Sex role similarity and sexual selection predict male and female song elaboration and dimorphism in fairy‐wrens

Historically, bird song complexity was thought to evolve primarily through sexual selection on males; yet, in many species, both sexes sing and selection pressure on both sexes may be broader. Previous research suggests competition for mates and resources during short, synchronous breeding seasons leads to more elaborate male songs at high, temperate latitudes. Furthermore, we expect male–female song structure and elaboration to be more similar at lower, tropical latitudes, where longer breeding seasons and year‐round territoriality yield similar social selection pressures in both sexes. However, studies seldom take both types of selective pressures and sexes into account. We examined song in both sexes in 15 populations of nine‐fairy‐wren species (Maluridae), a Southern Hemisphere clade with female song. We compared song elaboration (in both sexes) and sexual song dimorphism to latitude and life‐history variables tied to sexual and social selection pressures and sex roles. Our results suggest that song elaboration evolved in part due to sexual competition in males: male songs were longer than female songs in populations with low male survival and less male provisioning. Also, female songs evolved independently of male songs: female songs were slower paced than male songs, although only in less synchronously breeding populations. We also found male and female songs were more similar when parental care was more equal and when male survival was high, which provides strong evidence that sex role similarity correlates with male–female song similarity. Contrary to Northern Hemisphere latitudinal patterns, male and female songs were more similar at higher, temperate latitudes. These results suggest that selection on song can be sex specific, with male song elaboration favored in contexts with stronger sexual selection. At the same time, selection pressures associated with sex role similarity appear to favor sex role similarity in song structure.     

Early constraints in sexual dimorphism: survival benefits of feminized phenotypes

Sexual dimorphism (SD) has evolved in response to selection pressures that differ between sexes. Since such pressures change across an individual's life, SD may vary within age classes. Yet, little is known about how selection on early phenotypes may drive the final SD observed in adults. In many dimorphic species, juveniles resemble adult females rather than adult males, meaning that out of the selective pressures established by sexual selection feminized phenotypes may be adaptive. If true, fitness benefits of early female‐like phenotypes may constrain the expression of male phenotypes in adulthood. Using the common kestrel Falco tinnunculus as a study model, we evaluated the fitness advantages of expressing more feminized phenotypes at youth. Although more similar to adult females than to adult males, common kestrel fledglings are still sexually dimorphic in size and coloration. Integrating morphological and chromatic variables, we analysed the phenotypic divergence between sexes as a measure of how much each individual looks like the sex to which it belongs (phenotypic sexual resemblance, PSR). We then tested the fitness benefits associated with PSR by means of the probability of recruitment in the population. We found a significant interaction between PSR and sex, showing that in both sexes more feminized phenotypes recruited more into the population than less feminized phenotypes. Moreover, males showed lower PSR than females and a higher proportion of incorrect sex classifications. These findings suggest that the mechanisms in males devoted to resembling female phenotypes in youth, due to a trend to increase fitness through more feminized phenotypes, may provide a mechanism to constrain the SD in adulthood.     

Female‐ and male‐specific signals of quality in the barn owl

Most bird studies of female signalling have been confined to species in which females display a male‐ornament in a vestigial form. However, a great deal of benefit may be gained from considering phenotypic traits that are specific to females. This is because (1) sex‐specific traits may signal sex‐specific qualities and (2) females may develop a male‐ornament not because they are selected to do so, but because fathers transmit to daughters the underlying genes for its expression (genetic correlation between the sexes). We investigated these two propositions in the barn owl Tyto alba, a species in which male plumage is lighter in colour and less marked with black spots than that of females. Firstly, we present published evidence that female plumage spottiness reflects parasite resistance ability. We also show that male plumage coloration is correlated with reproductive success, male feeding rate and heart mass. Secondly, cross‐fostering experiments demonstrate that plumage coloration and spottiness are genetically correlated between the sexes. This implies that if a given trait value is selected in one sex, the other sex will indirectly evolve towards a similar value. This prediction is supported by the observation that female plumage coloration and spottiness resembled that of males, in comparisons at the level of Tyto alba alba populations, Tyto alba subspecies and Tyto species. Our results therefore support the hypothesis that sex‐specific traits signal sex‐specific qualities and that a gene for a sex‐specific trait can be expressed in the other sex as the consequence of a genetic correlation between the sexes.     

