Horn length is not correlated with calling efforts in the horn‐headed cricket Loxoblemmus doenitzi (Orthoptera: Gryllidae)

Field cricket species are ideal model organisms for the study of sexual selection because cricket calling songs, used to attract mating partners, are pronouncedly sexually dimorphic. However, few studies have focused on other sexually dimorphic traits of field crickets. The horn‐headed cricket, Loxoblemmus doenitzi, exhibits exaggerated sexual dimorphism in head shape: males have flat heads with triangular horns, while females lack horns. This study examines the relationship between horn length, male calling efforts and diet quality. Horn length was not found to be significantly correlated with calling efforts. When diet was manipulated for late‐stage nymphs, calling efforts in the group with poor‐quality diet treatment was significantly lower than that of crickets in the group with high‐quality diet treatment. However, horn length was not affected by diet quality. The implication of these results in the context of the evolution of multiple signals and sexual dimorphism is discussed.     

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 and male horn dimorphism in Copris ochus (Coleoptera: Scarabaeidae)

Copris ochus (Coleoptera: Scarabaeidae), an endangered species, is the largest dung beetle in Japan. In C. ochus, males have a long head horn, while females lack this long horn (sexual dimorphism). Very large males of C. ochus have disproportionately longer head horns than small males, suggesting male horn dimorphism, although the dimorphism has not been investigated quantitatively. To clarify sexual and male horn dimorphism in C. ochus quantitatively, we examined the scaling relationship between body size (prothorax width) and head horn length in 94 females and 76 males. These beetles were captured during July 1978 from a natural population on Mt. Aso in southwestern Japan using a light trap. Although the horn length of the females and males scaled with prothorax width, the scaling relationship differed between the sexes, i.e., the relationship was linear in females and nonlinear in males. Statistical tests for dimorphism in male horn length showed a significant discontinuous relationship, thus indicating distinct sexual and male dimorphism in head horns. Long- and short-horned C. ochus males may have different reproductive behaviors, as described in other horned dung beetles.     

Horn Length Is the Determining Factor in the Outcomes of Escalated Fights Among Male Japanese Horned Beetles, <Emphasis Type="Italic">Allomyrina dichotoma</Emphasis> L. (Coleoptera: Scarabaeidae)

Male horn length in some horned beetles shows a sigmoidal relationship with body size. This has often been considered as the reflection of alternative reproductive tactics of males based on body size. Large males should possess long horns to acquire females through fights with other males using their horns, whereas small males do not require long horns because they usually avoid intermale fights and adopt alternative tactics such as sneaking. This may lead to a prediction that horn length is a reliable indicator of the fighting ability of the male. We examined the effects of both male horn length and body size of Allomyrina dichotoma on the outcomes of escalated fights. Results indicate that male horn length was more important than body size in predicting the outcomes of fight, and this may support the hypothesis that the evolution of the horn dimorphism in male horned beetles is the result of different reproductive tactics.     

ARTIFICIAL SELECTION ON HORN LENGTH-BODY SIZE ALLOMETRY IN THE HORNED BEETLE ONTHOPHAGUS ACUMINATUS (COLEOPTERA: SCARABAEIDAE)

Males of the horned beetle Onthophagus acuminatus Har. (Coleoptera: Scarabaeidae) exhibit horn length dimorphism due to a sigmoidal allometric relationship between horn length and body size: the steep slope of the allometry around the inflection of the sigmoid curve separates males into two groups; those larger than this inflection possess long horns, and those smaller than this inflection have short horns or lack horns. I examined the genetic basis of the allometric relationship between horn length and body size by selecting males that produced unusually long horns, and males that produced unusually short horns, for their respective body sizes. After seven generations of selection, lines selected for relatively long horns had significantly longer horn lengths for a given body size than lines selected for relatively short horns, indicating a heritable component to variation in the allometry. The sigmoidal shape of the allometry was not affected by this selection regime. Rather, selected lines differed in the position of the allometry along the body size axis. One consequence of lateral shifts in this allometric relationship was that the body size separating horned from hornless males (the point of inflection of the sigmoid curve) differed between selection lines: lines in which males were selected for relatively long horns began horn production at smaller body sizes than lines selected for relatively short horns. These results suggest that populations can evolve in response to selection on male horn length through modification of the growth relationship between horn length and body size.     

