Tail Length and Mutual Mate Choice in Bearded Tits (Panurus biarmicus)

Direct sexual selection via mutual mate choice can result in both sexes showing conspicuous traits. We experimentally tested whether this hypothesis can explain tail length in the bearded tit (Panurus biarmicus). In this species, both sexes have a long, graduated tail. Males have, however, a longer tail than females, suggesting perhaps that females are choosier than males in selecting mates. We used two choice set‐ups for each sex: shortened vs. control tail individuals and elongated vs. control tail individuals. We found that direct sexual selection seems to operate differently in the two sexes. In both set‐ups, females spent more time with the male with the longest tail, and they also showed sexual display behaviour only towards these males. Males spent more time with control than with short‐tailed females, but they did not discriminate between control and long‐tailed females. Moreover, males displayed preference towards both short‐ and long‐tailed females. Thus, females preferred long‐tailed males, whereas males did not always prefer long‐tailed females. Our study suggests that mutual mate choice has played a role in the evolution of long tails in bearded tits. It also suggests that the sexual dimorphism in tail length has evolved because mate choice exerts a stronger sexual selection pressure on males than on females.     

WHY SHOULD LEK-BREEDERS BE MONOMORPHIC?

Approximately one-quarter of all lek-breeding bird species are sexually monomorphic. Understanding the significance, if any, of this exception to the usual correlation between sexual selection and dimorphism requires detailed data on the mating systems of both monomorphic and dimorphic species. The capuchinbird (Perissocephalus tricolor) is a sexually monomorphic, lek-breeding member of the cotinga family. I studied the social and sexual behavior of this species, and compared it with the Guianan cock-of-the-rock (Rupicola rupicola), a dimorphic, lekking member of the same family. Male–male competition in capuchinbirds involved direct contests for dominance, rather than territorial displays as in classic lek species. In each year, one dominant individual was able to control the most desired display site on the 8-male lek, and was the only male that copulated. In contrast to dimorphic lek birds, female as well as male capuchinbirds engaged in frequent and intense aggression at the lek, and both males and females engaged in sexual mimicry. I suggest that plumage monomorphism in lek birds has evolved as a result of social competition affecting both sexes. This hypothesis accounts for the exaggerated plumage characters shared by males and females in capuchinbirds and a number of other monomorphic lek birds. The evolution of plumage can best be analyzed as an arms race, in which the balance of selective forces acting on each sex can produce a variety of equilibrium states, ranging from sexual indistinguishability to extreme dimorphism.     

Sexual dimorphism in relation to current selection in the house finch

Abstract.— Sexual dimorphism is thought to have evolved in response to selection pressures that differ between males and females. Our aim in this study was to determine the role of current net selection in shaping and maintaining contemporary sexual dimorphism in a recently established population of the house finch ( Carpodacus mexicanus ) in Montana. We found strong differences between sexes in direction of selection on sexually dimorphic traits, significant heritabilities of these traits, and a close congruence between current selection and observed sexual dimorphism in Montana house finches. Strong directional selection on sexually dimorphic traits and similar intensities of selection in each sex suggested that sexual dimorphism arises from adaptive responses in males and females, with both sexes being far from their local fitness optimum. This pattern is expected when a recently established population experiences continuous immigration from ecologically distinct areas of a species range or as a result of widely fluctuating selection pressures, as found in our study. Strong and sexually dimorphic selection pressures on heritable morphological traits, in combination with low phenotypic and genetic covariation among these traits during growth, may have accounted for close congruence between current selection and observed sexual dimorphism in the house finch. This conclusion is consistent with the profound adaptive population divergence in sexual dimorphism that accompanied very successful colonization of most of the North America by the house finch over the last 50 years.     

Multivariate intralocus sexual conflict in seed beetles

Intralocus sexual conflict (IaSC) is pervasive because males and females experience differences in selection but share much of the same genome. Traits with integrated genetic architecture should be reservoirs of sexually antagonistic genetic variation for fitness, but explorations of multivariate IaSC are scarce. Previously, we showed that upward artificial selection on male life span decreased male fitness but increased female fitness compared with downward selection in the seed beetle Callosobruchus maculatus. Here, we use these selection lines to investigate sex‐specific evolution of four functionally integrated traits (metabolic rate, locomotor activity, body mass, and life span) that collectively define a sexually dimorphic life‐history syndrome in many species. Male‐limited selection for short life span led to correlated evolution in females toward a more male‐like multivariate phenotype. Conversely, males selected for long life span became more female‐like, implying that IaSC results from genetic integration of this suite of traits. However, while life span, metabolism, and body mass showed correlated evolution in the sexes, activity did not evolve in males but, surprisingly, did so in females. This led to sexual monomorphism in locomotor activity in short‐life lines associated with detrimental effects in females. Our results thus support the general tenet that widespread pleiotropy generates IaSC despite sex‐specific genetic architecture.     

