Sexually dimorphic male horns and their use in agonistic behaviors in the horn-headed cricket <Emphasis Type="Italic">Loxoblemmus doenitzi</Emphasis> (Orthoptera: Gryllidae)

Sexual dimorphism, the difference between the sexes in secondary sexual characters, is in general driven by processes of sexual selection. The horn-headed cricket, Loxoblemmus doenitzi, exhibits sexual dimorphism in head shape. Males have flat heads and triangular horns on both sides of their heads, whereas females have rounded heads and no horns. We hypothesized that male horns have evolved due to intra-sexual selection, in which males use these horns as weapons in aggressive interactions. We tested two predictions of this hypothesis by conducting agonistic trials with field-caught males of L. doenitzi: (1) the horns should be used in agonistic interactions between males, and (2) the asymmetry in horn size or horn use may determine contest outcome. Horn length was significantly correlated with thorax length and hind femur length. During agonistic interactions, males aggressively used their horns by beating the opponent’s horns with their own or by poking the opponent’s body. However, logistic regression analysis revealed that neither horn length nor horn use were significant factors for contest outcome. Instead, body size was significant for determining contest outcome. We discuss possible scenarios for evolution of male horns in L. doenitzi.     

Contrasting patterns of sexually selected traits in Mediterranean and continental populations of European mouflon

The expression of sexually selected traits in highly dimorphic ungulates may be influenced by environmental quality. Variations in habitat conditions can impose different constraints on the allocation of energy resources to male life‐history traits, and possibly alter the female preferences for specific features. Here, we compared the horn growth patterns in male European mouflon Ovis aries musimon living in different habitats (Mediterranean vs. continental) but sharing a common genetic origin. We hypothesized that the expression of sexually selected traits such as horn development should be promoted in more favorable habitat conditions (i.e., Mediterranean). Using linear mixed models on data retrieved from individuals harvested under the same hunting regime, we found longer horns and greater individual variance in horn segment length in the Mediterranean population than in the continental one. Furthermore, Mediterranean rams showed no evidence of compensatory horn growth, as opposed to the continental rams. Unexpectedly, horn base circumference was greater in the continental habitat than in the Mediterranean one. The overall results suggest different patterns of investment in horns in the two populations, with seemingly stronger pressure and consequences of sexual selection on mouflon rams living in more favorable environments. Although the role of hunters' selectivity cannot be excluded a priori, our data suggest that the differences in the expression of sexually selected traits in our study populations may be influenced by environmental conditions. Because sexual selection can impose substantial fitness costs on individuals, further investigations on the trade‐offs between reproduction and survival would improve our understanding of the dynamics of mouflon populations living in different environmental conditions.     

The role of doublesex in the evolution of exaggerated horns in the Japanese rhinoceros beetle

Male‐specific exaggerated horns are an evolutionary novelty and have diverged rapidly via intrasexual selection. Here, we investigated the function of the conserved sex‐determination gene doublesex (dsx) in the Japanese rhinoceros beetle (Trypoxylus dichotomus) using RNA interference (RNAi). Our results show that the sex‐specific T. dichotomus dsx isoforms have an antagonistic function for head horn formation and only the male isoform has a role for thoracic horn formation. These results indicate that the novel sex‐specific regulation of dsx during horn morphogenesis might have been the key evolutionary developmental event at the transition from sexually monomorphic to sexually dimorphic horns.     

Dimorphism of both head and prothoracic horn morphologies in male Copris acutidens (Coleoptera: Scarabaeoidea)

Allometric scaling of male horn morphologies of Copris acutidens, including the head and prothoracic horns, was analyzed. The analyses of scaling relationships indicated a discontinuous increase in head horn length and prothoracic horn height but a linear increase in prothoracic horn length relative to body size. The different results for length and height of the prothoracic horn suggest that the height is functionally more important for ensuring mates in the nests. Furthermore, both the head and prothoracic horns were dimorphic, and this characteristic was not found in the scarabaeoid beetle Onthophagus. Similarity in the switch point values for the head and prothoracic horns suggests that the dimorphism of the two kinds of horns may result from the same developmental threshold mechanism.     