Sexual Selection,Ontogenetic Acceleration,and Hypermorphosis Generates Male Trimorphism in Wellington Tree Weta

Strong sex-specific selection on traits common to both sexes typically results in sexual dimorphism. Here we find that Wellington tree weta (Hemideina crassidens) are sexually dimorphic in both head shape and size due to differential selection pressures on the sexes: males use their heads in male-male combat and feeding whereas females use theirs for feeding only. Remarkably, the sexes share a common ontogenetic trajectory with respect to head growth. Male head shape allometry is an extension of the female’s trajectory despite maturing two instars earlier, a feat achieved through ontogenetic acceleration and hypermorphosis. Strong sexual selection also favours the evolution of alternative reproductive strategies in which some males produce morphologically different weapons. Wild-caught male H. crassidens are trimorphic with regard to weapon size, a rare phenomenon in nature, and weapon shape is related to each morph’s putative mating strategy.     

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.     

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.     

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.     

Proximate determination of male horn dimorphism in the beetle Onthophagus taurus (Coleoptera: Scarabaeidae)

The existence of discrete phenotypic variation within one sex poses interesting questions regarding how such intrasexual polymorphisms are produced and modified during the course of evolution. Approaching these kinds of questions requires insights into the genetic architecture underlying a polymorphism and an understanding of the proximate mechanisms determining phenotype expression. Here we explore the genetic underpinnings and proximate factors influencing the expression of beetle horns – a dramatic sexually selected trait exhibiting intramale dimorphism in many species. Two relatively discrete male morphs are present in natural populations of the dung beetle Onthophagus taurus (Scarabaeidae, Onthophagini). Males exceeding a critical body size develop a pair of long, curved horns on their heads, while those smaller than this critical body size remain essentially hornless. We present results from laboratory breeding experiments designed to assess the relative importance of inherited and environmental factors as determinants of male morphology. Using father–son regressions, our findings demonstrate that horn length and body size of male progeny are not predicted from paternal morphology. Instead, natural variation in an environmental factor, the amount of food available to larvae, determined both the body sizes exhibited by males as adults and the presence or absence of horns. The nonlinear scaling relationship between the body size and horn length of males bred in the laboratory did not differ from the pattern of variation present in natural populations, suggesting that nutritional conditions account for variation in male morphology in natural populations as well. We discuss our results by extending ideas proposed to explain the evolution of conditional expression of alternative phenotypes in physically heterogeneous environments toward incorporating facultative expression of secondary sexual traits. We use this synthesis to begin characterizing the potential origin and subsequent evolution of facultative horn expression in onthophagine beetles.     

No fecundity cost of female secondary sexual trait expression in the horned beetle Onthophagus sagittarius

Typically males bear the products of sexual selection in the form of ornaments and/or weapons used to compete for and attract females. Secondary sexual traits in females have been thought of as the product of correlated responses to sexual selection on males. However, there is increasing phylogenetic evidence that female secondary sexual traits can arise independently of selection on males, and may be subject to sexual selection. Theoretical models of the evolution of female ornamentation via male mate choice have assumed that females suffer a cost of ornament expression via reduced fecundity, and hence female ornaments are less likely to evolve than male ornaments. In the dung beetle Onthophagus sagittarius, there has been an independent evolutionary origin of horns in females that are qualitatively different from the horns produced by males. We use this system as a model to examine the costs of horn expression for females within a life-history context. We identified a longevity cost of reproduction for females that was independent of horn expression. Large females lived longer, and after controlling for lifespan, had a higher lifetime fecundity, and invested more heavily in maternal provisioning than did small females. We found no evidence of a cost to females of investment in horns. Rather, the rate of increase in fecundity and horn expression with body size were equal, so that absolute horn size provides an accurate indicator of body size and maternal quality. The effects we observe were independent of female contest competition and/or male mate choice, which were excluded in our experimental protocol. However, we speculate on the potential functional contributions female horns might make to female fitness.     

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.     

Evolution of sexual dimorphism and male dimorphism in the expression of beetle horns: phylogenetic evidence for modularity, evolutionary lability, and constraint

Beetle horns are enlarged outgrowths of the head or thorax that are used as weapons in contests over access to mates. Horn development is typically confined to males (sexual dimorphism) and often only to the largest males (male dimorphism). Both types of dimorphism result from endocrine threshold mechanisms that coordinate cell proliferation near the end of the larval period. Here, we map the presence/absence of each type of dimorphism onto a recent phylogeny for the genus Onthophagus (Coleoptera: Scarabaeidae) to explore how horn development has changed over time. Our results provide empirical support for several recent predictions regarding the evolutionary lability of developmental thresholds, including uncoupled evolution of alternative phenotypes and repeated fixation of phenotypes. We also report striking evidence of a possible developmental constraint. We show that male dimorphism and sexual dimorphism map together on the phylogeny; whenever small males have horns, females also have horns (and vice versa). We raise the possibility that correlated evolution of these two phenomena results from a shared element in their endocrine regulatory mechanisms rather than a history of common selection pressures. These results illustrate the type of insight that can be gained only from the integration of developmental and evolutionary perspectives.     