Strategic concealment of sexual identity in an estrilid finch

One explanation for the evolution of sexual monomorphism is the sexual indistinguishability hypothesis, which argues that in group-living species individuals might benefit by concealing their sex to reduce sexual competition. We tested this hypothesis in long-tailed finches Poephila acuticauda. Males and females could not be reliably distinguished morphologically or by analysis of the reflectance spectra (300 to 700 nm) from the plumage and bill. Males seemed unable to distinguish the sex of an unfamiliar individual in the absence of behavioural cues; they were equally likely to court and copulate with unfamiliar males and females but rarely courted familiar males. Here we report the first experimental evidence that sexual monomorphism enables strategic concealment of sex. Males were more likely to reveal their sex when faced with a solitary unfamiliar individual than a group of unfamiliar individuals. When encountering an unfamiliar male that revealed his sex, subordinate males were more likely to conceal their sex than dominant males.     

Acoustical variations in sexually dimorphic features of distance calls in domesticated zebra finches (Taeniopygia guttata castanotis)

Species-specific distance calls (DCs) were recorded from Zebra finches (Taeniopygia guttata castanotis) obtained from three different breeding stocks: Japanese breeders that use Bengalese finches as fostering parents, and Japanese and American breeders that let natural parents rear Zebra finches. These calls were analyzed for five acoustic parameters that were shown to be sexually dimorphic in wild Zebra finches. Male Zebra finches had DCs that were variable among breeding stocks and among individuals. Female DCs recorded from Bengalese-fostered birds were generally longer in duration and higher in pitch than those recorded from Zebra-finch-reared birds, males and females in each breeding stock differed in at least one acoustic parameter, but that parameter was unique in each of the breeding stocks. These results suggest that although sexual dimorphism in Zebra finch DCs has gradually disappeared during the process of domestication, at least one acoustic attribute which allows discrimination between the calls of the sexes has been preserved.     

Sexually dimorphic immune response in the harem polygynous Wellington tree weta Hemideina crassidens

Abstract Adult males are often less immunocompetent than females. One explanation for this is that intense sexual selection causes males to trade‐off investment in immunity with traits that increase mating success. This hypothesis is tested in the Wellington tree weta (Hemideina crassidens), a large, sexually dimorphic orthopteran insect in which males possess enormous mandibular weaponry used during fights for access to female mates. Field‐collected males have a significantly greater immune response (greater melanotic encapsulation) than females, suggesting that body condition, longevity or an allied trait is important to male fitness, or that females require materials for egg production that would otherwise be used to boost immunity. Although immunity is expected to trade‐off against reproductive traits in both sexes, there is no significant relationship between immune response and weapon or testes size in males, nor fecundity in females.     

An eye for detail: selective sexual imprinting in zebra finches

To investigate the idea that sexual imprinting creates incipient reproductive isolation between phenotypically diverging populations, I performed experiments to determine whether colony-reared zebra finches would imprint on details of artificial white crests. In the first experiment, adults in one breeding colony wore white crests with a vertical black stripe, while in another colony adults wore crests having a horizontal black stripe; except for their crests, breeders possessed wild-type plumage and conformation. Offspring of both sexes reared in these colonies developed mate preferences for opposite-sexed birds wearing the crest type with which they were reared; neither sex developed a social preference for crested individuals of the same sex. In a second experiment, females reared by crested parents preferred crested males versus males with red leg bands, while control females (reared in a colony of wild-type, uncrested birds) preferred red-banded males in the same test. Results of a third experiment that used sexually dimorphic crest phenotypes indicate that both sexes of offspring imprinted on maternal crest patterns. Results support the hypothesis that sexual imprinting can facilitate isolation both by engendering a preference for population-typical traits and by prioritizing such an imprinting-based preference over species-typical preferences for other traits used in mate choice. Comparison with results of other recent studies indicates that imprinting tendencies of both sexes vary with the characteristics of traits presented as an imprinting stimuli. Tendency to imprint may vary with the perceived information content (e.g., kin, sex, or population indicator) of parental traits, a process dubbed selective sexual imprinting.     

