SPECIES ISOLATION,GENITAL MECHANICS,AND THE EVOLUTION OF SPECIES-SPECIFIC GENITALIA IN THREE SPECIES OF MACRODACTYLUS BEETLES (COLEOPTERA,SCARABEIDAE, MELOLONTHINAE)

The question asked was why male genitalic structures have diverged in three syntopic species of Macrodactylus beetles. Four hypotheses were evaluated: 1. The ways in which male genitalia mesh with internal female structures indicate that selection for species isolation via mechanical exclusion (“lock and key”) is unlikely to explain the genitalic differences. 2. The specific mate recognition hypothesis also clearly fails to explain genitalic differences due to the implausibility of postulated environmental effects on genitalia, and lack of postulated coevolution of male and female morphologies. 3. Selection for species isolation via differences in genitalic stimulation (sensory lock and key) is unlikely due to relatively infrequent cross-specific pair formation and intromission in the field, and “excessive” numbers of species-specific genitalic structures and male courtship behavior patterns which nevertheless occasionally fail. It also fails to explain the frequent failure of intraspecific copulations to result in sperm transfer. This hypothesis cannot, however, be rejected as confidently as the previous hypotheses. 4. Conditions under which sexual selection by cryptic female choice could take place are common. Females frequently exercise their ability to prevent sperm transfer by conspecific males even after intromission has occurred, and females generally mate repeatedly, probably with different males. Males behave as if cryptic female choice is occurring, courting assiduously while their genitalia are within the female. Sexual selection by female choice could thus contribute to the divergence in genitalic structures.     

Hairstreak butterflies (Lepidoptera,Lycaenidae) and evolution of their male secondary sexual organs

Hairstreak butterflies in the Atlides Section of the Eumaeini are biologically notable for a diverse array of male secondary sexual organs. A “species recognition” hypothesis postulates that females use these organs to choose between conspecific and non-conspecific males, thereby promoting reproductive isolation. Alternately, a “sexual selection” hypothesis posits that females use these organs to choose among conspecific males. These hypotheses need not be mutually exclusive but make different predictions about the evolutionary gain and loss of male secondary sexual organs. We analysed most of the Atlides Section (Theclinae, Eumaeini) phylogenetically. Sister lineages were sympatric at 22 of 37 nodes. Nine evolutionary gains occurred in lineages that were sympatric with their phylogenetic sister, and one occurred in a lineage that was allopatric/parapatric with its sister. By contrast, seven of ten evolutionary losses occurred in lineages that were allopatric/parapatric with their sisters. These results are significantly different from those predicted by a sexual selection hypothesis. We conclude that male secondary sexual organs in the Atlides Section function primarily for species recognition and thereby promote sympatric diversification.     

The evolution of tail-length in widowbirds (Ploceidae): tests of alternatives to sexual selection

The long tails of male widowbirds (Euplectes , Ploceidae) have been used to test sexual selection theory, but alternatives to sexual selection have not been investigated. This study tests three alternative hypotheses for the evolution of tail-length in widowbirds: apose-matism, the unprofitable prey hypothesis and species recognition. Using museum specimens, geographic patterns of tail-length were examined for evidence of character convergence (as predicted by the aposematism and unprofitable prey hypotheses) or character divergence (as predicted by the species recognition hypothesis) in areas where two pairs of species were sympatric. There was no consistent trend: one species showed evidence of character convergence and the other some evidence of divergence. Experimental manipulation of tail-length in the Yellow-shouldered Widowbird Euplectes macrourus also failed to support the species recognition hypothesis: there was no preference for males with species-typical tail-lengths but only a slight and non-significant trend to favour short-tailed males. There was also no evidence that mistaken identity led to territory loss following these experimental manipulations. Tail-length of six Euplectes species did not correlate with unpalatability scores, as predicted by the aposematism hypothesis. These hypotheses do not appear to explain the evolution of long tails in widowbirds, suggesting that sexual selection is the sole factor favouring long tails. Interspecific variation in tail-length remains unexplained.     