DENSITY-DEPENDENT SEXUAL SELECTION IN THE FUNGUS BEETLE,BOLITOTHERUS CORNUTUS

The hypothesis that population density can affect sexual selection on male horn size was tested in a three-year study of a fungus beetle, Bolitotherus cornutus. Males of this species have horns that vary greatly in length. These horns are used in fights over females; longer-horned males win the majority of fights, regardless of population density. However, density does affect the relationship between horn length and access to females. In six populations of naturally and experimentally varying densities, longer-horned males gained a greater advantage in access to females in low-density populations than at high density. This increase in access to females causes an increase in the number of females inseminated by longer-horned males; thus, sexual selection for longer horns is stronger at lower densities.     

Sexual dimorphism in conspicuousness and ornamentation in the enigmatic leaf‐nosed lizard Ceratophora tennentii from Sri Lanka

Measures of physiological performance capacity, such as bite force, form the functional basis of sexual selection. Information about fighting ability may be conveyed through a structural feature such as a rostrum (i.e. horn) or a colour signal and thereby help reduce costly conflict. We quantified sexual dimorphism in key traits likely to be the targets of sexual selection in Tennent's leaf‐nosed lizard (Ceratophora tennentii) from Sri Lanka, and examined their relationship to bite force and body condition. We found body length and bite force to be similar for males and females. However, head length was significantly greater in males and they had significantly more conspicuous throats and labials (chromatic contrast and luminance) than females. Males also had a proportionally larger rostrum, which we predicted could be an important source of information about male quality for both sexes. Rostrum length was correlated with throat chromatic contrast in males but not females. Nonetheless, the rostrum and aspects of coloration did not correlate with bite force or body condition as we predicted. We have no information on contest escalation in this species but if they rarely bite, as suggested by a lack of difference in bite force between males and females, then bite force and any associated signals would not be a target of selection. Finally, males and females had similar spectral reflectance of the mouth and tongue and both had a peak in the ultra‐violet, and were conspicuous to birds. Lizards only gaped their mouths during capture and not when threatened by a potential predator (hand waving). We hypothesize that conspicuous mouth colour may act as a deimatic signal, startling a potential predator, although this will need careful experimental testing in the future.     

Males with short horns spent more time mating in the Japanese horned beetle Allomyrina dichotoma

We examined the relationships between male body and horn sizes and mating duration in the Japanese horned beetle, Allomyrina dichotoma. Smaller males possessing shorter horns spent more time for copulation with a female and mounting the female without copulation. The results of multiple regression analyses indicate that the horn length is a determining factor for the time spent by the males during mating. A previous study has documented that the horn length of male A. dichotoma primarily determined the outcomes of aggressive male–male interactions; hence, predicts access to females. Therefore, instead of fighting for females, males possessing short horns may maximize their fertilization success by mating longer with the few females they have access to.     

Male Choice and Male-male Competition in Idotea baitica (Crustacea,Isopoda)

Sexual selection in mate-guarding Crustacea may involve several processes: male choice, male-male competition, and female choice. To evaluate the relative importance of the different processes in mate choice of the aquatic isopod I. baitica we studied 1) the mate-choice criteria of males, 2) effects of sex ratio on the outcome of the mating contest, and 3) the role of size in male-male interactions. When given a choice between a small and a large female, males most often chose the one that matured earlier for parturial ecdysis. Maturity was a more important choice criterion than female size, but these also correlated positively. Large males had a mating advantage in both male- and female-biased sex ratios; pairing was size-assortative only in the male-biased ratio where guarding was also longer. If an extra male was placed with a precopulatory pair, 30 % take-overs occurred, large males surpassing. Present and earlier work suggests that male size is an asset in both intra- and intersexual interactions. There is little or no direct phenotypic sexual selection on female size: sexual selection for large males presumably contributes to the evolution of sexual size dimorphism in I. baitica.     

