Activity budgets and sociality in a monomorphic ungulate: the African oryx (Oryx gazella).(Report)

Introduction

In most ruminant species, males are larger than females and the sexes tend to live in unisex groups or as solitary individuals (Mysterud 2000). Mixed-sex groups exist but are mostly restricted to the breeding season (but see CluttonBrock et al. (1982) or Kie and Bowyer (1999) for some deer species in which mixed-sex groups are more common, but where the proportion of males in female groups is small). It has been argued that sexual dimorphism in body size is a key factor in the evolution of sexual segregation (Weckerly 1998; Mysterud 2000; Perez-Barberia and Gordon 2000). The type of mating system and the social environment of the species therefore likely has an important role to play in shaping body sizes in males.

Not many species of ruminants exist in which males and females are monomorphic in body size, with some of the exceptions being the alcelaphine antelopes, tribe Hippotragini (roan (Hippotragus equinus (Desmarest, 1804)), sable (Hippotragus niger (Harris, 1838)), and oryx), Kirk's dik-dik (Madoqua kirkii (Gunther, 1880)), the oribi (Ourebia ourebi (Zimmermann, 1783)) (Estes 1991), or western roe deer (Capreolus capreolus (L., 1758)). In these species, sexual differences in body mass are below 15% and often less than 9%; in the dik-dik, the female is even 2% bigger than the male (Owen-Smith 1988). While dik-diks and oribis usually live in monogamous pairs within a territory (Brotherton and Manser 1997; Brotherton et al. 1997; Jongejan et al. 1991; Kranz 1991), male roe deer defend mating territories (Bresinski 1982; Brashares and Arcese 2002). Oryx are special because there seem to be two types of males: some males establish territories, which they rigorously defend against intruders, and some reproductively active males freely mix with females and form mixed-sex groups (Estes 1991). The fact that males frequently from mixed-sex groups with females sets them apart from most other ruminants in general, and is likely due to their adaptation to a semidesert environment with low population densities and sparse, patchily distributed forage and female groups (Estes 1991). African buffalo (Syncerus caffer (Sparrman, 1779)) are often cited as an exception, being a dimorphic species (body mass dimorphism >20%), in which adult males can often be found in mixed-sex groups. However, such males form a tending bond with receptive females, mate with them, and then leave the groups to join bachelors. In bachelor groups, they often regain body condition lost during their time in mixed-sex groups. Once recovered they rejoin females (Prins 1989; Turner et al. 2005). A similar pattern of switching between mixed-sex groups (and being a large territorial male) and bachelor groups is also found in impalas (Aepyceros melampus (Lichtenstein, 1812)) and attributed to the fact that a single male cannot hold a territory for an extended period of time. In our study we were particularly interested in those oryx males that readily associate with females. The study of activity patterns in a monomorphic species could shed light on the debate on which factors cause sexual segregation in dimorphic ruminants.

There has been a lot of research on the factors that could govern the association patterns of males and females, and particularly why in most sexually dimorphic (>20%, for a definition see Ruckstuhl and Neuhaus 2002) social ruminants and other ungulates the sexes segregate into unisex groups outside the breeding season. Sexual segregation in ungulates can occur on the habitat or social scale (Conradt 1999; Ruckstuhl and Neuhaus 2002; Bowyer 2004). Four main hypotheses to explain sexual segregation have emerged and been most supported: predation risk (PR), forage selection (FS), social factors (SF), and activity budget (AB) hypotheses (Demment and Van Soest 1985; Bon 1991; Main et al. 1996; Barboza and Bowyer 2000, 2001; Conradt and Roper 2000; Jiang et al. 2000; Mysterud 2000; Perez-Barberia and Gordon 2000; Weckerly et al. 2001; Bowyer et al. 2002; Ruckstuhl and Kokko 2002; Ruckstuhl and Neuhaus 2002; Michelena et al. 2005; Shi et al. 2005; Turner et al. 2005). According to the PR and FS hypotheses, males and females should segregate into different habitats because they have different reproductive strategies and vulnerability to predators (PR hypothesis) or because males and females have different energy requirements and digestive abilities (FS hypothesis). Males and females are hypothesized to segregate socially within the same habitats owing to same-sex attraction (SF hypothesis) or incompatibilities of activity budgets (AB hypothesis). Although these predictions have been made for dimorphic ungulates, some of them should equally apply to non-dimorphic ungulates, notably the predictions for PR, FS, and SF hypotheses (see Ruckstuhl and Neuhaus 2000; Ruckstuhl 2007). When cared-for young are vulnerable or energy demands in females are high, one should expect females to segregate from males to hide from predators (PR hypothesis) or select high-quality forage because of much elevated energy demands owing to lactation (FS hypothesis). If, on the other hand, social factors were driving social segregation, we should expect males to prefer males and to segregate socially from females even in the absence of sexual size dimorphism (Ruckstuhl and Neuhaus 2000). One example would be if males were to segregate from females to develop their fighting skills and establish dominance hierarchies (SF hypothesis). The predictions of SF would then also apply to non-dimorphic species, with established male dominance hierarchies. However, according to the activity budget hypothesis, same-sized males and females should have similar activity budgets and habitat preferences, and thus should not be segregated socially (Ruckstuhl 1998; Ruckstuhl and Neuhaus 2000).

In addition to these well-researched hypotheses, Bowyer (1984) and Conradt et al. (2000b) suggested an entirely different hypothesis, namely, that differential sensitivity to weather might be driving habitat segregation in dimorphic species. The more vulnerable sex (in their study male red deer (Cervus elaphus L., 1758)) would select habitat that will protect them from inclement weather, whereas the less vulnerable sex (here females) would not be biased towards that habitat type. If sensitivity to weather affects habitat selection or behaviour, we would expect to see weather affecting habitat selection, but we would not expect any sex bias in the reaction to different weather in non-dimorphic species.

In this study, we were primarily interested in investigating social segregation and activity budgets of oryx. African oryx are mostly found in mixed-sex groups, although it seems that some males are territorial and try to defend females that come into their territory. We investigated to what degree oryx of different age-sex classes and reproductive state differ in their activity budgets and bite rates when they are in mixed-sex groups. We predicted that males …