No evidence for a genetic association between female mating preference and male secondary sexual trait in a Lake Victoria cichlid fish

Sexual selection by female mating preference for male nuptial coloration has been suggested as a driving force in the rapid speciation of Lake Victoria cichlid fish. This process could have been facilitated or accelerated by genetic associations between female preference loci and male coloration loci. Preferences, as well as coloration, are heritable traits and are probably determined by more than one gene. However, little is known about potential genetic associations between these traits. In turbid water, we found a population that is variable in male nuptial coloration from blue to yellow to red. Males at the extreme ends of the phenotype distribution resemble a reproductively isolated species pair in clear water that has diverged into one species with blue-grey males and one species with bright red males. Females of the turbid water population vary in mating preference coinciding with the male phenotype distribution. For the current study, these females were mated to blue males. We measured the coloration of the sires and male offspring. Parents-offspring regression showed that the sires did not affect male offspring coloration, which confirms earlier findings that the blue species breeds true. In contrast, male offspring coloration was determined by the identity of the dams, which suggests that there is heritable variation in male color genes between females. However, we found that mating preferences of the dams were not correlated with male offspring coloration. Thus, there is no evidence for strong genetic linkage between mating preference and the preferred trait in this population [Current Zoology 56 （1）： 57-64 2010].

Male mate choice in Tibetan macaques Macaca thibetana at Mt.Huangshan,China

Though females are generally more selective in mate choice, males may also benefit from mate choice if male reproductive success is limited by factors other than simply the number of female mates, and if females differ in short-term reproductive potential. We studied male mate choice in a free-ranging troop of Tibetan macaques Macaca thibetana at Mt. Huangshan, China, from August 2007 to April 2008. We employed focal animal sampling and all occurrence sampling to record sexual related behaviors. Eight adult females were divided into three female quality categories according to the females＇ age, rank and parity. Using male mating effort as a proxy for male mate choice, we found that males do distinguish female quality and show time-variant mating strategies. Specifically, females with dominant rank, high fecundity, and middle age attracted significantly more males. Our results suggest that female short-term reproductive potential appears to be an important variable in determining male mating effort. Male Tibetan macaques do exercise mate choice for higher quality females as well as reduce useless reproductive cost, which is consistent with the direct benefits theory of mate choice.

Female mate choice and the potential for ornament evolution in túngara frogs Physalaemus pustulosus

The potential for ornament evolution in response to sexual selection rests on the interaction between the permissive- ness or selectivity of female preferences and the constraints on male development of signaling related traits. We investigate the former by determining how latent female preferences either exaggerate the magnitude of current traits （i.e. elaborations） or favor novel traits （i.e. innovations）. In tt^ngara frogs, females prefer complex mating calls （whine-chucks） to simple calls （whine only）. The whine is critical for mate recognition while the chuck further enhances the attractiveness of the call. Here we use a combina- tion of synthetic and natural stimuli to examine latent female preferences. Our results show that a diversity of stimuli, including conspecific and heterospecific calls as well as predator-produced and human-made sounds, increase the attractiveness of a call when added to a whine. These stimuli do not make simple calls more attractive than a whine-chuck, however. In rare cases we found stimuli that added to the whine decrease the attractiveness of the call. Overall, females show strong preferences for both elaborations and innovations of the chuck. We argue that the emancipation of these acoustic adornments from mate recognition allows such female permissiveness, and that male constraints on signal evolution are probably more important in explaining why males evolved their specific adornment. Experimentally probing latent female preferences for stimuli out of the species＇ range is a useful means to gain insights about the potential of female choice to influence signal evolution and thus the astounding diversity in male sexually-selected traits [Current Zoology 56 （3）： 343-357, 2010].

Kinship alters the effects of forced cohabitation on body weight,mate choice and fitness in the rat-like hamster Tscheskia triton