Juvenile hormone mediates sexual dimorphism in horned beetles

The causes and consequences of sexual dimorphism are major themes in biology. Here we explore the endocrine regulation of sexual dimorphism in horned beetles. Specifically, we explore the role of juvenile hormone (JH) in regulating horn expression in females of two species with regular sexual dimorphism for pronotal horns (females have much shorter horns than males) and a third species with a rare reversed sexual dimorphism for both pronotal and head horns (females have much larger horns in both body regions compared with males). Applications of the JH analog methoprene caused females of the two more typical species to grow significantly shorter pronotal horns than control females, whereas no consistent effect on pronotal horn development was detected in the third, sex-reversed species. Instead, females in this species showed an unexpected and significant increase in head horn expression in response to methoprene treatment. Lastly, horn shape was also affected in females of one of the regularly sexually dimorphic species, but in the opposite direction than horn length. Although methoprene exerted a feminizing effect on female horn length in this species, it significantly masculinized horn shape by inducing a peculiar shape change observed naturally only in males. Our results suggest that JH influences both overall size and shape of female horns, but does so flexibly and as a function of species, sex and horn location. We use our results to review current models on the role of endocrine mechanisms in development and evolution of horned beetle diversity.     

Darwinian balancing selection: Predation counters sexual selection in a wild insect

The potential viability costs of sexually selected traits are central to hypotheses about the evolution of exaggerated traits. Estimates of these costs in nature can come from selection analyses using multiple components of fitness during the same time frame. For a population of tree crickets (Oecanthus nigricornis: Gryllidae), we analyzed viability and sexual selection on male traits by comparing Oecanthus prey of a solitary wasp to those that survived, and comparing mating individuals to solitary males. We measured forewing width (sexually size dimorphic and used for singing), head width, pronotum length, and size of hind jumping legs as potential targets of selection. Supporting the hypothesis that sexually selected traits have viability costs, we found that significant directional sexual selection for wider heads was opposed by significant viability selection for narrower heads. Nonlinear selection revealed that individuals with wide heads and small legs were most attractive, but individuals with narrow heads, large legs, and intermediate pronotum length were most likely to survive. Successful mating may put males at greater risk of predation, especially if copulation per se is risky. Such balancing selection in tree crickets may have constrained the evolution of sexual dimorphism in head size—a condition seen in other gryllids and orthopterans.     

Among‐population variation and correlations in sexually dimorphic traits of Silene latifolia

Abstract The degree of sexual dimorphism in a trait may be determined directly by disruptive selection, as well as by correlations with other traits under selection. We grew seeds from nine populations of the dioecious plant Silene latifolia in a common‐garden experiment to determine whether phenotypic variation and correlations existed for floral, leaf and resource allocation traits, and whether this variation had a genetic component. We also determined the traits which were sexually dimorphic, the degree of dimorphism, and whether it varied among populations. Seven traits exhibited among‐population variation and sexual dimorphism. Variation in the degree of dimorphism occurred only for two traits, suggesting that dimorphism may be evolving more slowly than trait means. Males had more, smaller flowers, shorter leaves, and allocated less of their total biomass to stems and more to leaves than females. Flower production was the most sexually dimorphic trait and was correlated with all measured traits. Most traits exhibited significant correlations between the sexes. The pattern of correlations and the degree of sexual dimorphism among traits lead us to suggest that intrasexual selection for an exaggerated floral display in males has indirectly led to sexual dimorphism in a host of other traits.     

Heterogeneity in male horn growth and longevity in a highly sexually dimorphic ungulate

In sexually dimorphic ungulates, sexual selection favoring rapid horn growth in males may be counterbalanced by a decrease in longevity if horns are costly to produce and maintain. Alternatively, if early horn growth varied with individual quality, it may be positively correlated with longevity. We studied Alpine ibex Capra ibex in the Gran Paradiso National Park, Italy, to test these alternatives by comparing early horn growth and longevity of 383 males that died from natural causes. After accounting for age at death, total horn length after age 5 was positively correlated with horn growth from two to four years. Individuals with the fastest horn growth as young adults also had the longest horns later in life. Annual horn growth increments between two and six years of age were independent of longevity for ibex whose age at death ranged from 8 to 16 years. Our results suggest that growing long horns does not constrain longevity. Of the variability in horn length, 22% could be explained by individual heterogeneity, suggesting persistent differences in phenotypic quality among males. Research on unhunted populations of sexually dimorphic ungulates documents how natural mortality varies according to horn or antler size, and can help reduce the impact of sport hunting on natural processes.     