Intrasexual competition and body weight dimorphism in anthropoid primates

Body weight dimorphism in anthropoid primates has been thought to be a consequence of sexual selection resulting from male-male competition for access to mates. However, while monogamous anthropoids show low degrees of weight dimorphism, as predicted by the sexual selection hypothesis, polygynous anthropoids show high variation in weight dimorphism that is not associated with measures of mating system or sex ratio. This observation has led many to debate the role of other factors such as dietary constraints, predation pressure, substrate constraints, allometric effects, and phylogeny in the evolution of anthropoid weight dimorphism. Here, we re-evaluate variation in adult body weight dimorphism in anthropoids, testing the sexual selection hypothesis using categorical estimates of the degree of male-male intrasexual competition (“competition levels”). We also test the hypotheses that interspecific variation in body weight dimorphism is associated with female body weight and categorical estimates of diet, substrate use, and phylogeny. Weight dimorphism is strongly associated with competition levels, corroborating the sexual selection hypothesis. Weight dimorphism is positively correlated with increasing female body weight, but evidence suggests that the correlation reflects an interaction between overall size and behavior. Arboreal species are, on average, less dimorphic than terrestrial species, while more frugivorous species tend to be more dimorphic than folivorous or insectivorous species. Several alternative hypotheses can explain these latter results. Weight dimorphism is correlated with taxonomy, but so too are competition levels. We suggest that most taxonomic correlations of weight dimorphism represent “phylogenetic niche conservatism”; however, colobines show consistently low degrees of weight dimorphism for reasons that are not clear. Am J Phys Anthropol 103:37–68, 1997. © 1997 Wiley-Liss, Inc.     

SEXUAL DIMORPHISM,MATING SYSTEM AND BODY SIZE IN NEW WORLD BLACKBIRDS (ICTERINAE)

Although sexual selection is widely accepted as a primary functional cause of sexual size dimorphism in birds and mammals, results from some comparative studies have cast doubt on this conclusion. Chief among these contradictory results is the widespread association between body size and size dimorphism—large species tend to be more dimorphic than small species. This correlation is not directly predicted by the normal sexual selection scenario, and many hypotheses have been advanced to explain it. This paper reviews these hypotheses and evaluates them using data for the New World blackbirds (Icterinae). In this avian subfamily, (1) body size correlates with the intensity of sexual selection (as measured by mean harem size), and (2) size does not correlate with dimorphism if the effects of mating system are removed. Similar results are obtained when controlling for the confounding influence of phylogeny. Further, body size and mating system are associated with nesting dispersion. These results strongly argue that sexual dimorphism is a product of sexual selection in this subfamily, and suggest that either: (1) large body size itself, or the ecology of large species, promotes the development of coloniality and a polygynous mating system; or (2) polygyny and/or coloniality lead to the evolution of large size in both males and females. None of the other hypotheses examined predict an association between size and mating system, and all predict that size will correlate with dimorphism after the effects of mating system are removed. Thus, none of the other hypotheses seem applicable in this case. These results are compared to those obtained for other avian and mammalian taxa. Difficulties of analysis present in previous studies are discussed. I argue that it is inappropriate to assume that associations between a trait and body size or phylogeny are evidence of nonadaptive evolutionary “constraints.”     

Sexual Selection in the Loud Calls of Male Primates: Signal Content and Function

Researchers have used sexual selection theory and hypotheses based on intersexual mate choice and intrasexual mate competition to explain the role of spontaneous long-distance vocalizations emitted by adult male primates, relying on the tacit assumption that assessment or identity cues are encoded in the vocalizations. I review the published literature and aim to substantiate a relationship between sexual selection and long-distance vocal communication in primates. First, I review findings from nonprimate taxa to determine the relative importance of inter- and intrasexual selection and to provide a background for examining primates. Next, I describe several hypotheses for signal content and function in adult male loud calls. Then, I examine the available data across Primates for evidence to support or to refute these hypotheses and to determine if they meet proposed criteria for demonstrating sexual selection [Snowdon, C. T. (2004). Sexual Selection in Primates: New and Comparative Perspectives]. Signal content refers to patterns of acoustic features within vocalizations from which listeners might extract cues or information about the signaler. I interpret signal function, in turn, from behavioral responses of receivers and assume it has ultimate effects on the evolution and design of acoustic signals if direct fitness consequences exist. After the general review across primates, I propose orangutans as a candidate species for further evaluation of sexual selection in vocal communication. The available evidence corroborates a demonstrable relationship between sexual selection and adult male loud calls based on individual recognition, but it is necessary to obtain additional data to affirm a direct benefit to reproductive success.     

