The Origins of Sexual Dimorphism in Body Size in Ungulates

Abstract

Abstract. Jarman (1974) proposed a series of relationships between habitat use, food dispersion, and social behavior and hypothesized a series of evolutionary steps leading to sexual dimorphism in body size through sexual selection in African antelope species. The hypothesis states that sexual size dimorphism evolved in a three-step process. Initially, ancestral monomorphic and monogamous ungulate species occupying closed habitats radiated into open grassland habitats. Polygynous mating systems then rapidly evolved in response to the aggregation of males and females, perhaps in relation to the clumped distribution of food resources in open habitats. Subsequently, size dimorphism evolved in those species occupying open habitats, but not in species that remained in closed habitats or retained monogamy. This hypothesis has played an important role in explaining the origins of sexual dimorphism in mammals. However, the temporal sequence of the events that Jarman proposed has never been demonstrated. Here we use a phylogeny of extant ungulate species, along with maximum-likelihood statistical techniques, to provide a test of Jarman's hypothesis. In most mammalian groups males are larger than females (Mace 1979;Reiss 1989;Abouheif and Fairbairn 1997;Weckerly 1998;Loison et al. 1999; Pérez-Barbería and Gordon 2000). Explanations for the degree of sexual dimorphism in body mass (SD) fall into three main groups (Fairbairn 1997). First, sexual selection hypotheses suggest that SD is a consequence of either intrasexual competition for mates, mainly males fighting amongst themselves for mating opportunities with females, or epigamic selection in which females choose among males to get mating partners (Trivers 1972). Second, large body mass may facilitate the evolution of SD by making polygyny more likely and thus increasing the intensity of sexual selection between the sexes (Lack 1968;Selander 1972;Clutton-Brock et al. 1977;Webster 1992;Abouheif and Fairbairn 1997;Loison et al. 1999). A third group of explanations suggests that SD arises from natural selection by favoring different optimal size for males and females because they occupy different ecological niches (Selander 1972;Shine 1989).Darwin (1871) was the first to link sexual dimorphism to polygyny. Sexual dimorphism among birds (Lack 1968;Wiley 1974), primates (Clutton-Brock et al. 1977;Ralls 1977;Clutton-Brock and Harvey 1978;Clutton-Brock 1985;Lindenfors andTullberg 1998) and ungulates (Geist 1974;Jarman 1974;Ralls 1977;Bubenik 1985;Loison et al. 1999) is associated with polygyny. However, less is known about the causes of the evolution of sexual size dimorphism within any given taxa (Jarman 1974;Clutton-Brock and Harvey 1977a;Clutton-Brock et al. 1977). Fairbairn (1997) pointed out that most attempts to explain the evolution of SD were based on correlations among variables, which make it difficult to separate cause from effect. Fairbairn went on to suggest that a more promising approach would be to use directional comparative analyses that estimate the order of a...

Patterns of sexual size dimorphism in cattle breeds support Rensch’s rule

“…Webb and Freckleton 2007;Stuart-Fox 2009). Although, the even-toed ungulates (Artiodactyla) are sexually dimorphic and mostly male-larger (Glucksmann 1974;Jarman 1983;Pérez-Barbería et al 2002), previous studies failed to confirm Rensch's rule in this mammalian group (Alexander et al 1979;Abouheif and Fairbairn 1997;Lindenfors et al 2007; for the only exception see Weckerly 1998; but the effect of phylogeny was not controlled in this analysis).…”

Does the Jarman–Bell principle at intra-specific level explain sexual segregation in polygynous ungulates? Sex differences in forage digestibility in Soay sheep

“…Sheep were selected to meet the criterion of 20% minimum sexual dimorphism in body mass that Ruckstuhl and Neuhaus (2002) claimed necessary to induce segregation in ruminant species. We defined dimorphism as the difference between average male and average female body masses, as a proportion of average male body mass (Pérez-Barbería et al 2002). In fact, sexual dimorphism in our sheep led to a sex body mass difference of 39% (females = 26.5 kg, SE = 0.67, n = 13; males = 43.8, SE = 0.70, n = 13).…”

Gregariousness increases brain size in ungulates

“…It is generally accepted that, for many vertebrates, gregariousness provides protection against predators (Pullian and Caraco 1984;Cowlishaw et al 2004), which could select for a reduction of body size in these species, especially in environments where large bodies are costly to maintain. However, there is some evidence that gregariousness selects for larger bodies in ungulates (Pe´rez-Barberı´a et al 2002) since it appears that increasing group size, as a consequence of habitat use, triggered the development of polygyny and via polygyny male body size increased. Due to the close relationship between gregariousness and polygyny we believe that it is more likely that group living selects for an increase in brain size rather than for a decrease in body size.…”

“…Developmental hypotheses (a) energy strategy, selection for larger brain size operates, primarily, through maternal metabolic turnover, as most of the brain's growth takes place in utero. Consequently, we expect a positive relationship between gestation length and brain size after accounting for the effect of body size; we consider that gestation length is an indicator of maternal investment and accordingly gestation length should be related to diet quality; (b) sex hypothesis, brain size relative to body size, should be larger in females than in males, since, as a general rule, males are larger than females in ungulates (Pe´rez- Barberı´a and Gordon 1998;Pe´rez-Barberı´a et al 2002), because, unlike females, males grow for most of their lifetime (Georgiadis 1985). In addition, the brain grows relatively little after birth in both sexes (Martin and Harvey 1985;Willner and Martin 1985;Wemmer and Wilson 1987).…”