The tale of the shrinking weapon: seasonal changes in nutrition affect weapon size and sexual dimorphism,but not contemporary evolution

Sexually selected traits are often highly variable in size within populations due to their close link with the physical condition of individuals. Nutrition has a large impact on physical condition, and thus, any seasonal changes in nutritional quality are predicted to alter the average size of sexually selected traits as well as the degree of sexual dimorphism in populations. However, although traits affected by mate choice are well studied, we have a surprising lack of knowledge of how natural variation in nutrition affects the expression of sexually selected weapons and sexual dimorphism. Further, few studies explicitly test for differences in the heritability and mean‐scaled evolvability of sexually selected traits across conditions. We studied Narnia femorata (Hemiptera: Coreidae), an insect where males use their hind legs as weapons and the femurs are enlarged, to understand the extent to which weapon expression, sexual dimorphism and evolvability change across the actual range of nutrition available in the wild. We found that insects raised on a poor diet (cactus without fruit) are nearly monomorphic, whereas those raised on a high‐quality diet (cactus with ripe fruit) are distinctly sexually dimorphic via the expression of large hind leg weapons in males. Contrary to our expectations, we found little evidence of a potential for evolutionary change for any trait measured. Thus, although we show weapons are highly condition dependent, and changes in weapon expression and dimorphism could alter evolutionary dynamics, our populations are unlikely to experience further evolutionary changes under current conditions.     

A negative association between horn length and survival in a weakly dimorphic ungulate

While all models of sexual selection assume that the development and expression of enlarged secondary sexual traits are costly, males with larger ornaments or weapons generally show greater survival or longevity. These studies have mostly been performed in species with high sexual size dimorphism, subject to intense sexual selection. Here, we examined the relationships between horn growth and several survival metrics in the weakly dimorphic Pyrenean chamois (Rupicapra pyrenaica). In this unhunted population living at high density, males and females were able to grow long horns without any apparent costs in terms of longevity. However, we found a negative relationship between horn growth and survival during prime age in males. This association reduces the potential evolutionary consequences of trophy hunting in male chamois. We also found that females with long horns tended to have lower survival at old ages. Our results illustrate the contrasting conclusions that may be drawn when different survival metrics are used in analyses. The ability to detect trade‐off between the expression of male secondary sexual traits and survival may depend more on environmental conditions experienced by the population than on the strength of sexual selection.     

The intensity of sexual selection predicts weapon size in male bovids

As a classical example of a sexually selected trait, the horns of male bovids offer a prime opportunity to identify predictors of the intensity of sexual selection. Here I use the comparative method to quantify sexual and natural selection pressures behind interspecific variation in horn length. I show that male horn length depends on factors proposed to affect the mean mate number per mating male, correlating positively with group size and negatively with male territoriality. This suggests that whereas group size increases the opportunity for sexual selection, territoriality reduces it because territorial males are unable to follow and monopolize female groups as effectively as males in nonterritorial species. Sexual body size dimorphism also correlates positively with group size and negatively with territoriality, corroborating these factors as predictors of the intensity of sexual selection on males. Female horn length was unaffected by the factors related to mating system, suggesting that this trait is mainly under natural selection. Using female horn length as a proxy for forces of natural selection revealed a negative effect on male horn length. Thus where natural selection favors female horns, possibly as effective weapons against predators, a similar selection pressure on males might prevent them from evolving too elaborate horns through sexual selection. There was no correlation found between horn length and latitude, thus providing no support for the hypothesis that horns have a thermoregulatory function.     