Sex and destiny: The role of mating signals in speciation and macroevolution

A growing body of research posits a central role for mating signals in speciation and the reproductive isolation of species, yet there has been relatively little consideration of mating signal evolution within macroevolutionary theory. Factors that influence the divergence of fertilization systems generally, and mating signals specifically, may incidentally influence rates of speciation and patterns of species sorting. Potential key processes include: genetic drift, natural selection (differential survival), selection for mate recognition, and sexual selection. This paper explores the integration of mating signal evolution into macroevolution and hierarchy theory, arguing that speciational patterns may frequently result from “effect sorting”; in which microevolutionary processes operating at the organismal level have macroevolutionary effects at the clade level. Preliminary evidence indicates that sexual selection is a widespread and potent evolutionary force that, together with other mechanisms, may have a large, though incidental impact on species sorting. The Mate Competition Hypothesis is here proposed to account for this possibility, postulating that heritable, clade‐specific variations in the intensity of sexual selection and the potential breadth of signal‐receiver systems contribute to divergent patterns of species‐richness. Several examples from the vertebrate fossil record are consistent with this hypothesis.     

Ancestrality and evolution of trait syndromes in finches (Fringillidae)

Species traits have been hypothesized by one of us (Ponge, 2013) to evolve in a correlated manner as species colonize stable, undisturbed habitats, shifting from “ancestral” to “derived” strategies. We predicted that generalism, r‐selection, sexual monomorphism, and migration/gregariousness are the ancestral states (collectively called strategy A) and evolved correlatively toward specialism, K‐selection, sexual dimorphism, and residence/territoriality as habitat stabilized (collectively called B strategy). We analyzed the correlated evolution of four syndromes, summarizing the covariation between 53 traits, respectively, involved in ecological specialization, r‐K gradient, sexual selection, and dispersal/social behaviors in 81 species representative of Fringillidae, a bird family with available natural history information and that shows variability for all these traits. The ancestrality of strategy A was supported for three of the four syndromes, the ancestrality of generalism having a weaker support, except for the core group Carduelinae (69 species). It appeared that two different B‐strategies evolved from the ancestral state A, both associated with highly predictable environments: one in poorly seasonal environments, called B1, with species living permanently in lowland tropics, with “slow pace of life” and weak sexual dimorphism, and one in highly seasonal environments, called B2, with species breeding out‐of‐the‐tropics, migratory, with a “fast pace of life” and high sexual dimorphism.     

Neuropsychological implications for preadaptation and language evolution

Findings from neuropsychology are used to support the hypotheses that selection for increased “sound recognition” abilities preadapted certain neural structures for aural language comprehension and that selection for gestural communication preadapted other neural structures for reading comprehension.     

The evolution of courtship display repertoire size in the dabbling ducks (Anatini)

Many animals possess multiple ornaments or behaviours that seem to have evolved via sexual selection. A complete understanding of sexual selection requires an explanation for such multiple traits. The dabbling ducks (Tribe: Anatini) exhibit considerable variation among species in the number of displays in the male courtship repertoire. I tested five hypotheses concerning the evolution of the variation in display repertoire size of dabbling ducks: (1) species recognition, (2) courtship habitat, (3) sexual selection intensity, (4) display media tradeoff and (5) time constraints on pair formation. I tested these hypotheses, using an explicit phylogenetic hypothesis developed from DNA sequences for the dabbling ducks, with two types of statistical comparative methods (discrete and continuous character). The variation observed in male courtship display repertoire size in dabbling ducks was consistent with the courtship habitat and sexual selection intensity hypotheses. Specifically, the size of the display repertoire was larger in species that exhibit courtship exclusively on water and larger in species with dimorphic plumage. These results suggest that ecological (habitat) as well as social (sexual selection) factors may be important in driving the evolution of displays in the dabbling ducks.     

Sexual selection accelerates signal evolution during speciation in birds

Sexual selection is proposed to be an important driver of diversification in animal systems, yet previous tests of this hypothesis have produced mixed results and the mechanisms involved remain unclear. Here, we use a novel phylogenetic approach to assess the influence of sexual selection on patterns of evolutionary change during 84 recent speciation events across 23 passerine bird families. We show that elevated levels of sexual selection are associated with more rapid phenotypic divergence between related lineages, and that this effect is restricted to male plumage traits proposed to function in mate choice and species recognition. Conversely, we found no evidence that sexual selection promoted divergence in female plumage traits, or in male traits related to foraging and locomotion. These results provide strong evidence that female choice and male–male competition are dominant mechanisms driving divergence during speciation in birds, potentially linking sexual selection to the accelerated evolution of pre-mating reproductive isolation.     

