Female mate choice and species recognition between two closely related cichlid fish of Lake Malawi,Metriaclima estherae and M. callainos

Cichlids are one of the most diverse and colourful groups of freshwater fishes in the world. Despite much investigation, the factors that promote speciation in these fishes are still uncertain. However, previous studies suggest that sexual selection on male colour is one of the main drivers of speciation among these fishes. Metriaclima estherae is a polymorphic cichlid species from Lake Malawi, and thus provides an ideal model for the investigation of the importance of colour as a species recognition cue. M. callainos is a closely related and morphologically similar species, with male colour pattern very similar to that of M. estherae. We tested female choice by giving females of the two species a choice between conspecific and heterospecific males in the presence and absence of visual (colour) and chemical cues. The results show that females of M. callainos were able to reliably recognize conspecific males, even when colour was eliminated as a cue. However, females of M. estherae did not prefer conspecific males, although they were able to discriminate between red and blue conspecific colour morphs by using chemical cues. These results suggest that species recognition cues may differ even among closely related species of cichlid fish, and that female preferences for male coloration may be weak in certain species.     

Predator preference for brightly colored males in the guppy: a viability cost for a sexually selected trait

Although conspicuous visual sexual signals, such as bright colors,in males serve to attract females in numerous species, theymay also attract the attention of potential predators and thusmay be costly in terms of increasing individual risk of mortalityto predation. Most models of the evolution of extravagant malesexual traits and female preferences for them assume that thesexually preferred male trait is costly to produce and maintain.However, there is surprisingly little empirical evidence fordirect fitness costs associated with sexually selected visualtraits that enhance male mating success. In the present study,we report a direct fitness cost for sexually selected, brightbody-color patterns in males in the form of an associated greaterrisk of mortality to predation. By using the guppy (Poeciliareticulata) and the blue acara cichlid fish (Aequidens pulcher)as a model prey–predator system, we demonstrate experimentallythat individual cichlids preferentially and consistently approached,attacked, and captured the more brightly colored of two size-matchedmale guppies presented simultaneously in staged encounters.This resulted in the brightly colored male incurring, on average,a significantly higher risk of mortality given an encounterwith the predator than with the drabber male in matched pairs.Our results constitute strong behavioral evidence for a directviability cost associated with bright coloration in male guppies,and they corroborate the generally accepted paradigm that directionalpredation by visual fish predators against brightly colored,adult male guppies underlies the evolution of the known divergentcolor patterns in natural guppy populations that experiencedifferent intensities of predation. The viability cost associatedwith bright conspicuous coloration in male guppies potentiallyreinforces for females the reliability of this sexually selectedtrait as an indicator trait of male quality.     

Cross-fostering Does Not Influence the Mate Preferences and Territorial Behaviour of Males in Lake Victoria Cichlids

The ability to recognise conspecifics in contexts of mate choice and territorial defence may have large effects on an individual's fitness. Understanding the development of assortative behaviour may shed light on how species assortative behaviour evolves and how it may influence reproductive isolation. This is the case not only for female mate preferences, but also for male mate preferences and male territorial behaviour. Here we test with a cross-fostering experiment whether early learning influences male mate preferences and male–male aggression biases in two closely related, sympatrically occurring cichlid species Pundamilia pundamilia and Pundamilia nyererei from Lake Victoria. Males that had been fostered, either by a conspecific female or by a heterospecific female, were tested for their aggression bias, as well as for their mate preferences, in two-way choice tests. Males cross-fostered with conspecific and heterospecific foster mothers selectively directed their aggression towards conspecific intruders. The cross-fostering treatment also did not affect male mate preferences. These results are in striking contrast with the finding that females of these species show a sexual preference for males of the foster species.     

Laboratory mating trials indicate incipient speciation by sexual selection among populations of the cichlid fish Pseudotropheus zebra from Lake Malawi

It has been suggested that sexual selection may have played a major role in the rapid evolution of hundreds of species of cichlid fishes in Lake Malawi. We report the results of a laboratory test of assortative mating among Lake Malawi cichlid fishes from five closely related geographical populations differing in male courtship colour. Paternity of clutches was tested using microsatellite DNA typing of offspring. Out of 1955 offspring typed, 1296 (66.3%) were sired by the male from the same population as the female, which is more than three times the rate expected if females do not differentiate among males of the different populations (20%). This result indicates that mate preferences of geographical races are strongly differentiated, consistent with the races representing incipient geographical species diverging under sexual selection exerted by female preferences for different male courtship colours.     

