Foraging costs drive female resistance to a sensory trap

Male ornaments can evolve through the exploitation of female perceptual biases such as those involved in responding to cues from food. This type of sensory exploitation may lead to confusion between the male signals and the cues that females use to find/recognize food. Such interference would be costly to females and may be one reason why females evolve resistance to the male ornaments. Using a group of species of viviparous fish where resistance to a sensory trap has evolved, we demonstrate that females exposed to an ornament that resembles food have a diminished foraging efficiency, that this effect is apparent when foraging on a food item with which the ornament shares visual attributes, and that not all species are equally affected by such confusion. Our results lend support to the model of ornamental evolution through chase-away sexual conflict.     

Preferences for coloured bower decorations can be explained in a nonsexual context

The sensory bias model of sexual selection suggests that elaborate male secondary sexual traits evolved to exploit biases in the female's sensory system. Such biases may have evolved in a nonsexual context. Male bowerbirds build and decorate elaborate structures, bowers, which function as targets of female choice. The colour of decorations used on bowers appears to be important in determining a male's mating success. We tested two predictions made by the sensory bias model, using cache presentation experiments of artificially coloured grapes made to captive bowerbirds of five species. We first searched for evidence of ancestral biases for certain colours that could explain current colour preferences across species. We found no single parsimonious explanation for ancestral patterns of colour preference across the family. We next tested whether female preference patterns could be explained in a nonsexual, foraging, context. For both of the species where sufficient data could be collected, female foraging preferences for grapes were significantly related to male preferences for grapes used as bower decorations. Our results suggest that choice of bower decoration colour may have evolved to exploit a bias in the female's sensory system, originally shaped by selection on foraging practices. Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.      

Does female feeding motivation affect the response to a food‐mimicking male ornament in the swordtail characin Corynopoma riisei?

Female response to various aspects of male trait morphology and the effect of female feeding motivation were investigated in the swordtail characin Corynopoma riisei, a species where males are equipped with a flag‐like food‐mimicking ornament that grows from the operculum. Unfed females responded more strongly to the male ornament and showed a stronger preference for larger ornaments than did fed females. Females were shown not to discriminate between artificial male ornaments of either undamaged or damaged shape.     

A possible non-sexual origin of mate preference: are male guppies mimicking fruit?

In most animals, the origins of mating preferences are not clear. The "sensory-bias" hypothesis proposes that biases in female sensory or neural systems are important in triggering sexual selection and in determining which male traits will become elaborated into sexual ornaments. Subsequently, other mechanisms can evolve for discriminating between high- and low-quality mates. Female guppies (Poecilia reticulata) generally show a preference for males with larger, more chromatic orange spots. It has been proposed that this preference originated because it enabled females to obtain high-quality mates. We present evidence for an alternative hypothesis, that the origin of the preference is a pleiotropic effect of a sensory bias for the colour orange, which might have arisen in the context of food detection. In field and laboratory experiments, adult guppies of both sexes were more responsive to orange-coloured objects than to objects of other colours, even outside a mating context. Across populations, variation in attraction to orange objects explained 94% of the inter-population variation in female mate preference for orange coloration on males. This is one of the first studies to show both an association between a potential trigger of a mate-choice preference and a sexually selected trait, and also that an innate attraction to a coloured inanimate object explains almost all of the observed variation in female mate choice. These results support the "sensory-bias" hypothesis for the evolution of mating preferences.     

Innate and Learned Colour Preference in the Zebrafish, Danio rerio

Receiver biases towards specific sensory signals have been demonstrated in insects, birds and fish, both in the context of foraging and mate choice. In some cases, signals important in sexual selection appear to have evolved by exploiting a pre-existing bias in the sensory system. For instance, female preferences for male nuptial colouration may have arisen from selection on foraging practices. Using the zebrafish ( Danio rerio ), a species in which red is not a factor in mate choice, we tested for a foraging bias towards the colour red. We further investigated the plasticity of foraging biases by raising groups of fish on diets consisting solely of red, blue, green or white food. When we subsequently tested their colour preferences in a foraging context, each group responded most strongly to red, irrespective of the colour of food with which they had been conditioned. We also detected a significant effect of conditioning on colour preferences; fish responded more strongly to the colour that matched diet colour than to other colours. The observed receiver bias towards red may have evolved as an adaptive preference for carotenoid compounds in their diet. While the bias to red appears to be innate, our results indicate that learning is also important in shaping foraging biases.     