No evidence for color or size preference in either sex of a dichromatic stream fish, Percina roanoka

Sexual dimorphism is hypothesized to be the result of differential selection pressures between the sexes. Dimorphic traits can serve as indicators of mate quality, altering mate preferences in the opposite sex in favor of a conspicuous trait. Common indicators of mate quality include color and size, with traditional assumptions and evidence predicting a preference for more colorful and/or larger sized mates in many species. Both male and female preferences for more colorful and larger mates within a species are rarely examined simultaneously, however. We examined a sexually dichromatic freshwater fish, Percina roanoka and found that male coloration is positively correlated with size, suggesting color may function as an indicator of viability. We tested preferences for coloration and size in both sexes in a dichotomous mate choice setup in which only visual signals were exchanged. Neither females nor males exhibited a color or size preference in individuals of the opposite sex. Visual cues alone therefore appear to be insufficient to elicit a significant preference in both sexes of this species. Male coloration in P. roanoka does not appear to be driven solely by female preference.     

No Evidence that Male Choice Contributes to the Maintenance of a Shared, Sex-Limited Trait in Mimetic and Non-mimetic Female Tiger Swallowtail Butterflies, Papilio glaucus

In animal species that have morphological polymorphisms maintained by unique or divergent selection pressures, understanding the preservation of shared traits is important for identifying the factors that are influencing overall evolutionary processes. In the Eastern tiger swallowtail butterfly, Papilio glaucus, females are dimorphic. One morph (‘dark-morph’) is mostly black and mimics the toxic pipevine swallowtail, Battus philenor. These females have large amounts of blue coloration on the dorsal hind wings that enhances their mimetic resemblance. Conversely, the alternate female type (‘yellow-morph’) is similar to males in coloration with the exception of extensive dorsal blue coloration, comparable to dark-morph females. Such coloration is almost completely absent in males. We examined dorsal blue coloration in P. glaucus to determine if mimetic resemblance in dark morphs is predominantly responsible for the maintenance of dorsal blue color in both female types, or whether mate recognition and/or sexual selection by males has a stronger influence on this trait. We measured the relative amount and variance of dorsal and ventral blue coloration in females of both color morphs, as well as males. We also compared these measurements to similar ones taken in the sister species, P. canadensis (which does not exhibit female dimorphism). Lastly, we investigated mate recognition and preferences of wild males. Our results suggest that mimetic resemblance may be more important than sexual selection for sustaining dorsal blue coloration in dark-morph females and that yellow-morphs could have elevated levels of blue due to currently unknown genetic associations. Although trait correlation between sexes is common, intrasexual trait correlation in a sex-limited, polymorphic species has not been frequently observed.     

Mycophagous beetle females do not behave competitively during intrasexual interactions in presence of a fungal resource

Intrasexual interactions can determine which individuals within a population have access to limited resources. Despite their potential importance on fitness generally and mating success especially, female–female interactions are not often measured in the same species where male–male interactions are well‐defined. In this study, we characterized female–female interactions in Bolitotherus cornutus, a mycophagous beetle species native to Northeastern North America. We used dyadic, behavioral assays to determine whether females perform directly aggressive or indirectly exclusionary competitive behaviors. Polypore shelf fungus, an important food and egg‐laying resource for B. cornutus females, is patchily distributed and of variable quality, so we tested for competition over fungus as a resource. Behavior of females was assessed in three sets of dyadic trials with randomly paired female partners. Overall, females did not behave aggressively toward their female partner or perform exclusionary behaviors over the fungal resource. None of the behaviors performed by females were individually repeatable. Two scenarios may explain our lack of observed competition: our trial context may not induce competition, or female B. cornutus simply may not behave competitively in the wild. We compare our results to a similar study on male–male interactions in the same species and propose future studies on female–female interactions under different competitive contexts to expand the understanding of female competition.     