The evolution of immune defense and song complexity in birds

Abstract There are three main hypotheses that explain how the evolution of parasite virulence could be linked to the evolution of secondary sexual traits, such as bird song. First, as Hamilton and Zuk proposed a role for parasites in sexual selection, female preference for healthy males in heavily parasitized species may result in extravagant trait expression. Second, a reverse causal mechanism may act, if sexual selection affects the coevolutionary dynamics of host-parasite interactions per se by selecting for increased virulence. Third, the immuno-suppressive effects of ornamentation by testosterone or limited resources may lead to increased susceptibility to parasites in species with elaborate songs. Assuming a coevolutionary relationship between parasite virulence and host investment in immune defense we used measures of immune function and song complexity to test these hypotheses in a comparative study of passerine birds. Under the first two hypotheses we predicted avian song complexity to be positively related to immune defense among species, whereas this relationship was expected to be negative if immuno-suppression was at work. We found that adult T-cell mediated immune response and the relative size of the bursa of Fabricius were independently positively correlated with a measure of song complexity, even when potentially confounding variables were held constant. Nestling T-cell response was not related to song complexity, probably reflecting age-dependent selective pressures on host immune defense. Our results are consistent with the hypotheses that predict a positive relationship between song complexity and immune function, thus indicating a role for parasites in sexual selection. Different components of the immune system may have been independently involved in this process.     

Variations in sexual behavior among capuchin monkeys function for conspecific mate recognition: a phylogenetic analysis and a new hypothesis for female proceptivity in tufted capuchins

Researchers of capuchin monkeys have noted stereotyped body postures, facial expressions, and vocalizations that accompany copulations in this genus. Notable variations in these sexual behaviors are observed across capuchin species. Although several hypotheses exist to explain variation in the duration and vigor of sexual behaviors across species, there is no proposed explanation for variation in the forms of these behaviors. I hypothesized that the forms of sexual behaviors function as recognition signals of conspecific mates. Such signals are adaptive when F1 hybrids exhibit reduced fitness compared with nonhybrid offspring. Recent evidence from nonprimate taxa supports the existence of species recognition signals during mating. Using newly observed sexual behaviors for Cebus albifrons and a recent phylogeny of capuchins, I found significant support for a key prediction of the conspecific mate recognition hypothesis: evolutionary changes in sexual behaviors were associated with speciation. Given the resultant best model for evolution of sexual behaviors, I reconstructed the ancestral pattern of sexual behaviors for extant capuchin species (genera Cebus and Sapajus). This reconstruction suggests that the extreme female proceptivity of tufted capuchin monkeys may function to increase female reproductive choices in the context of sperm-limited males (genus Sapajus).     

The evolution of sexual dimorphism in animals: Hypotheses and tests

Three major hypotheses, based upon mechanisms of sexual selection, intersexual food competition and reproductive role division, have been advanced to explain the evolution of sexual dimorphism in body size and morphology of animals. Genetic models suggest that all of the hypotheses are plausible, and empirical studies demonstrate that each of the three mechanisms operates in natural populations. However, problems arise in testing hypotheses for the evolution of sexual dimorphism: more than one mechanism may be operating simultaneously, and the demonstrated occurrence of a mechanism does not indicate that it actually results in selection for dimorphism. A recent statistical technique offers a solution to these problems and provides a promising new approach to the study of sexual dimorphism, in which researchers can assess the relative importance of each mechanism in present-day selection for sexual dimorphism within a species.     

Sexual selection and animal personality

Consistent individual behavioural tendencies, termed “personalities”, have been identified in a wide range of animals. Functional explanations for personality have been proposed, but as yet, very little consideration has been given to a possible role for sexual selection in maintaining differences in personality and its stability within individuals. We provide an overview of the available literature on the role of personality traits in intrasexual competition and mate choice in both human and non‐human animals and integrate this into a framework for considering how sexual selection can generate and maintain personality. For this, we consider the evolution and maintenance of both main aspects of animal personality: inter‐individual variation and intra‐individual consistency.     

'Concealed ovulation' and sexual signals in primates.

The absence of conspicuous sexual signals in some primates, particularly humans and vervets, has been interpreted as evidence that females of these species are 'concealing' ovulation from males. This conclusion is unjustified: the null hypothesis of no adaptation, that the absence of conspicuous sexual signals has resulted from the absence of selective pressures maintaining such adaptations, is both more parsimonious and better fits the facts. The related suggestion that there has been adaptation among females to conceal ovulation from their own consciousness is also unjustified. What, then, maintains sexual signals in those species that do have them? Many proposed hypotheses for the function of sexual signals do not account for their most puzzling feature: their conspicuousness. According to current theory on the evolution of communication, two explanations seem most plausible: conspicuous sexual signals function to communicate to distant receivers and/or to convince reluctant receivers. There is some empirical support for both hypotheses, but not overwhelming support for either.     

