Sexual segregation in ungulates: a comparative test of three hypotheses

Abstract

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.

Multiple causes of sexual segregation in European red deer: enlightenments from varying breeding phenology at high and low latitude

“…(d ) Lack of evidence for the 'forage-selection' hypothesis We found no support for the 'forage-selection' hypothesis in accordance with other recent results (Conradt 1999;Conradt et al 1999), although red deer are one of the most dimorphic ungulate species (more dimorphic than the 20% threshold needed for sexual segregation to occur; Ruckstuhl & Neuhaus 2002) and several empirical studies provide evidence for males having higher energy needs than females. However, the Jarman-Bell principle was coined at the interspecific level to explain variations in energy requirements and differences in diet selection between the African elephant (Loxodonta africana) and dik-dik (Madoqua kirki ) and more recently between the cow (Bos taurus) (750 kg) and Thomson's gazelle (Gazella thomsoni ) (20 kg) (Wilmshurst et al 2000).…”

“…Energetic needs are highest when the females lactate because the production of milk is extremely costly (Sadleir 1969;Loudon 1985;Robbins 1993). Lactating females can therefore have a different activity budget from other females (Ruckstuhl & Neuhaus 2002). Females with calves were more socially segregated from males than were females without calves.…”

Consensus decision making in human crowds

“…In many situations conflicts may exist between the preferences of different group members (Conradt 1998;Ruckstuhl 1998;Ruckstuhl & Neuhaus 2000;Ruckstuhl & Neuhaus 2002;Couzin et al 2005). If these conflicts cannot be resolved and no consensus can be reached then the group will split and individuals will forego the benefits associated with being part of a large group (Krause & Ruxton 2002).…”

Female mountain goats, Oreamnos americanus , associate according to kinship and reproductive status

“…It is possible that variation in mass was insufficient to affect associations. For example, Ruckstuhl and Neuhaus (2002) showed that there was a threshold that maleefemale body mass differences should exceed for sexes to segregate. Alternatively, female differences in body mass may be heritable as in many other ungulate species (Pelletier, Reale, Garant, Coltman, & Festa-Bianchet, 2007;Wilson & Reale, 2006), and associations between mothers and daughters may hide potential association patterns of females with different body masses.…”

“…Reproductive status could also influence female associations and group formation. In ungulates, lactating females forage longer than nonlactating females (Hamel & Côte, 2008;Ruckstuhl & Neuhaus, 2002). Therefore, segregation of females of different reproductive status may result from a lack of synchronization generated by different time budgets.…”