Unexplained sexual segregation in polygamous ungulates: a defense of an ontogenetic approach

Two processes are generally mixed when considering ungulate sexual segregation: the social segregation which appears to be a rule in polygamous ungulate species and the spatial segregation which is facultative. Early in life, there is evidence that males and females exhibit different levels of activity and patterns of interaction which could lead to behavioural incompatibility. It is proposed that juvenile females, differently motivated than males to interact socially, may soon avoid the pseudo-sexual and agonistic male behavioural components. as body and behavioural dimorphism increase with age, the behavioural incompatibility would be intensified. Since adults often interact agonistically with or avoid them, juveniles will attend to interact with same-sex peers of similar status. Because social experiences are occurring since early stages of development, i.e., during sensitive periods, a social auto-segregation by sex/age classes is expected. Then population would be structured in coherent socio-spatial sets of compatible individuals, sharing similar constraints and properties. Around the birth period, females become asocial promoting a temporal social segregation. Costs linked to gestation and lactation may make females more dependent on water and rich food resources. Female with offspring are also likely to select areas where the risks of predation are reduced, sometimes at detriment to energy intake, while other animals exhibit opportunistic feeding behaviour, resulting in seasonally ecological segregation. Body growth channels a lot of energy for males which are expected to use the best feeding areas. However, indirect competition for the same resources may result in a spatial segregation between the sexes. Sex- and age-specific activity budgets are suspected to favour social and spatial segregations. The social segregation among males and females use exclusive ranges. Whether spatial segregation results in ecological segregation would however depend on habitat heterogeneity, population density, local ecological conditions and the biological cycle periods. It is suggested that spatial segregation will occur not only between the sexes but also among males. If populations are structured in coherent socio-spatial units, spatial segregation among individuals of age and sex classes may result from indirect interactions between sets of individuals (of same age and sex class), that may be translated as an indirect competition between individuals. Studies on ethogenesis are considered as fundamental as they allow to know on what basis social and spatial segregations are set up. In particular, studies on behavioural development could highlight how social segregation between adults may result from a process that originates much earlier in the life of animals. By considering the role of behaviour, we defend the idea that social and spatial segregations may be logically distinguished.