Sexual dimorphism in bill morphology and feeding ecology in Cory's shearwater (Calonectris diomedea)

The bill is a sexually dimorphic structure in many bird species and implicated in numerous functions. Sexual differences may arise from sexual selection or ecological divergence. Here, we examined differences in bill size and shape between males and females and explored to what extent these relate to feeding ecology of each sex in Cory's shearwater (Calonectris diomedea). We applied linear measurements and geometric morphometric methods to examine sexual differences in bill size and shape. We investigated feeding ecology by tracking foraging movements during the breeding period and by analysing stable isotope signatures in blood during the breeding period and in feathers grown during the non-breeding period. Bill traits were all sexually dimorphic, both in absolute and relative terms, and scaled hypermetrically with body mass in several characters in males. However, males and females did not differ in their feeding areas or isotopic signatures and no significant correlation was observed between these traits and bill dimorphism. Therefore, we discard the foraging-niche divergence hypothesis, and suggest that sexual dimorphism in bill size in this species is more likely driven by sexual selection related to antagonistic interactions.     

Ecology,sexual dimorphism,and jumping evolution in anurans

Sexual dimorphism (SD) is a common feature of animals, and selection for sexually dimorphic traits may affect both functional morphological traits and organismal performance. Trait evolution through natural selection can also vary across environments. However, whether the evolution of organismal performance is distinct between the sexes is rarely tested in a phylogenetic comparative context. Anurans commonly exhibit sexual size dimorphism, which may affect jumping performance given the effects of body size on locomotion. They also live in a wide variety of microhabitats. Yet the relationships among dimorphism, performance, and ecology remain underexamined in anurans. Here, we explore relationships between microhabitat use, body size, and jumping performance in males and females to determine the drivers of dimorphic patterns in jumping performance. Using methods for predicting jumping performance through anatomical measurements, we describe how fecundity selection and natural selection associated with body size and microhabitat have likely shaped female jumping performance. We found that the magnitude of sexual size dimorphism (where females are about 14% larger than males) was much lower than dimorphism in muscle volume, where females had 42% more muscle than males (after accounting for body size). Despite these sometimes-large averages, phylogenetic t-tests failed to show the statistical significance of SD for any variable, indicating sexually dimorphic species tend to be closely related. While SD of jumping performance did not vary among microhabitats, we found female jumping velocity and energy differed across microhabitats. Overall, our findings indicate that differences in sex-specific reproductive roles, size, jumping-related morphology, and performance are all important determinants in how selection has led to the incredible ecophenotypic diversity of anurans.     

Sexual differences and sex ratios of dioecious plants under stressful environments

雌雄异株植物对环境胁迫响应的性别差异与性别比例 雌雄异株植物在性特征(繁殖器官)和次级性特征(植物的特征)均表现出性二态。形态、生理与生态特征等次级性特征的性别差异,通常在繁殖成本和其他功能性状之间存在着权衡。尽管有证据表明性二态对环境胁迫的响应不一定存在于所有植物中,但次级性特征的权衡可能受到环境胁迫的影响。当植物表现出性二态时,不同的物种与胁迫因子可以导致性别特异性的响应。因此,胁迫作用对雌雄异株植物影响的概括性研究是必须的。另外,性二态可能会影响雌雄异株植物沿着环境梯度的频率和分布,引起生态位分化与性别空间分异。目前,控制性别比例偏差的原因和机制还知之甚少。本综述旨在讨论不利环境下的性别特异性响应与性别比例偏差,有利于深入的理解性二态对环境胁迫的响应。     

Insights into the genetic architecture of sexual dimorphism from an interspecific cross between two diverging Silene (Caryophyllaceae) species

The evolution of sexual dimorphism in species with separate sexes is influenced by the resolution of sexual conflicts creating sex differences through genetic linkage or sex‐biased expression. Plants with different degrees of sexual dimorphism are thus ideal to study the genetic basis of sexual dimorphism. In this study we explore the genetic architecture of sexual dimorphism between Silene latifolia and Silene dioica. These species have chromosomal sex determination and differ in the extent of sexual dimorphism. To test whether QTL for sexually dimorphic traits have accumulated on the sex chromosomes and to quantify their contribution to species differences, we create a linkage map and performed QTL analysis of life history, flower and vegetative traits using an unidirectional interspecific F2 hybrid cross. We found support for an accumulation of QTL on the sex chromosomes and that sex differences explained a large proportion of the variance between species, suggesting that both natural and sexual selection contributed to species divergence. Sexually dimorphic traits that also differed between species displayed transgressive segregation. We observed a reversal in sexual dimorphism in the F2 population, where males tended to be larger than females, indicating that sexual dimorphism is constrained within populations but not in recombinant hybrids. This study contributes to the understanding of the genetic basis of sexual dimorphism and its evolution in Silene.     