It has been documented that social isolation imparts deleterious effects on gregarious rodents species, but caging in group imparts such effects on solitary rodents. This study was attempted at examining how kinship to affect body weight, behavioral interaction, mate choice and fitness when we caged male and female rat-like hamsters Tscheskia triton in pair, a solitary species. We found that females paired with nonsibling males became heavier than the females paired with sibling males, but both agonistic and amicable behavior between paired males and females did not differ between sibling and nonsibling groups. This indicated that kinship might reduce females＇ obesity in response to forced cohabitation, and dissociation might exist between physiological and behavioral responses. Furthermore, binary choice tests revealed that social familiarity between either siblings or nonsiblings decreased their investigating time spent in opposite sex conspeeifie of cage mates and/or their scents as compared with those of nonmates, suggesting effects of social association on mate and kin selection of the hamsters. On the other side, both females and males caged in pair with siblings show a preference between unfamiliar siblings or their scents and the counterparts of nonsiblings after two month separation, indicating that the kin recognition of the hamsters might also rely on phenotype matching. In addition, cohabitation （or permanent presence of fathers） elicited a lower survival of pups in nonsibling pairs than sibling pairs, but did not affect litter size, suggesting that kinship affects fitness when housing male and female ratlike hamsters together. Therefore, inbreeding might be adapted for rare and endangered animals.

Large males have a mating advantage in a species of darter with smaller,allopaternal males Etheostoma olmstedi

Theory suggests that males that are larger than their competitors may have increased mating success, due to both greater competitive ability and increased attractiveness to females. We examined how male mating suceess varies with male size in the tessellated darter Etheostoma olmstedi. Previous work has shown that large males tend to move around and breed in vacant breeding sites, and consequently provide less care for their eggs, while smaller individuals can be allopaternal, caring for the eggs of other males as well as for their own. We studied female egg deposition in a natural breeding population using artificial breeding sites and in the laboratory, where female choice of spawning site was restricted to two breeding sites tended by two males of different sizes. In both the field and the laboratory, nests tended by larger males were more likely to receive new eggs. Additionally, the mean size of males associated with a nest was positively correlated with both the maximum coverage of eggs at the nest and the number of times new eggs were deposited. We discuss how the increased mating success of larger males, despite their decreased parental care, may help explain aUopaternal care in this species [Current Zoology 56 （1）： 1-5, 2010].

Mate choice for major histocompatibility complex(MHC)complementarity in the Yellow-rumped Flycatcher(Ficedula zanthopygia)

Background:Genes of the major histocompatibility complex(MHC)are an important component of the vertebrate immune system and play a significant role in mate choice in animal populations.However,the MHC genetic targets of female mate choice have not been clearly identified,and whether female mate choice is based on neutral genetic characteristics remains an open question.Here,we focus on the effects of morphological traits and genetic similarity among individuals in MHC class IIB(MHC IIB)exon 2 on mating in a sexually dimorphic songbird that exhibits social monogamy with extra-pair paternity(EPP).Methods:We sequenced 64 parent-offspring triads sampled over a 3-year period using two MHC class II loci to detect disassortative mating in the Yellow-rumped Flycatcher(Ficedula zanthopygia).Results:We found that MHC similarity in social pairs was lower than that in random pairs.Extra-pair mate choice according to MHC IIB was observed,in which females’extra-pair mates had fewer MHC alleles than their within-pair mates,but there was no significant band-sharing between extra-pair sires and potential extra-pair mates.However,the interaction between the MHC diversity of females and that of the social males affected the occurrence of EPP.Conclusions:Our results support the“optimality hypothesis”of MHC-based social and extra-pair choice.Female choice probably maintains a certain level of MHC diversity in offspring in the Yellow-rumped Flycatcher.

Can simple songs express useful signals for mate choice?

Background:As one of the most elaborate and diverse sexual signals,bird songs are prominent among mate choice criteria.Females generally prefer mates with larger repertoire size,which promotes the evolution of song complex?ity.However,there are also some songbirds that have far simpler and less diverse vocalizations,which have not been the focus of scientific scrutiny.Most Phylloscopus warblers are accomplished singers with complex songs.In contrast,Hume's Warbler(P.humei) has extremely simple songs.In order to explore the song's function,its evolutionary sig?nificance and particularly to assess its possible relationship with parental investment,we studied mate choice of the subspecies P.h.mandellii in Lianhuashan National Nature Reserve,Gansu,China.Methods:We recorded body measurements and songs of breeding males and then explored their relationships with the date of clutch initiation,reasoning that the characteristics of males that are involved with early nesting activities reflect female mate preferences.We also recorded egg size and body measurements of nestlings to assess the rela?tionship between parental investment and mate choice.Results:We found that male wing and tail lengths were positively correlated with early clutch initiation as were songs characterized by short duration and rapid rise to maximum amplitude.We also found that early?breeding females did not lay large eggs,but produced more surviving young,which grew up faster.Conclusions:Female mate choice criteria in this bird include both visual signals and song characteristics.Our study supports the hypothesis that females may judge male quality from quite subtle differences.In order to reduce the risk of predation,a preference for such inconspicuous male characteristics may be partially driven by high vulnerability of this warbler to predators as a ground?nesting species.

