Ruckstuhl K E, Neuhaus P
University of Cambridge, Dept of Zoology, LARG, UK.
Biol Rev Camb Philos Soc. 2002 Feb;77(1):77-96. doi: 10.1017/s1464793101005814.
In most social ungulate species, males are larger than females and the sexes live in separate groups outside the breeding season. It is important for our understanding of the evolution of sociality to find out why sexual segregation is so widespread not only in ungulates but also in other mammals. Sexual body size dimorphism was proposed as a central factor in the evolution of sexual segregation in ungulates. We tested three hypotheses put forward to explain sexual segregation: the predation-risk, the forage-selection, and the activity budget hypothesis. We included in our analyses ungulate species ranging from non-dimorphic to extremely dimorphic in body size. We observed oryx, zebra, bighorn sheep and ibex in the field and relied on literature data for 31 additional species. The predation-risk hypothesis predicts that females will use relatively predator-safe habitats, while males are predicted to use habitats with higher predation risk but better food quality. Out of 24 studies on different species of ungulates, females and their offspring chose poorer quality but safer habitat in only eight cases. The forage-selection hypothesis predicts that females would select habitat based on food quality, while males should prefer high forage biomass. In fact, females selected higher quality food in only six out of 18 studies where males and females segregated, in eight studies there was no difference in forage quality and in four studies males were in better quality habitat. The activity budget hypothesis predicts that with increasing dimorphism in body size males and females will increasingly differ in the time spent in different activities. Differences in activity budgets would make it difficult for males and females to stay in mixed-sex groups due to increased costs of synchrony to maintain group cohesion. The predictions of the activity budget hypothesis were confirmed in most cases (22 out of 23 studies). The heavier males were compared to females, the more time females spent foraging compared to males. The bigger the dimorphism in body mass, the more males spent time walking compared to females. Lactating females spent more time foraging than did non-lactating females or males. Whether species were mainly bulk or intermediate feeders did not affect sexual differences in time spent foraging. We conclude that sexual differences in activity budgets are most likely driving sexual segregation and that sexual differences in predation risk or forage selection are additive factors.
在大多数群居有蹄类动物物种中，雄性比雌性体型更大，且在繁殖季节之外，两性分群生活。弄清楚为何性别隔离不仅在有蹄类动物中，而且在其他哺乳动物中如此普遍，对于我们理解群居行为的进化至关重要。性体型二态性被认为是有蹄类动物性别隔离进化的一个核心因素。我们检验了为解释性别隔离而提出的三个假说：捕食风险假说、觅食选择假说和活动预算假说。我们的分析纳入了体型从无二态性到极度二态性的有蹄类动物物种。我们在野外观察了大羚羊、斑马、大角羊和野山羊，并依靠文献数据研究另外31个物种。捕食风险假说预测，雌性会选择相对安全、不易被捕食的栖息地，而雄性则预计会选择捕食风险较高但食物质量较好的栖息地。在对不同有蹄类动物物种的24项研究中，只有8项研究表明雌性及其后代选择了质量较差但更安全的栖息地。觅食选择假说预测，雌性会根据食物质量选择栖息地，而雄性则应偏好高觅食生物量的栖息地。事实上，在18项雄性和雌性分群的研究中，只有6项研究表明雌性选择了质量更高的食物，8项研究中觅食质量没有差异，4项研究中雄性处于质量更好的栖息地。活动预算假说预测，随着体型二态性的增加，雄性和雌性在不同活动上花费的时间差异会越来越大。活动预算的差异会使雄性和雌性难以维持混合性别群体，因为维持群体凝聚力所需同步的成本增加。活动预算假说的预测在大多数情况下得到了证实（23项研究中的22项）。与雌性相比，雄性体重越重，雌性用于觅食的时间比雄性就越多。体重二态性越大，与雌性相比，雄性用于行走的时间就越多。哺乳期雌性比非哺乳期雌性或雄性花更多时间觅食。物种主要是大量进食者还是中等进食者并不影响觅食时间的性别差异。我们得出结论，活动预算的性别差异很可能是导致性别隔离的原因，而捕食风险或觅食选择的性别差异是附加因素。