The evolution of sexual size dimorphism in the house finch. III. Developmental basis

Sexual size dimorphism of adults proximately results from a combination of sexually dimorphic growth patterns and selection on growing individuals. Yet, most studies of the evolution of dimorphism have focused on correlates of only adult morphologies. Here we examined the ontogeny of sexual size dimorphism in an isolated population of the house finch (Carpodacus mexicanus). Sexes differed in growth rates and growth duration; in most traits, females grew faster than males, but males grew for a longer period. Sexual dimorphism in bill traits (bill length, width, depth) and in body traits (wing, tarsus, and tail length; mass) developed during different periods of ontogeny. Growth of bill traits was most different between sexes during the juvenile period (after leaving the nest), whereas growth of body traits was most sexually dimorphic during the first few days after hatching. Postgrowth selection on juveniles strongly influenced sexual dimorphism in all traits; in some traits, this selection canceled or reversed dimorphism patterns produced by growth differences between sexes. The net result was that adult sexual dimorphism, to a large degree, was an outcome of selection for survival during juvenile stages. We suggest that previously documented fast and extensive divergence of house finch populations in sexual size dimorphism may be partially produced by distinct environmental conditions during growth in these populations.     

Test of the activity budget hypothesis on Asiatic ibex in Tian Shan Mountains of Xinjiang,China

Asiatic ibex (Capra sibirica) is a threatened species in China and is distributed in the mountains of Central Asia. It is a sexually dimorphic ungulate. The males are much larger than females, and except for the breeding season, both sexes live in separate groups. Many hypotheses have been developed to explain the sexual segregation in sexually dimorphic ungulates. These hypotheses are not mutually exclusive, and in recent years, the activity budget hypothesis has received special attention. To test this hypothesis, we studied the activity budget of Asiatic ibex in the autumn of 2005. According to the activity budget hypothesis, females should spend more time feeding than the males, and the degree of activity synchronization should be higher in same-sex groups than in mixed-sex groups. Our results are consistent with the hypothesis. Both sexes of the Asiatic ibex spent most of the time feeding, and females spent a significantly longer time feeding than males, and males spent a significantly longer time standing. Activity synchronization indexes of both the female groups and males groups were significantly higher than mixed-sex groups. These results indicated that in Asiatic ibex, the activity budget hypothesis about sexual segregation is supported.     

Ornaments or offspring: costs to reproductive success restrict sexual selection processes

If, in their partner choice, males seek direct benefits (fecund females), the result will be selection for traits indicating female quality rather than for arbitrary (Fisherian) traits. However, the costs of developing and maintaining the sexually selected traits (ornaments) may reduce the resources available to the female for allocation to reproduction and hence result in lower reproductive success per brood. This hitherto unrecognized fecundity cost of sexually selected traits will constrain both the potency of sexual selection mechanisms and the degree of elaboration of sexually selected traits in females, and can also apply to males which invest in their offspring: sexual selection becomes self-limiting. The fitness implications of these costs are examined for both sexes in a variety of mating and parental care patterns. Sexual selection acting on both sexes may lead to either dimorphism or monomorphism, the latter being the case when the quality indicators chosen by both sexes coincide. Ways of evasion or reduction of these reproductive costs of allocations to sexually selected traits include using different resource components for the ornament and for reproduction, or partitioning the two allocations in time.     

Patterns of sexual dimorphism in Mexican alligator lizards,Barisia imbricata

We compare morphological characteristics of male and female Barisia imbricata, Mexican alligator lizards, and find that mass, head length, coloration, incidence of scars from conspecifics, tail loss, and frequency of bearing the color/pattern of the opposite sex are all sexually dimorphic traits. Overall size (measured as snout–vent length), on the other hand, is not different between the two sexes. We use data on bite scar frequency and fecundity to evaluate competing hypotheses regarding the selective forces driving these patterns. We contend that sexual selection, acting through male‐male competition, may favor larger mass and head size in males, whereas large females are likely favored by natural selection for greater fecundity. In addition, the frequency of opposite‐sex patterning in males versus females may indicate that the costs of agonistic interactions among males are severe enough to allow for an alternative mating strategy. Finally, we discuss how sexual and natural selective forces may interact to drive or mask the evolution of sexually dimorphic traits.     