Shared preferences by predators and females for male ornaments in swordtails

Sexually dimorphic traits in many mate recognition systems have evolved in response to preexisting female biases. These biases are often quite general in form and are likely to be shared by predators, thereby imposing a cost on male trait expression. The Mexican tetra Astyanax mexicanus (Pisces: Characidae), a visual predator of swordtail fishes, exhibits the same visual preferences for male body size morphs as do females. Furthermore, tetras in populations where swordtails are absent prefer males with sword ornaments over males with swords removed. The predator preference is thus likely to have arisen prior to contact with fishes bearing the ornament, as has also been suggested for mating preferences for swords.     

Variation in preference for a male ornament is positively associated with female eyespan in the stalk-eyed fly Diasemopsis meigenii

There is currently much interest in mate preferences for sexual ornaments. However, few studies have focused on individual variation in mate preference despite its importance for the rate and direction of sexual selection. Females of the sexually dimorphic stalk-eyed fly, Diasemopsis meigenii, exhibit an unambiguous rejection response towards unattractive males bearing small ornaments. We investigated individual mate preferences using repeated sequential sampling of female rejection or acceptance responses to a wide range of male ornament phenotypes. We found significant variation in the strength of individual preference. In addition, preference was positively associated with female eyespan, a condition-dependent trait putatively linked to visual acuity.     

Female bluethroats prefer males with symmetric colour bands

Fluctuating asymmetry in male ornaments may be related to male quality and therefore may be used by females as a guide in mate choice. Experiments on different species have shown female preferences for symmetry, but few previous studies have addressed preferences for symmetry in coloration. Using the bluethroat,Luscinia s. svecicaa passerine with a conspicuous blue and chestnut throat patch, as a model species, we devised an experiment in which the females could choose between males with either symmetric or asymmetric coloured leg bands. The females associated more with symmetric than asymmetric males, indicating a preference for symmetry.     

Do Female Western Mosquitofish,Gambusia affinis,Prefer Ornaments That Males Lack?

Some species in the family Poeciliidae are known for extravagant male ornaments and courtship behavior (e.g., guppies), but the majority of poeciliids are characterized by coercive male copulation attempts that seem to circumvent female choice. In some lineages with male ornaments, female sensory bias may have preceded the evolution of corresponding male signals. We examined female preferences for colorful ornaments in Western mosquitofish, Gambusia affinis, in which males lack ornamentation and reproduce primarily through coercive mating attempts. We found that females exhibited a positional affinity for males that were artificially ornamented with blue coloration over males that had been treated with a transparent ornament. Females exhibited the opposite effect for males treated with red ornaments. In contrast, focal females did not exhibit behavioral discrimination between two live stimulus females or two models (silver fishing lures) with blue vs. transparent ornaments. This suggests a sexual context for female discrimination between males based on ornament color and whether an ornament was present. Because tribe Gambusiini is the basal branch of family Poeciliidae, the results of this study are consistent with the hypothesis that female responsiveness to male coloration is the ancestral poeciliid character state.     

Preference for conspecifics evolves earlier in males than females in a sexually dimorphic radiation of fishes

Speciation by sexual selection is generally modeled as the coevolution of female preferences and elaborate male ornaments leading to behavioral (sexual) reproductive isolation. One prediction of these models is that female preference for conspecific males should evolve earlier than male preference for conspecific females in sexually dimorphic species with male ornaments. We tested that prediction in darters, a diverse group of freshwater fishes with sexually dimorphic ornamentation. Focusing on the earliest stages of divergence, we tested preference for conspecific mates in males and females of seven closely related species pairs. Contrary to expectation, male preference for conspecific females was significantly greater than female preference for conspecific males. Males in four of the 14 species significantly preferred conspecific females; whereas, females in no species significantly preferred conspecific males. Relationships between the strength of preference for conspecifics and genetic distance revealed no difference in slope between males and females, but a significant difference in intercept, also suggesting that male preference evolves earlier than females’. Our results are consistent with other recent studies in darters and suggest that the coevolution of female preferences and male ornaments may not best explain the earliest stages of behavioral isolation in this lineage.     

I'm sexy and I glow it: female ornamentation in a nocturnal capital breeder

In many species, males rely on sexual ornaments to attract females. Females, by contrast, rarely produce ornaments. The glow-worm (Lampyris noctiluca) is an exception where wingless females glow to attract males that fly in search of females. However, little is known about the factors that promote the evolution of female ornaments in a sexual selection context. Here, we investigated if the female ornament of the glow-worm is a signal of fecundity used in male mate choice. In support of this, we found brightness to correlate with female fecundity, and males to prefer brighter dummy females. Thus, the glow emitted by females is a reliable sexual signal of female fecundity. It is likely that male preference for the fecundity-indicating ornament has evolved because of large variation among females in fecundity, and because nocturnal males cannot directly assess female size and fecundity. These results indicate that female ornamentation may evolve in capital breeders (i.e. those in which stored resources are invested in reproduction) when females vary significantly in fecundity and this variation cannot be assessed directly by males.     