Sexual dimorphism in the tusked frog, Adelotus brevis (Anura:Myobatrachidae): the roles of natural and sexual selection

At least two adaptive processes can lead to the evolution of sexual dimorphism: sexual selection (e.g. male-male combat) or natural selection (e.g. dietary divergence). We investigated the adaptive significance of a distinctive pattern of sexual dimorphism in a south-eastern Australian frog, Adelotus brevis. Male Adelotus grow larger than female conspecifics, have larger heads relative to body size, and have large paired projections (‘tusks’) in the lower jaw. All of these traits are rare among anurans. We quantified the degree of dimorphism in Adelotus, and gathered data on diets and mating systems of this species to evaluate the possible roles of sexual selection and dietary divergence in favoring die evolution of these sexually dimorphic traits. Analysis of prey items in alimentary tracts revealed significant sex differences in prey types. For example, females ate proportionally more arthropods and fewer molluscs than did males. However, this difference is likely to be a secondary consequence of habitat differences between the sexes (due in turn to their different reproductive roles) rather than a selective force for the evolution of sexual dimorphism. Calling males spend their time in moist habitats where pondsnails are abundant, whereas females are more often encountered in the drier arthropod-rich woodlands. A three-year behavioural ecology study on a field population revealed that reproductive males engage in agonistic interactions, with the sexually dimorphic tusks used to attack rivals. Larger body size contributed to male reproductive success. Small males were excluded from calling sites and, among the calling males, larger animals had higher reproductive success (numbers of matings) than did smaller individuals. Hence, the atypical pattern of sexual dimorphism in Adelotus brevis seems to have resulted from sexual selection for larger body size and tusk size in males, in the context of male-male agonistic behaviour, rather than natural selection for ecological divergence between the sexes.     

Ontogenetic stage‐specific quantitative trait loci contribute to divergence in developmental trajectories of sexually dimorphic fins between medaka populations

Sexual dimorphism can evolve when males and females differ in phenotypic optima. Genetic constraints can, however, limit the evolution of sexual dimorphism. One possible constraint is derived from alleles expressed in both sexes. Because males and females share most of their genome, shared alleles with different fitness effects between sexes are faced with intralocus sexual conflict. Another potential constraint is derived from genetic correlations between developmental stages. Sexually dimorphic traits are often favoured at adult stages, but selected against as juvenile, so developmental decoupling of traits between ontogenetic stages may be necessary for the evolution of sexual dimorphism in adults. Resolving intralocus conflicts between sexes and ages is therefore a key to the evolution of age‐specific expression of sexual dimorphism. We investigated the genetic architecture of divergence in the ontogeny of sexual dimorphism between two populations of the Japanese medaka (Oryzias latipes) that differ in the magnitude of dimorphism in anal and dorsal fin length. Quantitative trait loci (QTL) mapping revealed that few QTL had consistent effects throughout ontogenetic stages and the majority of QTL change the sizes and directions of effects on fin growth rates during ontogeny. We also found that most QTL were sex‐specific, suggesting that intralocus sexual conflict is almost resolved. Our results indicate that sex‐ and age‐specific QTL enable the populations to achieve optimal developmental trajectories of sexually dimorphic traits in response to complex natural and sexual selection.     

Role of cell death in the formation of sexual dimorphism in the Drosophila central nervous system

Currently, sex differences in behavior are believed to result from sexually dimorphic neural circuits in the central nervous system (CNS). Drosophila melanogaster is a common model organism for studying the relationship between brain structure, behavior, and genes. Recent studies of sex‐specific reproductive behaviors in D. melanogaster have addressed the contribution of sexual differences in the CNS to the control of sex‐specific behaviors and the development of sexual dimorphism. For example, sexually dimorphic regions of the CNS are involved in the initiation of male courtship behavior, the generation of the courtship song, and the induction of male‐specific muscles in D. melanogaster. In this review, I discuss recent findings about the contribution of cell death to the formation of sexually dimorphic neural circuitry and the regulation of sex‐specific cell death by two sex determination factors, Fruitless and Doublesex, in Drosophila.     