Honest signals and sexual conflict: Female lizards carry undesirable indicators of quality

Sex differences in animal coloration often result from sex‐dependent regulatory mechanisms. Still, some species exhibit incomplete sexual dimorphism as females carry a rudimentary version of a costly male trait, leading to intralocus sexual conflict. The underlying physiology and condition dependence of these traits can inform why such conflicts remain unresolved. In eastern fence lizards (Sceloporus undulatus), blue iridophore badges are found in males and females, but melanin pigmentation underneath and surrounding badges is male‐exclusive. We track color saturation and area of badges across sexual maturity, and their relationship to individual quality (body condition and immunocompetence) and relevant hormones (testosterone and corticosterone). Saturation and testosterone were positively correlated in both sexes, but hormone and trait had little overlap between males and females. Saturation was correlated with body condition and immunocompetence in males but not in females. Co‐regulation by androgens may have released females from resource allocation costs of color saturation, even when in high condition. Badge area was independent of testosterone, but associated with low corticosterone in females, indicating that a nonsex hormone underlies incomplete sexual dimorphism. Given the evidence in this species for female reproductive costs associated with ornamentation, this sex‐nonspecific regulation of an honest signal may underlie intralocus sexual conflict.     

Thermobiological effects of temperature‐induced color variations in Aglais urticae (Lepidoptera,Nymphalidae)

Coloration of animals is important for camouflage, for social behavior, or for physiological fitness. This study investigates the color variation in adults of Aglais urticae obtained on subjecting some pre‐imaginal stages to different temperature conditions and their thermobiological consequences. To investigate the evolutionary–ecological interactions of temperature and pigmentation in butterflies, caterpillars, and pupae of the small tortoiseshell, Aglais urticae (Lepidoptera, Nymphalidae), larvae from Central Europe and Scandinavia were reared at temperatures between 7 and 34°C in the laboratory or in the field. After emergence, the intensity of pigmentation of the imagines and their increase in body temperature under defined full‐spectrum light irradiation were quantified by image analysis and thermal imaging. At constant conditions, ambient rearing temperature and pigmentation intensity of imagines were negatively and linearly correlated in Central European butterflies, regardless of whether the pupal stage alone or, additionally, the last period of the larval stage was exposed to these conditions: low temperatures induced darker coloration and high temperatures led to lighter individuals. A thermal pulse of a few days alone at the beginning of pupal dormancy led to a similar, albeit weakened, effect. Caterpillars of the Scandinavian subspecies A. urticae polaris, whose pupal dormancy took place under Central European field conditions, developed into strongly pigmented imagines. The thermobiological relevance of more intense pigmentation was shown by significantly higher absorption of light, and thus stronger increased body temperature after 5 min of defined illumination, but this difference ceased after 15 min. Our results show that phenotypic plasticity in wing coloration is adaptive since temperature‐induced developmental changes provide thermobiological benefit in adult butterflies. We propose that, in subpolar latitudes, darker coloration likely has a selection advantage favoring individuals with reaction norms gradually shifted to stronger pigmented phenotypes, possibly leading to the establishment of a pigmentation cline.     

Losses of female song with changes from tropical to temperate breeding in the New World blackbirds

Birds in which both sexes produce complex songs are thought to be more common in the tropics than in temperate areas, where typically only males sing. Yet the role of phylogeny in this apparent relationship between female song and latitude has never been examined. Here, we reconstruct evolutionary changes in female song and breeding latitude in the New World blackbirds (Icteridae), a family with both temperate and tropical representatives. We provide strong evidence that members of this group have moved repeatedly from tropical to temperate breeding ranges and, furthermore, that these range shifts were associated with losses of female song more often than expected by chance. This historical perspective suggests that male-biased song production in many temperate species is the result not of sexual selection for complex song in males but of selection against such songs in females. Our results provide new insights into the differences we see today between tropical and temperate songbirds, and suggest that the role of sexual selection in the evolution of bird song might not be as simple as we think.     