Heterospecific mating preferences for a feather ornament in least auklets

Auklets (Alddae, Aethiini) include five species of small, sociallymonogamous, sexually monomorphic seabirds that display a varietyof feather and bare-part ornaments during the breeding season.Previous experimental work on two auklet species has demonstratedthat some ornaments are likely to be favored by sexual selectionbecause mutual male and female mating preferences benefit individualswith the most elaborate expression of these traits. In thisstudy we experimentally investigated whether naturally crestlessleast auklets Aethia pusilla have a maring preference for foreheadcrests similar to the most prominent ornament of two other species,crested A. cristatella and whiskered auklets A. pygmaea. Ourobjective was to investigate the function of this ornament asa species-recognition mechanism or as a product of one or moreof three proposed sexual selection models that address the originof elaborate traits and preferences. During the experiment,least auklets reacted to realistic models equipped with artificialforehead crests with approximately an order of magnitude morefrequent sexual displays and greater interest, consistent withthe idea that they have a mating preference for crests, eventhough they do not naturally express this ornament This heterospecificpreference also favored large crest size. These results refutethe possibility that least auklet forehead ornamentation alonedetermines species recognition at present Among models of sexualselection considered, the results are consistent with the sensoryexploitation model, although this could not be established unequivocallybecause a viability indicator or Fisherian mechanism could havebeen involved if least auklets had an ancestor with a foreheadcrest.     

Patterns of fluctuating asymmetry in sexual ornaments predict female choice

Extravagant secondary sexual characters are assumed to have arisen and be maintained by sexual selection. While traits like horns, antlers and spurs can be ascribed to intrasexual competition, other traits such as extravagant feather ornaments, displays and pheromones have to be ascribed to mate choice. A number of studies have tested whether females exert selection on the size of male ornaments, but only some of these have recorded female preferences for the most extravagantly ornamented males. Here I demonstrate that female choice can be directly predicted from the relationship between the degree of fluctuating asymmetry and the size of a secondary sexual character. Fluctuating asymmetry is an epigenetic measure of the ability of individuals to cope with stress, and it occurs when an individual is unable to undergo identical development of an otherwise bilaterally symmetric trait on both sides of its body. There is a negative relationship between the degree of fluctuating asymmetry and the absolute size of an ornament in those bird species with a female preference for the largest male sex trait, while there is a flat or U-shaped relationship among species without a female preference. These results suggest that females prefer exaggerated secondary sexual characters if they reliably demonstrate the ability of males to cope with genetic and environmental stress. Some species may demonstrate a flat or U-shaped relationship between the degree of fluctuating asymmetry and the absolute size of an ornament because (i) the genetic variance in viability signalled by the secondary sex trait has been depleted; (ii) the secondary sex trait is not particularly costly and therefore does not demonstrate condition dependence; or because (iii) the sex traits can be considered arbitrary traits rather than characters reflecting good genes.     

Evolution of a mating preference for a dual‐utility trait used in intrasexual competition in genetically monogamous populations

The selection pressures by which mating preferences for ornamental traits can evolve in genetically monogamous mating systems remain understudied. Empirical evidence from several taxa supports the prevalence of dual‐utility traits, defined as traits used both as armaments in intersexual selection and ornaments in intrasexual selection, as well as the importance of intrasexual resource competition for the evolution of female ornamentation. Here, we study whether mating preferences for traits used in intrasexual resource competition can evolve under genetic monogamy. We find that a mating preference for a competitive trait can evolve and affect the evolution of the trait. The preference is more likely to persist when the fecundity benefit for mates of successful competitors is large and the aversion to unornamented potential mates is strong. The preference can persist for long periods or potentially permanently even when it incurs slight costs. Our results suggest that, when females use ornaments as signals in intrasexual resource competition, males can evolve mating preferences for those ornaments, illuminating both the evolution of female ornamentation and the evolution of male preferences for female ornaments in monogamous species.     

Sexual selection and condition-dependent mate preferences

The last decade has witnessed considerable theoretical and empirical investigation of how male sexual ornaments evolve. This strong male-biased perspective has resulted in the relative neglect of variation in female mate preferences and its consequences for ornament evolution. As sexual selection is a co-evolutionary process between males and females, ignoring variation in females overlooks a key aspect of this process. Here, we review the empirical evidence that female mate preferences, like male ornaments, are condition dependent. We show accumulating support for the hypothesis that high quality females show the strongest mate preference. Nonetheless, this is still an infant field, and we highlight areas in need of more research, both theoretical and empirical. We also examine some of the wider implications of condition-dependent mating decisions and their effect on the strength of sexual selection.     