Sexual dimorphism of head morphology in three‐spined stickleback Gasterosteus aculeatus

This study examined sexual dimorphism of head morphology in the ecologically diverse three‐spined stickleback Gasterosteus aculeatus. Male G. aculeatus had longer heads than female G. aculeatus in all 10 anadromous, stream and lake populations examined, and head length growth rates were significantly higher in males in half of the populations sampled, indicating that differences in head size increased with body size in many populations. Despite consistently larger heads in males, there was significant variation in size‐adjusted head length among populations, suggesting that the relationship between head length and body length was flexible. Inter‐population differences in head length were correlated between sexes, thus population‐level factors influenced head length in both sexes despite the sexual dimorphism present. Head shape variation between lake and anadromous populations was greater than that between sexes. The common divergence in head shape between sexes across populations was about twice as important as the sexual dimorphism unique to each population. Finally, much of the sexual dimorphism in head length was due to divergence in the anterior region of the head, where the primary trophic structures were found. It is unclear whether the sexual dimorphism was due to natural selection for niche divergence between sexes or sexual selection. This study improves knowledge of the magnitude, growth rate divergence, inter‐population variation and location of sexual dimorphism in G. aculeatus head morphology.     

Male horn dimorphism in the scarab beetle, Onthophagus taurus: do alternative reproductive tactics favour alternative phenotypes?

In a variety of organisms morphological variation is discrete rather than continuous. Discrete variation within a sex has attracted particular interest as it is thought to reflect the existence of alternative adaptations to a heterogeneous selection environment. The beetle Onthophagus taurus shows a dimorphism for male horns: males that exceed a critical body size develop a pair of long, curved horns on their heads, while smaller males remain hornless. In this study we report on the alternative reproductive tactics used by males with these two morphologies, and present experimental and behavioural data suggesting that these alternative tactics selectively favour discretely different male phenotypes. Horned males aggressively defended tunnel entrances containing breeding females. Fights involved the use of horns, and males with longer horns were more likely to win fights. In contrast, hornless males employed nonaggressive sneaking behaviours when faced with competitively superior males. Sneaking behaviours appeared to require high degrees of manoeuvrability inside tunnels to access and mate with females despite the presence of a guarding male. Comparisons of running performances of males with identical body sizes but different horn lengths suggest that the possession of horns reduces male agility inside tunnels. Thus, horn possession confers a clear advantage to males using fighting behaviours to access females, whereas hornlessness may be favoured in males that rely primarily on sneaking behaviours. Combined, the two alternative reproductive tactics used by male O. taurus appear to favour opposite horn phenotypes, which may explain the paucity of intermediate morphologies in natural populations of O. taurus. Copyright 2000 The Association for the Study of Animal Behaviour.     

Sexual Dimorphism and Mortality Bias in a Small Miocene North American Rhino, <Emphasis Type="Italic">Menoceras arikarense</Emphasis>: Insights into the Coevolution of Sexual Dimorphism and Sociality in Rhinos

Rhinos are the only modern perissodactyls that possess cranial weapons similar to the horns, antlers and ossicones of modern ruminants. Yet, unlike ruminants, there is no clear relationship between sexual dimorphism and sociality. It is possible to extend the study of the coevolution of sociality and sexual dimorphism into extinct rhinos by examining the demographic patterns in large fossil assemblages. An assemblage of the North American early Miocene (∼22 million years ago) rhino, Menoceras arikarense, from Agate Springs National Monument, Nebraska, exhibits dimorphism in incisor size and nasal bone size, but there is no detectible dimorphism in body size. The degree of dimorphism of the nasal horn is greater than the degree of sexual dimorphism of any living rhino and more like that of modern horned ruminants. The greater degree of sexual dimorphism in Menoceras horns may relate to its relatively small body size and suggests that the horn had a more sex-specific function. It could be hypothesized that Menoceras evolved a more gregarious type of sociality in which a fewer number of males were capable of monopolizing a larger number of females. Demographic patterns in the Menoceras assemblage indicate that males suffered from a localized risk of elevated mortality at an age equivalent to the years of early adulthood. This mortality pattern is typical of living rhinos and indicates that young males were susceptible to the aggressive behaviors of dominant individuals in areas conducive to fossilization (e.g., ponds, lakes, rivers). Menoceras mortality patterns do not suggest a type of sociality different from modern rhinos although a group forming type of sociality remains possible. Among both living and extinct rhinos, the severity of socially mediated mortality seems unrelated to the degree of sexual dimorphism. Thus, sexual dimorphism in rhinos is not consistent with traditional theories about the co-evolution of sexual dimorphism and sociality.