Male competition reverses female preference for male chemical cues

Females must choose among potential mates with different phenotypes in a variety of social contexts. Many male traits are inherent and unchanging, but others are labile to social context. Competition, for example, can cause physiological changes that reflect recent wins and losses that fluctuate throughout time. We may expect females to respond differently to males depending on the outcome of their most recent fight. In Bolitotherus cornutus (forked fungus beetles), males compete for access to females, but copulation requires female cooperation. In this study, we use behavioral trials to determine whether females use chemical cues to differentiate between males and whether the outcome of recent male competition alters female preference. We measured female association time with chemical cues of two size‐matched males both before and after male–male competition. Females in our study preferred to associate with future losers before males interacted, but changed their preference for realized winners following male competitive interactions. Our study provides the first evidence of change in female preference based solely on the outcome of male–male competition.     

Primate brain architecture and selection in relation to sex

Background  

Social and competitive demands often differ between the sexes in mammals. These differing demands should be expected to produce variation in the relative sizes of various brain structures. Sexual selection on males can be predicted to influence brain components handling sensory-motor skills that are important for physical competition or neural pathways involving aggression. Conversely, because female fitness is more closely linked to ecological factors and social interactions that enable better acquisition of resources, social selection on females should select for brain components important for navigating social networks. Sexual and social selection acting on one sex could produce sexual dimorphism in brain structures, which would result in larger species averages for those same brain structures. Alternatively, sex-specific selection pressures could produce correlated effects in the other sex, resulting in larger brain structures for both males and females of a species. Data are presently unavailable for the sex-specific sizes of brain structures for anthropoid primates, but under either scenario, the effects of sexual and social selection should leave a detectable signal in average sizes of brain structures for different species.     

Mating status affects female choice when females are signalers

Sexual selection in animals has been mostly studied in species in which males are signalers and females are choosers. However, in many species, females are (also) signalers. In species with non‐signaling females, virgin females are hypothesized to be less choosy than mated females, as virgins must mate to realize fitness and the number of available males is generally limited. Yet, when females signal to attract males, mate limitation can be overcome. We tested how virgin and mated females differ in their calling behavior, mating latency, and in mate choice, using the tobacco budworm Chloridea (Heliothis) virescens as an example for a species in which females are not only choosers but also signalers. We found that virgin females signaled longer than mated females, but virgin and mated signaling females were equally ready to mate, in contrast to non‐signaling females. However, we found that virgin signaling females showed weaker mate preference than mated females, which can be explained by the fact that females increase their fitness with multiple matings. Mated females may thus further increase their fitness by more stringent mate selection. We conclude that signaling is a crucial aspect to consider when studying female mate choice because signaling may affect the number of available mates to choose from.     

Year-round resource defence and the evolution of male and female song in suboscine birds: social armaments are mutual ornaments

The evolution of sexually monomorphic (i.e. mutual) ornamentation has attracted growing attention as a 'blind-spot' in evolutionary biology. The popular consensus is that female ornaments are subject to the same modes of sexual selection as males: intrasexual competition and mate choice. However, it remains unclear how these forces interact within and between sexes, or whether they fully capture selection on female traits. One possibility is that the 'armament-ornament' model - which proposes that traits used primarily in male-male contests are also co-opted by females as indicators of male quality - can be extended to explain signal evolution in both sexes. We examine this idea by testing the function of acoustic signals in two species of duetting antbirds. Behavioural observations and playback experiments suggest that male and female songs function primarily as armaments in competitive interactions. Removal experiments reveal that song is also a classic ornament used by unpaired males and females to advertise for mates. These results indicate that 'armament-ornament' processes may operate in reciprocal format, potentially explaining widespread mutual ornamentation in species with elevated intrasexual competition for resources. In addition, given that songs mediate competition between species outside the breeding season, our findings suggest that processes shaping monomorphic ornaments extend beyond the traditional definitions of sexual selection and are best understood in the broader framework of social selection.     

