Chapter

Sexual Dichromatism

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Abstract

Sexual dichromatism is a form of sexual dimorphism that refers to a difference in coloration between sexes within a species. While sexual dichromatism is common among birds, and to a lesser extent among lizards and fish, it is rare among mammals, with the exception of primates. Both male and female primates sometimes exhibit changes in skin color related to dominance and reproduction, but the term sexual dichromatism is typically reserved for differences in pelage, or fur, color. Among primates, the most striking examples of sexual dichromatism are found in black lemurs, black-and-gold howlers, saki monkeys, and several species of gibbon. In part because of the broad diversity of the species exhibiting sexual dichromatism, attempts to identify its functional significance have met with only limited success.

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... Further, sexual dichromatism of the overall coat (e.g. many gibbon species) has not been unequivocally linked to sexual selection [48,49] and, in fact, the often less conspicuous pelage in male compared with female primates would argue against sexual selection as its main driver [48,49]. For consistency with previous studies [35,43], we retained pelage dichromatism in our scores and confirmed in a comparison with analyses excluding this parameter (electronic supplementary material, tables S2 and S3) that our conclusions are largely independent of pelage dichromatism. ...
... Further, sexual dichromatism of the overall coat (e.g. many gibbon species) has not been unequivocally linked to sexual selection [48,49] and, in fact, the often less conspicuous pelage in male compared with female primates would argue against sexual selection as its main driver [48,49]. For consistency with previous studies [35,43], we retained pelage dichromatism in our scores and confirmed in a comparison with analyses excluding this parameter (electronic supplementary material, tables S2 and S3) that our conclusions are largely independent of pelage dichromatism. ...
Article
Males must partition their limited reproductive investments between traits that promote access to females (sexual ornaments and weapons) and traits that enhance fertilization success, such as testes and ejaculates. Recent studies show that if the most weaponized males can monopolize access to females through contest competition, thereby reducing the risk of sperm competition, they tend to invest less in sperm production. However, how males invest in sexual ornaments relative to sperm production remains less clear. If male ornaments serve as badges of status, with high-ranking males attaining near-exclusive access to females, similar to monopolizing females through combat, their expression should also covary negatively with investment in post-mating traits. In a comparative study across primates, which exhibit considerable diversification in sexual ornamentation, male weaponry and testes size, we found relative testes size to decrease with sexual ornaments but increase with canine size. These contrasting evolutionary trajectories might be driven by differential selection, functional constraints or temporal patterns of metabolic investment between the different types of sexual traits. Importantly, however, our results indicate that the theory of relative investments between weapons and testes in the context of monopolizing females can extend to male ornaments.
Article
Coloration is a diagnostic tool for identifying mammals, but inquiry into its function has lain dormant for almost a century. Recently, the topic has been revived and modern phylogenetic methods have been applied to large data sets, allowing researchers to assess, for the first time, the relative importance of three classic hypotheses for the function of coloration in mammals: concealment, communication, and regulation of physiological processes. Camouflage appears to be the single most important evolutionary force in explaining overall coloration in mammals, whereas patches of colored fur are used for intraspecific signaling. Sexual selection is associated with flamboyant ornamentation in a minority of primates and other restricted mammalian taxa, but to a far lesser extent than in birds. Interspecific signaling among mammals includes aposematic coloration, exaggeration of signals to deter pursuit, and lures for misdirecting predatory attack. Physiological causes of coloration, including melanism, are evident but poorly researched. The relative importance of evolutionary forces responsible for external coloration varies greatly between vertebrate taxa, but the reasons for this variation are not yet understood.
Article
Flip through The Pictorial Guide to the Living Primates1 and you will notice a striking yet generally underappreciated aspect of primate biology: primates are extremely colorful. Primate skin and pelage coloration were highlighted examples in Darwin's2 original discussions of sexual selection but, surprisingly, the topic has received little research attention since. Here we summarize the patterns of color variation observed across the primate order and examine the selective forces that might drive and maintain this aspect of primate phenotypic diversity. We discuss how primate color patterns might be adaptive for physiological function, crypsis, and communication. We also briefly summarize what is known about the genetic basis of primate pigmentation and argue that understanding the proximate mechanisms of primate coloration will be essential, not only for understanding the evolutionary forces shaping phenotypic variation, but also for clarifying primate taxonomies and conservation priorities.
Article
We experimentally tested the hypothesis that sexual dichromatism in the subspecies of Eulemur fulvus is the evolutionary result of female preference for brightly colored males. Ten female lemurs representing 6 different subspecies of Eulemur fulvus were subjects in the experiment; controls were 4 females of non-sexually dichromatic lemurid taxa. For each taxon we presented photographs of the face of a male of that taxon whose colors had been digitally altered to make him less and more colourful. Median viewing times of the pooled female Eulemur fulvus are significantly correlated with colorfulness. Viewing times in the control females are not correlated with color or brightness of the stimulus photographs. We concluded that the females of the Eulemur fulvus sspp. preferred to view photographs of more colorful males, which is consistent with the predictions of sexual selection theory.
Article
Recognized species of sakis, South American monkeys of genus Pithecia (Cebidae), are P. hirsuta Spix, P. monachus E. Geoffroy, P. albicans Gray, P. pithecia Linnaeus. Evolutionary stages in sexual dichromatism in sakis and other primates are noted.
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