Interaction between female mating preferences and predation may explain the maintenance of rare males in the pentamorphic fish Poecilia parae

Variation in mating preferences coupled with selective predation may allow for the maintenance of alternative mating strategies. Males of the South American live‐bearing fish Poecilia parae fall in one of five discrete morphs: red, yellow, blue, stripe‐coloured tail (parae) and female mimic (immaculata). Field surveys indicate that the red and yellow morphs are the rarest and that their rarity is consistent across years. We explored the role of variable female mating preference and selective predation by visual predators in explaining the rarity of red and yellow males, and more generally, the maintenance of this extreme colour polymorphism. We presented wild‐caught P. parae females and Aequidens tetramerus, the most common cichlid predator, with the five male colour morphs in separate trials to determine mating and prey preferences, respectively. We found that a large proportion of females shared a strong preference for the rare carotenoid‐based red and yellow males, but a distinct group also preferred the blue and parae morphs. The cichlid predator strongly preferred red and yellow males as prey. Together, these results suggest that the interaction between premating sexual selection favouring and predation acting against the red and yellow morphs may explain their rarity in the wild. The trade‐off between sexual and natural selection, accompanied by variation in female mating preferences, may therefore facilitate the maintenance of the striking colour polymorphism in P. parae.     

Signaling without the risk of illegitimate receivers: do predators respond to the acoustic signals of <Emphasis Type="Italic">Cyprinella</Emphasis> (Cyprinidae)?

Acoustic and visual signals are commonly used by fishes for communication. A significant drawback to both types of signals is that both sounds and visual stimuli are easily detected by illegitimate receivers, such as predators. Although predator attraction to visual stimuli has been well-studied in other animals, predator response to acoustic stimuli has received virtually no research attention among fishes and snakes. This study assessed whether the calls of male tricolor shiner (Cyprinella trichroistia) made during the breeding season would attract potential predators. We also examined the effect of visual stimulus of tricolor shiners on predators. Predators used were red eye bass (Micropterus coosae) and midland water snakes (Nerodia sipedon pleuralis). Neither predator was attracted to tricolor sounds when presented alone. Micropterus coosae responded significantly more to a visual stimulus, and to a combination of visual and acoustic stimuli, but with no greater intensity in the latter. Nerodia sipedon pleuralis did not responded to visual stimulus presented alone, but did respond to visual and acoustic stimuli presented simultaneously, and with greater intensity to the latter, indicating that acoustic signals may play a role in prey detection by N. sipedon pleuralis.     

The ecological drivers of nuptial color evolution in darters (Percidae: Etheostomatinae)

Closely related animal lineages often vary in male coloration, and ecological selection is hypothesized to shape this variation. The role of ecological selection in inhibiting male color has been documented extensively at the population level, but relatively few studies have investigated the evolution of male coloration across a clade of closely related species. Darters are a diverse group of fishes that vary in the presence of elaborate male nuptial coloration, with some species exhibiting vivid color patterns and others mostly or entirely achromatic. We used phylogenetic logistic regression to test for correlations between the presence/absence of color traits across darter species and the ecological conditions in which these species occur. Environmental variables were correlated with the presence of nuptial color in darters with colorful species tending to inhabit environments that would support fewer predators and potentially transmit a broader spectrum of natural light compared to species lacking male coloration. We also tested the color preferences of a common darter predator, largemouth bass, and found that it exhibits a strong preference for red, providing further evidence of predation as a source of selection on color evolution in darters. Ecological selection therefore appears to be an important factor in dictating the presence or absence of male coloration in this group of fishes.     

Anti-Predator Signals as Advertisements: Evidence in White-Throated Magpie-Jays

Calls and displays elicited by predators usually function as alarms or to inform predators of their detection. However, predator encounters may afford some individuals the opportunity to demonstrate quality or signal their availability. Here, I report on a class of vocal signals produced in predator-elicited displays that share many characteristics with sexually selected song. White-throated magpie-jays ( Calocitta formosa ) display at low-threat predators while producing 'loud display calls' (LDCs). I use this term because the calls occur primarily in two display contexts (see below) though occasionally in other contexts as well. Such calls and displays are primarily produced by males, and also occur in one other context, at dawn. Playback experiments showed that despite being elicited by predators, males were more likely than females to respond to LDCs, and more likely to respond when their mate was fertile. Over 134 different call types were produced in over 200 displays by 34 males; the largest minimum repertoire size was 67. Presentations of taxidermic raptor mounts elicited some LDCs, but fewer calls and lower diversity than at dawn or in predator approach displays. The male bias and high diversity suggest that LDCs are an outcome of intersexual selection, while their elicitation by predators suggests an alarm function. I propose that male magpie-jays use predator encounters as opportunities to advertise their presence and availability as mates; they use LDCs as songs. Such a communication system seems to have been favored by the unusual social system of magpie-jays, in which female groups defend territories and males have little opportunity to defend resources for mate attraction, forcing them to advertise when females are paying the most attention, during predator encounters.