Shift in competitive ability mediated by soil biota in an invasive plant

Understanding the shifts in competitive ability and its driving forces is key to predict the future of plant invasion. Changes in the competition environment and soil biota are two selective forces that impose remarkable influences on competitive ability. By far, evidence of the interactive effects of competition environment and soil biota on competitive ability of invasive species is rare. Here, we investigated their interactive effects using an invasive perennial vine, Mikania micrantha. The competitive performance of seven M. micrantha populations varying in their conspecific and heterospecific abundance were monitored in a greenhouse experiment, by manipulating soil biota (live and sterilized) and competition conditions (competition‐free, intraspecific, and interspecific competition). Our results showed that with increasing conspecific abundance and decreasing heterospecific abundance, (1) M. micrantha increased intraspecific competition tolerance and intra‐ vs. interspecific competitive ability but decreased interspecific competition tolerance; (2) M. micrantha increased tolerance of the negative soil biota effect; and (3) interspecific competition tolerance of M. micrantha was increasingly suppressed by the presence of soil biota, but intraspecific competition tolerance was less affected. These results highlight the importance of the soil biota effect on the evolution of competitive ability during the invasion process. To better control M. micrantha invasion, our results imply that introduction of competition‐tolerant native plants that align with conservation priorities may be effective where M. micrantha populations are long‐established and inferior in inter‐ vs. intraspecific competitive ability, whereas eradication may be effective where populations are newly invaded and fast‐growing.     

Sperm performance in conspecific and heterospecific female fluid

Divergent sexual selection within allopatric populations may result in divergent sexual phenotypes, which can act as reproductive barriers between populations upon secondary contact. This hypothesis has been most tested on traits involved in precopulatory sexual selection, with less work focusing on traits that act after copulation and before fertilization (i.e., postcopulatory prezygotic traits), particularly in internally fertilizing vertebrates. However, postcopulatory sexual selection within species can also drive trait divergence, resulting in reduced performance of heterospecific sperm within the female reproductive tract. Such incompatibilities, arising as a by‐product of divergent postcopulatory sexual selection in allopatry, can represent reproductive barriers, analogous to species‐assortative mating preferences. Here, we tested for postcopulatory prezygotic reproductive barriers between three pairs of taxa with diverged sperm phenotypes and moderate‐to‐high opportunity for postcopulatory sexual selection (barn swallows Hirundo rustica versus sand martins Riparia riparia, two subspecies of bluethroats, Luscinia svecica svecica versus L. s. namnetum, and great tits Parus major versus blue tits Cyanistes caeruleus). We tested sperm swimming performance in fluid from the outer reproductive tract of females, because the greatest reduction in sperm number in birds occurs as sperm swim across the vagina. Contrary to our expectations, sperm swam equally well in fluid from conspecific and heterospecific females, suggesting that postcopulatory prezygotic barriers do not act between these taxon pairs, at this stage between copulation and fertilization. We therefore suggest that divergence in sperm phenotypes in allopatry is insufficient to cause widespread postcopulatory prezygotic barriers in the form of impaired sperm swimming performance in passerine birds.     

Plumage coloration and personality in early life: sexual differences in signalling

Several studies have shown that melanin‐based traits play a crucial role in social contexts as they are associated with dominance, personality and social behaviour. However, most of these studies have focused on adults, and the role of these traits in juveniles has rarely been explored. Here, we explore the association between two melanin‐based traits and nestling personality in Common Kestrels Falco tinnunculus. Our results show that female nestlings with blacker plumages displayed bolder personalities, providing evidence of sex‐dependent phenotypic integration of these two traits in males and females. We consider that this differential integration may arise from different selection pressures acting on males and females on plumage coloration during adulthood and that nestling coloration can act as a status signal within the juvenile age‐class.     