Mate choice is affected by parasite infestation rate of the choosing individual as well as of potential mating partners

Parasites are known to be a key driving force in mate choice and are important for the expression and evolution of ornaments and behavioral traits being used.However,there is little experimental evidence on how the parasite’s burden of the choosing individual is integrated into the mate-choice process and how it affects decision-making,especially in relation to parasite infestation of potential mates.Thus,the aim of our study was to determine whether female house sparrows Passer domesticus adjust their mate preference according to their own as well as the parasite load of prospective partners.To do this,we experimentally manipulated female parasite load and determined their mate preferences prior to and after parasite treatment.We manipulated the chronic coccidian parasite burden of females either by initiating the acute infection phase via re-infecting them with coccidian or by temporally reducing the parasite load of coccidia.We then measured the effect of this manipulation on mate preference by presenting females with a choice of four stimuli:three males with similar ornaments,but unmanipulated,naturally varying chronic coccidiosis levels,and an unmanipulated control female.Additionally,we recorded some males’behavior in relation to their infection status pointing toward an increased or reduced interest in mating.We found that females preferred highly infested males prior to manipulation,regardless of their own infestation level.However,after manipulation,infested females avoided highly infested males probably in response to the deterioration of their health condition by parasites.Our study suggests that mate-choice decisions are more complex when they are mediated by parasites.The implications of parasites for evolutionary theories of sexual signaling and mate choice are discussed.

Chromatic and achromatic differences of melanin-and carotenoid-based plumage coloration in five minivet species(Pericrocotus spp.)under conspecific and predator visual systems

Adaptive mate choice has been accepted as the leading theory to explain the colorful plumage of birds.This theory hypothesizes that conspicuous colors act as signals to advertise the qualities of the owners.However,a dilemma arises in that conspicuous colors may not only attract mates,but also alert predators.The"private channels of communication"hypothesis proposes that some intraspecific signals may not be visible to heterospecific animals because of different visual systems.To better understand the evolution of plumage colors and sexual selection in birds,here we studied the chromatic difference and achromatic differences of melanin-and carotenoid-based plumage coloration in five minivet species(Pericrocotus spp.)under conspecific and predator visual systems.We found that either the chromatic or achromatic difference among male or female minivets’plumage was consistently higher under conspecific vision than under predator vision for all five studied species of minivets.This result indicated that individual differences in plumage colors of minivets were visible to the conspecific receivers and hidden from potential predators as a result of evolution under predation risk and conspecific communication.However,males were under a higher risk of predation because they were more conspicuous than females to the vision of a nocturnal predator.

Females adopt sexual catalepsy to facilitate mating

Theory predicts that males and females of dioecious species typically engage in an evolutionary sexual conflict over the frequency and choice of mating partner.Female sexual cannibalism,a particularly dramatic illustration of this conflict,is widespread in certain animal taxa including spiders.Nevertheless,females of some funnel weaving spiders that are generally aggressive to conspecifics enter a cataleptic state after male courtship,ensuring the males can mate without risk of attack.In this study,we demonstrated that the physical posture and duration,metabo-lites,and central neurotransmitters of females of Aterigena aculeata in sexual catalepsy closely resemble females in thanatosis but are distinct from those in anesthesia,indicating that the courted females feign death to eliminate the risk of potentially aggressive responses and thereby allow preferred males to mate.Unlike the taxonomically widespread thanatosis,which generally represents a deceptive visual signal that acts against the interest of the receivers,sexual catalepsy of females in the funnel weaving spiders may deliver a sexual-receptive signal to the courting males and thereby benefit both the signal senders and receivers.Therefore,sexual catalepsy in A.aculeata may not reflect a conflict butrather a confluence of interest betweenthe sexes.

