A note on group sizes of oribi (Ourebia ourebi, Zimmermann, 1783) from two contrasting sites in Zambia, with and without predation

Abstract

The oribi (Ourebia ourebi, Zimmermann, 1783) is a small species of antelope widely represented across open grasslands of sub‐Saharan Africa. Although largely territorial, differences in group size and mating systems (monogamy/polygamy), have been linked to habitat conditions and population density. At high population densities, additional males are recruited to assist in territorial defence. Other factors which might impact on group size, include predation threat, especially at low population densities. In this study (1995–98), we recorded group sizes of oribi across two contrasting areas (c. 300 km2 each) of Zambia with (Kafue), and without (Bangweulu), large vertebrate predators. A total of 412 groups was recorded across the two sites, with mean sizes of 2.31 (n = 217) at Bangweulu and 2.33 (n = 195) at Kafue. The modal group size was two throughout (range 1–6). Groups were more variable in size and sex composition at Kafue than at Bangweulu, but there was no significant difference in estimates of population density between sites. Mean estimates were 2.02 and 1.90 km−2 for Bangweulu and Kafue, respectively. Sex ratios (males : females) were biased towards females (1 : 1.72 and 1 : 3.55, respectively) at both sites. Adult males were particularly scarce at Kafue. However, values of density and group size, were both unexceptional for the species. Whilst a modal group size of two is consistent with a monogamous mating system, variations in group composition and size were more difficult to explain. Adult females appeared to retain or recruit additional animals to groups, irrespective of the needs of territorial defence. This suggests that vigilance is an important function of groups on large open plains, especially in the presence of predators. By extension, reversed sexual dimorphism in oribi could reflect increased selection for vigilance duties in females. Predators had no apparent effect on population density, but oribi groups were more variable in size and sex composition in their presence. Adult males may be particularly vulnerable to predation when defending territorial borders at low population densities. However, further work is needed to define group dynamics in this species and to identify causes of mortality amongst adult males.

Gregariousness increases brain size in ungulates

“…(Nowak 1999); b. (Brashares et al 2000); c. (Goldspink et al 2002). Gestation ref., sources used to estimate the gestation length: 1.…”

The anti-predator behaviour of wild white-handed gibbons (Hylobates lar)

“…Avoiding predation has been a central theme for the evolution of sociality in animals and hominids (Alexander 1974;Waterman 1997;Goldspink et al 2002;Alcock 2005;Deecke et al 2005;Hart and Sussman 2009). Across animal orders, there is evidence of both morphological and behavioural anti-predator adaptations.…”

“…Predation can influence the size and composition of groups, reproductive and vocal behaviour, cognitive abilities and habitat use (van Schaik and van Hooff 1983;Hill and Lee 1998;Zuberbühler and Jenny 2002;Janson 2003;van Schaik et al 2004). For example, preference for high forest canopies is a likely adaptation to increase protection from ground predators , while living in larger groups provides some safety in numbers and improves predator detection as well as chances of escape (Goldspink et al 2002;Zuberbühler and Jenny 2002;Zuberbühler and Byrne 2006). In addition, group living enables individuals to engage in cooperative defence behaviours that have a dissuasive effect on some predators, such as mobbing and alarm call chorusing (van Schaik and Hörstermann 1994;Zuberbühler et al 1999).…”

Population status of oribi (Ourebia ourebi Zimmermann, 1783) in Maze National Park, southern Ethiopia

“…Belovsky (1986) elaborated that population densities of a number of herbivore species depend upon the biomass of plants in the environment that satisfy the minimum characteristics sought by the herbivore. Goldspink et al (2002) recorded that the mean density estimates of oribi population from the two contrasting sites in Zambia were 2.02 and 1.90 per km 2 . Goldspink et al (2002) described that the population density of oribi in different parts of Africa vary from 2/km 2 up to 10/km 2 registered from the Serengeti National Park, Tanzania.…”

“…Goldspink et al (2002) recorded that the mean density estimates of oribi population from the two contrasting sites in Zambia were 2.02 and 1.90 per km 2 . Goldspink et al (2002) described that the population density of oribi in different parts of Africa vary from 2/km 2 up to 10/km 2 registered from the Serengeti National Park, Tanzania. The present estimate of population density at Maze National Park, Ethiopia was high compared to the data from the Bangweulu and Kafue areas, Zambia but low compared to the Serengeti National Park, Tanzania.…”