Female response song in the ephippigerines Steropleurus stali and Platystolus obvius (Orthoptera,Tettigoniidae)

Stridulation by females of Steropleurus stali and Platystolus obvius in response to the calling song of the males was observed and recorded. The response has only been stimulated by the appropriate male song, either directly or from a recording. The structure of the files and the form of stridulation in both sexes is described. The male song of S. stali is remarkable in that only a few teeth on the file are struck in each wing movement. It is also notable that both opening and closing wing strokes contribute more or less equally to the syllable. The female song is similar but distinct. The song of P. obvius male is a single chirp involving nearly all the teeth on the file and with the main emphasis on the closing syllable. The response song of the female is a very brief chirp. These species are only sporadic singers, but when the female responds they are stimulated into greater activity. They thus contrast with reiterative singers like Ephippiger in which there is no female response. The implications of these contrasting behavioural patterns is discussed.     

Divergent Preferences for Song Structure between a Field Cricket and its Phonotactic Parasitoid

In many animals, males produce signals to attract females for mating. However, eavesdropping parasites may exploit these conspicuous signals to find their hosts. In these instances, the strength and direction of natural and sexual selection substantially influence song evolution. Male variable field crickets, Gryllus lineaticeps, produce chirped songs to attract mates. The eavesdropping parasitoid fly Ormia ochracea uses cricket songs to find its hosts. We tested female preferences for song structure (i.e., chirped song vs. trilled song) in crickets and flies using choice experiments. Female crickets from a parasitized and a non-parasitized population significantly preferred the species-typical chirped song, whereas flies significantly preferred a trilled song, which is expressed by other hosts in different regions. Sexual selection due to female choice and natural selection due to fly predation both appear to favor the chirped song structure of G. lineaticeps in the parasitized population, whereas sexual selection favors the chirped structure in the non-parasitized population.     

Are good ornaments bad armaments? Male chaffinch perception of songs with varying flourish length

Birdsong functions as both a territorial and a mate-attracting signal, and is therefore subject to inter- and intrasexual selection through male-male competition and female mate choice. Consequently, differences in song perception between males and females could potentially lead to conflicting selection pressures on song structure. Chaffinch,Fringilla coelebs , songs have two distinctive parts: a series of rapidly repeated notes (the trill) followed by a sequence of more complex and often nonrepeated elements (the flourish). Males often spontaneously sing incomplete songs consisting mainly of the trill part, but their communicative function is unclear. We tested the reaction of males to songs with and without the terminal flourish in a dual speaker playback design by measuring their approach to the speakers. Like female chaffinches tested with the same stimuli in a previous study, males responded more to songs with both trill and flourish than to those consisting of a trill only. However, although females had preferred songs with a relatively longer flourish, males showed the strongest reaction (closest approach to the speaker) to songs with longer trills and shorter flourishes. The results suggest that males and females may perceive aspects of chaffinch song structure differently, in which case it could be under conflicting pressure of intra- and intersexual selection. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

The effect of body coloration and group size on social partner preferences in female fighting fish (Betta splendens)

Females of the fighting fish Betta splendens have been shown to associate with other B. splendens females in a manner reminiscent of shoaling behavior. Since body coloration varies dramatically in this species, and since body coloration has been shown to affect shoalmate choice in other species of fish, we examined the influence of body coloration on association preferences in female B. splendens. In dichotomous choice tests, B. splendens females spent more time swimming near groups of females (regardless of coloration) than swimming near an empty chamber, and chose to swim near fish of similar coloration to their own when choosing between two distinctly colored groups of females. When examining the interplay between body coloration and group size, focal fish spent more time swimming near larger groups (N = 5) of similarly colored fish than swimming near an individual female of similar coloration. However, focal fish showed no preference when presented with an individual female of similar coloration and a larger group of females of dissimilar coloration. These results suggest that association choices in B. splendens females are strongly affected by both body coloration and by group size.