A signal-substrate match in the substrate-borne component of a multimodal courtship display

The environment can impose strong limitations on the efficacy of signal transmission. In particular, for vibratory communication, the signaling environment is often extremely heterogeneous at small scales. Nevertheless, natural selection is expected to select for signals well-suited for effective transmission. Here, we test for substrate-dependent signal efficacy in the wolf spider Schizocosa stridulans Strattou 1991. We first explore the transmission characteristics of this important signaling mo- dality by playing recorded substrate-bome signals through three different substrates （leaf litter, pine litter, and red clay） and measuring the propagated signal. We found that the substrate-bome signal of S. stridulans attenuates the least on leaf litter, the substrate upon which the species is naturally found. Next, by assessing mating success with artificially muted and non-muted males across different signaling substrates （leaf litter, pine litter, and sand）, we explored the relationship between substrate-bome signaling and signaling substrate for mating success. We found that muted males were unsuccessful in obtaining copulations re- gardless of substrate, while mating success was dependent on the signaling substrate for non-muted males. For non-muted males, more males copulated on leaf litter than any other substrate. Taken together, these results confLrm the importance of sub- strate-bome signaling in S. stridulans and suggest a match between signal properties and signal efficacy - leaf litter transmits the signal most effectively and males are most successful in obtaining copulations on leaf litter [Current Zoology 56 （3）： 370-378, 2010].

Female and male visually based mate preferences are consistent with reproductive isolation between populations of the Lake Malawi endemic Labeotropheus fuelleborni

Sexual selection via female mate choice is thought to have played a key role in the speciation ofhaplochromine cichlids, but a dominant role for visual signals in such processes has lately been called into question. In addition, the possible role of male mating preferences in haplochromine speciation has been little studied. We studied patterns of both female and male mate choice, based exclusively on visual signals, in order to evaluate potential reproductive isolation between two populations of the Lake Malawi haplochromine Labeotropheus fuelleborni. In the first experiment, females were allowed to choose between two males, one from the same population and the other allopatric with respect to the female. Females in this experiment responded more frequently to males from their own population. Similarly, the males in these trials displayed more frequently when presented with females of their own population. In the second experiment, a female was allowed to choose between two males, either both from her own population or both allopatric. In these trials, both males and females from the Katale population interacted significantly more frequently in settings in which all three individuals were from the same population （＂same-population trios＂）, and those from the Chipoka population showed a similar trend. Thus, patterns in both male and female courtship behavior suggest that visual signals contribute to at least incipient reproductive isolation between populations of L. fuelleborni [Current Zoology 56 （ 1 ）： 65-72 2010].

Endocrine correlates of mate choice and promiscuity in females of a socially monogamous avian mating system with alternative male reproductive phenotypes

While our understanding of male reproductive strategies is informed by extensive investigations into endocrine mechanisms, the proximate mechanisms by which females compete for mates and adjust reproduction to social environment remains enigmatic. We set out to uncover endocrine correlates of mate choice, social environment, and reproductive investment in female red-backed fairy-wrens Malurus melanocephalus. In this socially monogamous, yet highly sexually promiscuous species, females experience discrete variation in the phenotype of their mates, which vary in both plumage signals and level of paternal care, and in the composition of their breeding groups, which consist of either the pair alone or with an additional cooperative auxiliary; fe- male investment varies according to these social parameters. We found that androgen, estrogen, and glucorticoid levels varied with reproductive stage, with highest androgen and estrogen concentrations during nest construction and highest corticosterone concentrations during the pre-breeding stage. These stage-dependent patterns did not vary with male phenotype or auxiliary presence, though androgen levels during pre-breeding mate selection were lower in females obtaining red/black mates than those obtaining brown mates. We found no evidence that androgen, estrogen, or corticosterone levels during the fertile period were re- lated to extra-pair young （EPY） frequency. This study demonstrates clear changes in steroid levels with reproductive stage, though it found little support for variation with social environment. We suggest hormonal responsiveness to social factors may be physiologically constrained in ways that are bypassed through exogenous hormone manipulations.