Quantitative genetics and sex-specific selection on sexually dimorphic traits in bighorn sheep

Sexual conflict at loci influencing traits shared between the sexes occurs when sex-specific selection pressures are antagonistic relative to the genetic correlation between the sexes. To assess whether there is sexual conflict over shared traits, we estimated heritability and intersexual genetic correlations for highly sexually dimorphic traits (horn volume and body mass) in a wild population of bighorn sheep (Ovis canadensis) and quantified sex-specific selection using estimates of longevity and lifetime reproductive success. Body mass and horn volume showed significant additive genetic variance in both sexes, and intersexual genetic correlations were 0.24+/-0.28 for horn volume and 0.63+/-0.30 for body mass. For horn volume, selection coefficients did not significantly differ from zero in either sex. For body weight, selection coefficients were positive in females but did not differ from zero in males. The absence of detectable sexually antagonistic selection suggests that currently there are no sexual conflicts at loci influencing horn volume and body mass.     

Sperm competition and sexually size dimorphic brains in birds

Natural selection may favour sexually similar brain size owing to similar selection pressures in males and females, while sexual selection may lead to sexually dimorphic brains. For example, sperm competition involves clear-cut sex differences in behaviour, as males display, mate guard and copulate with females, while females choose among males, and solicit or reject copulations. These behaviours may require fundamentally different neural government in the two sexes leading to sex-dependent brain evolution. Using two phylogenetic approaches in a comparative study, we tested for roles of both natural and sexual-selection pressures on brain size evolution of birds. In accordance with the natural-selection theory, relative brain size of males coevolved with that of females, which may be the result of adaptation to similar environmental constraints such as feeding innovation. However, the mode of brain size evolution differed between the sexes, and factors associated with sperm competition as reflected by extra-pair paternity may give rise to sexually size dimorphic brains. Specifically, species in which females have larger brains than males were found to have a higher degree of extra-pair paternity independently of potentially confounding factors, whereas species in which males have relatively larger brains than females appeared to have lower rates of extra-pair paternity. Hence, the evolution of sperm competition may select for complex behaviours together with the associated neural substrates in the sex that has a higher potential to control extra-pair copulations at the observed levels. Brain function may thus be affected differently in males and females by sexual selection.     

The evolution of sexual size dimorphism in the house finch. II. Population divergence in relation to local selection

Recent colonization of ecologically distinct areas in North America by the house finch (Carpodacus mexicanus) was accompanied by strong population divergence in sexual size dimorphism. Here we examined whether this divergence was produced by population differences in local selection pressures acting on each sex. In a long-term study of recently established populations in Alabama, Michigan, and Montana, we examined three selection episodes for each sex: selection for pairing success, overwinter survival, and within-season fecundity. Populations varied in intensity of these selection episodes, the contribution of each episode to the net selection, and in the targets of selection. Direction and intensity of selection strongly differed between sexes, and different selection episodes often favored opposite changes in morphological traits. In each population, current net selection for sexual dimorphism was highly concordant with observed sexual dimorphism--in each population, selection for dimorphism was the strongest on the most dimorphic traits. Strong directional selection on sexually dimorphic traits, and similar intensities of selection in both sexes, suggest that in each of the recently established populations, both males and females are far from their local fitness optimum, and that sexual dimorphism has arisen from adaptive responses in both sexes. Population differences in patterns of selection on dimorphism, combined with both low levels of ontogenetic integration in heritable sexually dimorphic traits and sexual dimorphism in growth patterns, may account for the close correspondence between dimorphism in selection and observed dimorphism in morphology across house finch populations.     

The evolution of sexual size dimorphism in the house finch. IV. Population divergence in ontogeny

Differences among taxa in sexual size dimorphism of adults can be produced by changes in distinct developmental processes and thus may reflect different evolutionary histories. Here we examine whether divergence in sexual dimorphism of adults between recently established Montana and Alabama populations of the house finch (Carpodacus mexicanus) can be attributed to population differences in growth of males and females. In both populations, males and females were similar at hatching, but as a result of sex-specific growth attained sexual size dimorphism by the time of independence. Timing and extent of growth varied between the sexes: Females maintained maximum rates of growth for a longer time than males, whereas males had higher initial growth rates and achieved maximum growth earlier and at smaller sizes than females. Ontogeny of sexual dimorphism differed between populations, but in each population, sexual dimorphism in growth parameters and sexual dimorphism at the time of nest leaving were similar to sexual dimorphism of adults. Variation in growth of females contributed more to population divergence than did growth of males. In each population, we found close correspondence between patterns of sexual dimorphism in growth and population divergence in morphology of adults: Traits that were the most sexually dimorphic in growth in each population contributed the most to population divergence in both sexes. We suggest that sex-specific expression of phenotypic and genetic variation throughout the ontogeny of house finches can result in different responses to selection between males and females of the same age, and thus produce fast population divergence in the sexual size dimorphism.