Sexual selection and maintenance of sex: evidence from comparisons of rates of genomic accumulation of mutations and divergence of sex-related genes in sexual and hermaphroditic species of Caenorhabditis

Several hypotheses have been proposed to explain the persistenceof dioecy despite the reproductive advantages conferred to hermaphrodites,including greater efficiency at purging deleterious mutationsin the former. Dioecy can benefit from both mutation purgingand accelerated evolution by bringing together beneficial mutationsin the same individual via recombination and shuffling of genotypes.In addition, mathematical treatment has shown that sexual selectionis also capable of mitigating the cost of maintaining separatesexes by increasing the overall fitness of sexual populations,and genomic comparisons have shown that sexual selection canlead to accelerated evolution. Here, we examine the advantagesof dioecy versus hermaphroditism by comparing the rate of evolutionin sex-related genes and the rate of accumulation of deleteriousmutations using a large number of orthologs (11,493) in thedioecious Caenorhabditis remanei and the hermaphroditic Caenorhabditisbriggsae. We have used this data set to estimate the deleteriousmutation rate per generation, U, in both species and find thatalthough it is significantly higher in hermaphrodites, bothspecies are at least 2 orders of magnitude lower than the valuerequired to explain the persistence of sex by efficiency atpurging deleterious mutations alone. We also find that genesexpressed in sperm are evolving rapidly in both species; however,they show a greater increase in their rate of evolution relativeto genes expressed in other tissues in C. remanei, suggestingstronger sexual selection pressure acting on these genes indioecious species. Interestingly, the persistence of a signalof rapid evolution of sperm genes in C. briggsae suggests arecent evolutionary origin of hermaphrodism in this lineage.Our results provide empirical evidence of increased sexual selectionpressure in dioecious animals, supporting the possibility thatsexual selection may play an important role in the maintenanceof sexual reproduction.     

Demonstrating sexual selection by cryptic female choice on male genitalia: What is enough?

Rapid divergence in external genital structures occurs in nearly all animal groups that practice internal insemination; explaining this pattern is a major challenge in evolutionary biology. The hypothesis that species‐specific differences in male genitalia evolved under sexual selection as courtship devices to influence cryptic female choice (CFC) has been slow to be accepted. Doubts may stem from its radical departure from previous ideas, observational difficulties because crucial events occur hidden within the female's body, and alternative hypotheses involving biologically important phenomena such as speciation, sperm competition, and male‐female conflicts of interest. We assess the current status of the CFC hypothesis by reviewing data from two groups in which crucial predictions have been especially well‐tested, Glossina tsetse flies and Roeseliana (formerly Metrioptera) roeselii bushcrickets. Eighteen CFC predictions have been confirmed in Glossina and 19 in Roeseliana. We found data justifying rejection of alternative hypotheses, but none that contradicted CFC predictions. The number and extent of tests confirming predictions of the CFC hypothesis in these species is greater than that for other generally accepted hypotheses regarding the functions of nongenital structures. By this criterion, it is reasonable to conclude that some genital structures in both groups likely involved sexual selection by CFC.     

Sexual dimorphism in chin shape: implications for adaptive hypotheses

The chin, or mentum osseum, is one of the most distinctive anatomical traits of modern humans. A variety of hypotheses for the adaptive value of the chin have been proposed, ranging from mechanical stress resistance to sexual selection via mate choice. While the sexual selection hypothesis predicts dimorphism in chin shape, most biomechanical hypotheses preclude it. Therefore determining the presence or absence of significant sexual dimorphism in chin shape provides a useful method for differentiating between various adaptive hypotheses; however, this has yet to be done due to a lack of quantitative data on chin shape. The goals of this study are therefore: (1) to introduce a new method for quantifying chin shape and (2) to determine the presence or absence of sexual dimorphism in chin shape in a diverse sample of modern humans. Samples were drawn from recent human skeletal collections representing nine geographic regions. Outlines of mentum osseum contours were quantified using elliptical Fourier function analysis (EFFA). Fourier coefficients were analyzed using principal components analysis (PCA). Sexual dimorphism in chin shape was assessed using PC loadings in the pooled geographic sample, and statistically significant differences were found. These findings provide the first quantitative, morphologically based evidence in support of adaptive hypotheses that predict dimorphism in chin shape, including the sexual selection hypothesis.