Nonlinear Phenomena Conveying Body Size Information and Improving Attractiveness of the Courtship Calls in the Males of Odorrana tormota

Nonlinear phenomena are commonly shown in the vocalization of animals and exerts different adaptive functions.Although some studies have pointed out that nonlinear phenomena can enhance the individual identification of male Odorrana tormota,whether the nonlinear phenomena play a specific role in the sexual selection of O.tormota remain unclear.Here we presented evidence that there was a significant negative correlation(Pearson:n=30,r=0.65,P<0.001)between the nonlinear phenomena content and snout-vent length in the male O.tormota,and two-choice amplexus experiments showed that female O.tormota preferred male with smaller body size containing higher nonlinear phenomena content in its calls.Phonotaxis experiments also revealed that females preferred calls with higher nonlinear phenomena content.Additionally,compared to the calls with lower nonlinear phenomena content and higher fundamental frequency,there was shorter response time in phonotactic behaviour of female induced by the calls with higher nonlinear phenomena content and lower fundamental frequency.We argue that the nonlinear phenomena content in the calls of male O.tormota can convey its body size information and may provide important clues for female frogs in darkened surroundings to identify males’body size during mate choice,meanwhile,higher nonlinear phenomena content in males’calls may increase the attractiveness of males to females.The results of this study provide confirmation that,for O.tormota,nonlinear phenomena have specific function in mate choice.

Mismatch between calf paternity and observed copulations between male and female reindeer:Multiple mating in a polygynous ungulate?

In polygynous systems,such as that exhibited by reindeer Rangifer tarandus,mate choice can be difficult to disentangle from male intrasexual competition because male behavior may constrain female choice.Multiple mating may provide an avenue for female mate choice,though it is difficult to identify using behavioral estimators alone.Molecular techniques address this issue by affording ecologists an opportunity to reassess mating systems from a genetic perspective.We assessed the frequency and possible explanations for multiple mating in reindeer using a genetic approach to determine the success of observed copulations in a semi-domesticated herd in Kaamanen,Finland.Behavioral and genetic data were synthesized with population characteristics over a 7-year period to test the hypothesis that,if present,polyandry in reindeer is driven by sexual harassment from sub-dominant males.We observed multiple mating in 42%of females,with as many as 60%exhibiting multiple mating in certain years.We found no evidence that multiple mating resulted from sexual harassment by sub-dominant males,suggesting that it is likely a deliberate strategy among females.Conversion rate of copulations into paternities varied with male size,with smaller males more likely to experience mismatch than larger males.Female preference for larger males persisted despite the occurrence of multiple mating,possibly suggesting a mechanism for cryptic post-copulatory selection.We suggest further research to delineate the possible influence of cryptic post-copulatory selection and multiple mating to defend against infertility in exhausted males.

Is bigger better？ Male body size affects wing-borne courtship signals and mating success in the olive fruit fly, Bactrocera oleae （Diptera： Tephritidae）

Variations in male body size are known to affect inter- and intrasexual selection outcomes in a wide range of animals. In mating systems involving sexual signaling before mating, body size often acts as a key factor affecting signal strength and mate choice. We evaluated the effect of male size on courtship displays and mating success of the olive fruit fly, Bactrocera oleae （Diptera： Tephritidae）. Wing vibrations performed during successful and unsuccessful courtships by large and small males were recorded by high-speed videos and analyzed through frame-by-frame analysis. Mating success of large and small males was investigated. The effect of male-male competition on mating success was evaluated. Male body size affected both male courtship signals and mating outcomes. Successful males showed wing-borne signals with high frequencies and short interpulse intervals. Wing vibrations displayed by successful large males during copulation attempt had higher frequencies over smaller males and unsuccessful large males. In no-competition conditions, large males achieved higher mating success with respect to smaller ones. Allowing large and small males to compete for a female, large males achieve more mating success over smaller ones. Mate choice by females may be based on selection of the larger males, able to produce high-frequency wing vibrations. Such traits may be indicative of ＂good genes,＂ which under sexual selection could means good social-interaction genes, or a good competitive manipulator of conspecifics.

Patterns of reproductive isolation in the Drosophila subquinaria complex： can reinforced premating isolation cascade to other species？

The reinforcement of premating barriers due to reduced hybrid fitness in sympatry may cause secondary sexual isolation within a species as a by-product. Consistent with this, in the fly Drosophila subquinaria, females that are sympatric with D. recens mate at very low rates not only with D. recens, but also with conspecific D. subquinaria males from allopatry. Here, we ask if these effects of re- inforcement cascade more broadly to affect sexual isolation with other closely related species. We assay reproductive isolation of these species with D. transversa and find that choosy D. subquinaria females from the region sympatric with D. recens discriminate strongly against male D. transversa, whereas D. subquinaria from the allopatric region do not. This increased sexual isolation cannot be explained by natural selection to avoid mating with this species, as they are allopatric in geographic range and we do not identify any intrinsic postzygotic isolation between D. subquinaria and D. transversa. Variation in epicuticular hydrocarbons, which are used as mating signals in D. subquinaria, follow patterns of premating isolation： D. transversa and allopatric D. subquinaria are most similar to each other and differ from sympatric D. subquinaria, and those of D. recens are distinct from the other two species. We suggest that the secondary effects of reinforcement may cascade to strengthen reproductive isolation with other species that were not a target of selection. These effects may enhance the divergence that occurs in allopatry to help explain why some species are already sexually isolated upon secondary contact.

