Mating System and Sexcul Dimorphism in Large Terrestrial, Mammalian Herbivores

Abstract

Summary Sexual dimorphism in mammals is not entirely satisfactorily explained by the models that are advanced to account for it among birds. This may be because species‐specific styles of being dimorphic, and of attaining mature dimorphic state, are not clearly recognized. Mature dimorphism is a syndrome involving body size, appearance and weaponry; each facet and the whole syndrome may have functions in both fighting and signalling. The mature dimorphic stage has to be reached by growth and change from juvenile and sub‐adult states. The occurrence of the separate facets of the dimorphic syndrome are reviewed in species of Bovidae, Cervidae and Macropodidae, large, diverse families of eutherian (the first two) and metatherian mammals, which have broadly similar ecological adaptations. In each family the smallest species tend to be homomorphic, with small, inconspicuous weapons. Greatest dimorphism in size is found in medium‐sized bovids and cervids, and the larger macropodids (in which no species exceeds 100 kg male weight); the range of species showing greatest dimorphism in size also shows the most exaggerated weapons. Mature dimorphism is reached by different patterns of growth, which may be determinate and similar in the sexes (leading to homomorphism), determinate but differing between the sexes, or indeterminate and differing, both of which lead to heteromorphism. The syndromes of dimorphism and patterns of growth are associated and a classification of styles of dimorphism is presented. The adaptiveness of the styles is suggested in terms of what is known of the socio‐ecology, in particular the male reproductive strategies, of the species. The various styles of heteromorphy appear to be associated with males' way of achieving polygyny: such as by non‐resource‐based territoriality, by dominance‐determined access to oestrous females, or by wandering and formation of a consortship with pro‐oestrous females. The relevance of the species' ecology of use of resources to these styles of dimorphism and mate‐acquisition is briefly discussed.

The Early Development of Gender Differences

“…The few exceptions are in species in which roles may be partially mixed and the sexes have less notable differences, and which more often have mating systems characterized as monogamous (Jarman 1983, Plavcan 2001. The primate order includes a relatively large number of monogamous species, often characterized by some level of male parental investment (Fuentes 1998).…”

The lemur syndrome unresolved: extreme male reproductive skew in sifakas (Propithecus verreauxi), a sexually monomorphic primate with female dominance

“…Moreover, as revealed by a comparative study in primates, male-male competition is also sensitive to the number of competitors, because male mating skew decreases as the number of males increases (Kutsukake and Nunn 2006). Importantly, as demonstrated for anthropoid primates, groups of up to five or six females can typically be monopolized by a single male (Mitani et al 1996;Nunn 1999), and these species tend to have the most pronounced sexual dimorphism (Alexander et al 1979;Jarman 1983;Plavcan 2001). Females may break this male monopolization by increasing their estrous synchrony (Nunn 1999), but a comparative study across primates found no relationship between the degree of male mating skew and the overlap in female receptivity (Kutsukake and Nunn 2006).…”

The development of sex differences in ring-tailed lemur feeding ecology

“…Sex differences in adult feeding are commonly linked to fluctuations in energetic and nutritional needs associated with sexual size dimorphism and costs of reproduction to females. Size dimorphism can have far reaching effects on growth, metabolism, and substrate or microhabitat use, as well as an individual's nutritional demands (Kleiber 1965;Fleagle and Mittermeier 1980;Jarman 1983;McGraw 1998;Beck et al 2005;O'Mara et al 2012). If size dimorphism is solely responsible for sex differences in feeding, then the divergence of female and male feeding ecology should parallel the developmental changes in size and strength (Jarman 1983;Watts 1988).…”

“…If size dimorphism is solely responsible for sex differences in feeding, then the divergence of female and male feeding ecology should parallel the developmental changes in size and strength (Jarman 1983;Watts 1988). However, sexual dimorphism can also determine the priority of access to foods, with the larger or dominant sex more likely to exclude others from desired feeding patches (Jarman 1983;Ménard and Vallet 1986;Wan et al 2013). Sexbased social dominance can determine priority of access and sex differences in feeding could be imposed through harassment or monopolization by the dominant individuals (Barton and Whiten 1993;Bonanni et al 2007), particularly in the absence of sexual dimorphism (Overdorff et al 2005;White and Wood 2007;Smith et al 2008).…”

“…Size dimorphism can have far reaching effects on growth, metabolism, and substrate or microhabitat use, as well as an individual's nutritional demands (Kleiber 1965;Fleagle and Mittermeier 1980;Jarman 1983;McGraw 1998;Beck et al 2005;O'Mara et al 2012). If size dimorphism is solely responsible for sex differences in feeding, then the divergence of female and male feeding ecology should parallel the developmental changes in size and strength (Jarman 1983;Watts 1988). However, sexual dimorphism can also determine the priority of access to foods, with the larger or dominant sex more likely to exclude others from desired feeding patches (Jarman 1983;Ménard and Vallet 1986;Wan et al 2013).…”