Male sexual ornament size but not asymmetry reflects condition in stalk-eyed flies

Models of sexual selection predict that females use ornament size to evaluate male condition. It has also been suggested that ornament asymmetry provides females with accurate information about condition. To test these ideas we experimentally manipulated condition in the stalk-eyed fly, Cyrtodiopsis dalmanni, by varying the amount of food available to developing larvae. Males of this species have greatly exaggerated eyestalk length and females prefer to mate with males with wider eyespans. Our experiments show that male ornaments (eyestalks) display a disproportionate sensitivity to condition compared with the homologous character in females, and to non-sexual traits (wing dimensions). In contrast, in neither sex did asymmetry reflect condition either in sexual ornaments or in non-sexual traits. We conclude that ornament size is likely to play a far greater role in sexual selection as an indicator of individual condition than does asymmetry.     

On the function of female ornaments: male bluethroats prefer colourful females

Female ornaments in animals with conventional sex roles have traditionally been considered non-functional, being merely a genetically correlated response to selection for male ornamentation. Alternatively, female ornaments may be influenced by selection acting directly on the females, either through female–female competition or male choice. We tested the latter hypothesis in mate choice experiments with bluethroats (Luscinia s. svecica), a passerine bird in which females vary considerably in coloration of an ornamental throat patch. In outdoor aviaries placed in prime breeding habitat, males were allowed to choose between a colourful and a drab female. We found that males associated more with, and performed more sexual behaviours towards, colourful females. Female coloration was not age-related, but correlated significantly with body mass and tarsus length. Thus, we have demonstrated both a male preference for female ornamentation, and a relationship between ornament expression and female body size, which may be indicative of quality. Our results refute the correlated response hypothesis and support the hypothesis that female ornamentation is sexually selected.     

Conspicuous Female Ornamentation and Tests of Male Mate Preference in Threespine Sticklebacks (Gasterosteus aculeatus)

Sexual selection drives the evolution of exaggerated male ornaments in many animal species. Female ornamentation is now acknowledged also to be common but is generally less well understood. One example is the recently documented red female throat coloration in some threespine stickleback (Gasterosteus aculeatus) populations. Although female sticklebacks often exhibit a preference for red male throat coloration, the possibility of sexual selection on female coloration has been little studied. Using sequential and simultaneous mate choice trials, we examined male mate preferences for female throat color, as well as pelvic spine color and standard length, using wild-captured threespine sticklebacks from the Little Campbell River, British Columbia. In a multivariate analysis, we found no evidence for a population-level mate preference in males, suggesting the absence of directional sexual selection on these traits arising from male mate choice. Significant variation was detected among males in their preference functions, but this appeared to arise from differences in their mean responsiveness across mating trials and not from variation in the strength (i.e., slope) of their preference, suggesting the absence of individual-level preferences as well. When presented with conspecific intruder males, male response decreased as intruder red throat coloration increased, suggesting that males can discriminate color and other aspects of phenotype in our experiment and that males may use these traits in intrasexual interactions. The results presented here are the first to explicitly address male preference for female throat color in threespine sticklebacks.     

A Test for Female Attraction to Male Orange Coloration in Poecilia picta

Examining the distribution of female preference and attractive male secondary sexual characters in closely related species can help evaluate the various models for the evolution of female preference. We examined whether orange coloration in males was attractive to females of Poecilia picta, a species in the same subgenus as the guppy, P. reticulata. Females did not respond differently to males with or without a conspicuous orange stripe on the dorsum of the tail, a characteristic ornamentation of the male of this species. This lack of preference in a taxon that possesses a conspicuous, secondary sexual character directly contradicts the predictions of the sensory exploitation hypothesis for the evolution of female preference, but may support a model of antagonistic sexual selection proposed by Rice (1996).     

Competition for access to mates predicts female-specific ornamentation and male investment in relative testis size

Sexually selected ornaments are highly variable and the factors that drive variation in ornament expression are not always clear. Rare instances of female-specific ornament evolution (such as in some dance fly species) are particularly puzzling. While some evidence suggests that such rare instances represent straightforward reversals of sexual selection intensity, the distinct nature of trade-offs between ornaments and offspring pose special constraints in females. To examine whether competition for access to mates generally favors heightened ornament expression, we built a phylogeny and conducted a comparative analysis of Empidinae dance fly taxa that display female-specific ornaments. We show that species with more female-biased operational sex ratios in lek-like mating swarms have greater female ornamentation, and in taxa with more ornate females, male relative testis investment is increased. These findings support the hypothesis that ornament diversity in dance flies depends on female receptivity to mates, which is associated with contests for nutritious nuptial gifts provided by males. Moreover, our results suggest that increases in female receptivity lead to higher levels of sperm competition among males. The incidence of both heightened premating sexual selection on females and postmating selection on males contradicts assertions that sex roles are straightforwardly reversed in dance flies.