Do males choose their mates in the lekking moth Achroia grisella？ Influence of female body mass and male reproductive status on male mate choice

Lekking males aggregate to attract females and contribute solely to egg fer- tilization, without any further parental care. Evolutionary theory therefore predicts them to be nonchoosy toward their mates, because any lost mating opportunities would out- weigh the benefits associated with such preferences. Nevertheless, due to time costs, the production of energetically costly sexual displays, and potential sperm limitation, the mat- ing effort of lekking males is often considerable. These factors, combined with the fact that many females of varying quality are likely to visit leks, could favor the evolution of male mate preferences. Here, we show that males of the lekking lesser wax moth, Achroia grisella, were indeed more likely to mate with heavier females in choice experiments, even at their virgin mating （i.e., when their reproductive resources have not yet been depleted by previous matings）. This differential female mating success could not be attributed to female behavior as heavy and light females showed similar motivation to mate （i.e., la- tency to approach the males） and time to copulate. Males seem to benefit from mating with heavier females, as fecundity positively correlated with female mass. This new em- pirical evidence shows that male mate choice may have been tmderestimated in lekking species

Male Drosophila melanogaster learn to prefer an arbitrary trait associated with female mating status

Although males are generally less discriminating than females when it comes to choosing a mate, they still benefit from distinguishing between mates that are receptive to courtship and those that are not, in order to avoid wasting time and ener- gy. It is known that males of Drosophila melanogaster are able to learn to associate olfactory and gustatory cues with female re- ceptivity, but the role of more arbitrary, visual cues in mate choice learning has been overlooked to date in this species. We there- fore carried out a series of experiments to determine： 1） whether males had a baseline preference for female eye color （red versus brown）, 2） if males could learn to associate an eye color cue with female receptivity, and 3） whether this association disappeared when the males were unable to use this visual cue in the dark. We found that naive males had no baseline preference for females of either eye color, but that males which were trained with sexually receptive females of a given eye color showed a preference for that color during a standard binary choice experiment. The learned cue was indeed likely to be truly visual, since the prefe- rence disappeared when the binary choice phase of the experiment was carried out in darkness.This is, to our knowledge l） the first evidence that male D. melanogaster can use more arbitrary cues and 2） the first evidence that males use visual cues during mate choice learning. Our findings suggest that that D. melanogaster has untapped potential as a model system for mate choice learning.

Can sexual selection and disassortative mating contribute to the maintenance of a shell color polymorphism in an inter- tidal marine snail？

Littorina fabalis is an intertidal snail commonly living on the brown algae Fucus vesiculosus and showing frequent shell-color polymorphisms in the wild. The evolutionary mechanism underlying this polymorphism is currently unknown. Shell color variation was studied in mated and non-mated specimens of this species from different microareas in one locality from NW Spain, in order to estimate sexual selection and assortative mating that may （still） be operating in this population. The analyses across microareas allowed us to investigate frequency-dependent selection and assortative mating components, mechanisms that could maintain the polymorphism. The presence of shell scars caused by crab attacks, an environmental variable not related with sexual selection or assortative mating, was used as experimental control. This study provides new evidence of significant disas- sortative mating and some degree of sexual selection against some shell colors, supporting the results found 21 years ago in a similar study, i.e. in the same species and locality. The similarity of these estimates during the studied period suggests that this experimental approach is consistent and valid to be extended to other populations and organisms. In addition, sexual selection and assortative mating estimates did not change across microareas differing in shell color frequencies, suggesting than the polymor- phism can not be maintained by a frequency-dependent （sexual selection-based） mechanism. Our main hypothesis is that negative assortative mating could contribute to the maintenance of the polymorphism, perhaps by males showing distinct female color preferences when searching for mates [Current Zoology 58 （3）： 463-474, 2012].