Male and female secondary sexual traits show different patterns of quantitative genetic and environmental variation in the horned beetle Onthophagus sagittarius

The expression of secondary sexual traits in females has often been attributed to a correlated response to selection on male traits. In rare cases, females have secondary sexual traits that are not homologous structures to secondary sexual traits in males and are thus less likely to have evolved in females because of correlated selection. In this study, we used the dung beetle Onthophagus sagittarius, a species with sex‐specific horns, to examine the environmental and quantitative genetic control of horn expression in males and females. Offspring subjected to different brood mass manipulations (dung addition/removal) were found to differ significantly in body size. Brood mass manipulation also had a significant effect on the length of male horns; however, female horn length was found to be relatively impervious to the treatment, showing stronger patterns of additive genetic variance than males. We found no correlations between horn expression in males and females. We therefore conclude that the horns of O. sagittarius females are unlikely to result from genetic correlations between males and females. Rather, our data suggest that they may be under independent genetic control.     

Sex‐linked inheritance,genetic correlations and sexual dimorphism in three melanin‐based colour traits in the barn owl

Theory states that genes on the sex chromosomes have stronger effects on sexual dimorphism than genes on the autosomes. Although empirical data are not necessarily consistent with this theory, this situation may prevail because the relative role of sex‐linked and autosomally inherited genes on sexual dimorphism has rarely been evaluated. We estimated the quantitative genetics of three sexually dimorphic melanin‐based traits in the barn owl (Tyto alba), in which females are on average darker reddish pheomelanic and display more and larger black eumelanic feather spots than males. The plumage traits with higher sex‐linked inheritance showed lower heritability and genetic correlations, but contrary to prediction, these traits showed less pronounced sexual dimorphism. Strong offspring sexual dimorphism primarily resulted from daughters not expressing malelike melanin‐based traits and from sons expressing femalelike traits to similar degrees as their sisters. We conclude that in the barn owl, polymorphism at autosomal genes rather than at sex‐linked genes generate variation in sexual dimorphism in melanin‐based traits.     

Condition dependence of female choosiness in a field cricket

Females generally choose mates that produce the loudest, brightest or most elaborate sexual displays, and these costly male displays are predicted to be condition dependent. However, mate choice itself is a costly behaviour also expected to be condition dependent. Male fall field crickets, Gryllus pennsylvanicus, produce a conspicuous long‐distance calling song that attracts females and is condition dependent. In this study, we tested the condition dependence of female preferences (preference function and choosiness) for male calling effort in G. pennsylvanicus. We manipulated female condition by raising crickets from hatching on either a low‐ or high‐quality diet. In a series of two‐speaker phonotaxis trials, both low‐ and high‐condition females preferred playbacks reflecting greater calling effort. However, relative to low‐condition females, high‐condition females took significantly longer to make a choice, were more likely to fail to choose within the time allotted for a phonotaxis trial and significantly increased their latency to choose over the course of multiple trials. We discuss these results with respect to the possibility that female G. pennsylvanicus may be foraging for direct benefits when they choose their mates.     

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.     

Using photogrammetry and color scoring to assess sexual dimorphism in wild western gorillas (Gorilla gorilla)

Investigating sexual dimorphism is important for our understanding of its influence on reproductive strategies including male-male competition, mate choice, and sexual conflict. Measuring physical traits in wild animals can be logistically challenging and disruptive for the animals. Therefore body size and ornament variation in wild primates have rarely been quantified. Gorillas are amongst the most sexually dimorphic and dichromatic primates. Adult males (silverbacks) possess a prominent sagittal crest, a pad of fibrous and fatty tissue on top of the head, have red crest coloration, their saddle appears silver, and they possess a silverline along their stomach. Here we measure levels of sexual dimorphism and within-male variation of body length, head size, and sexual dichromatism in a population of wild western gorillas using photogrammetry. Digital photogrammetry is a useful and precise method to measure sexual dimorphism in physical traits yielding sexual dimorphism indices (ISD), similar to those derived from traditional measurements of skeletal remains. Silverbacks were on an average 1.23 times longer in body length than adult females. Sexual dimorphism of head size was highest in measures of crest size (max ISD: 60.4) compared with measures of facial height (max ISD: 24.7). The most sexually dimorphic head size measures also showed the highest within-sex variation. We found no clear sex differences in crest coloration but there was large sexual dichromatism with high within-male variation in saddle coloration and silverline size. Further studies should examine if these sexually dimorphic traits are honest signals of competitive ability and confer an advantage in reproductive success.