WILDLIFE MANAGEMENT - ANTELOPE ABUNDANCE
Why aren`t all antelope equally abundant?

D T Rowe-Rowe
Why is it that antelopes such as the duikers and the steenbok do not occur in large herds, whereas species such as wildebeest and eland do?
The general rule is that small antelopes occur in small groups and that group size increases with the body size of the animal.
 
 In 1974 Peter Jarman, a biologist who had been involved in studies on a variety of African antelope, recognized five classes of social organization which he suggested were determined by ecological aspects of their way of life.
The main inter-related factors are feeding style, body style, group size, and spacing of individuals, all of which contribute to the social organization of each species.
In this technical guide I have dealt with the antelopes which occur in KwaZulu-Natal, dividing them into five classes (A to E), as indicated in Table 1, and the major factors influencing abundance and social organization are discussed.
I hope that this information will provide a clearer understanding of why antelopes such as oribi and steenbok do not thunder across the plains in herds numbering several hundred.
Feeding styles

Some species find only items of high protein content acceptable e.g. newly emerged leaves, and are thus termed selective feeders.
At the other extreme are unselective feeders that can tolerate low protein foods and are able to eat more of the plant.
All intermediate feeding styles exist and five general feeding categories are recognized, namely:

A  Antelope that feed very selectively on a wide range of plant species (both grasses and browse), being selective for particular parts of the plant. The food items are of the highest nutritive value. The antelopes belonging to this class are all small in size. 
B  Antelopes in this class feed selectively entirely on certain grasses or entirely on certain browse plants or parts of browse. Food is high in nutritive value. 
C  These antelope feed on a range of grasses and browse, fairly selectively. The diet and preference for vegetation type changes seasonally. 
D  All antelope in Class D are grazers, which are mainly unselective for grass species but are selective for growth stage. These antelope usually migrate to find grass of the right stage. 
E  This class includes the large-bodied mixed feeders, which feed fairly unselectively on a wide range of grasses and browse in a number of vegetation types.
 
Size
Smaller antelopes can feed more selectively that can large ones mainly because they have smaller mouths, therefore there is (broadly speaking) an increase in body size of the antelopes from the most selective feeders in Class A to the least selective feeders in Class E.

Table 1. Classification of some African antelopes according to their size, feeding style and social organization. Class Antelope A -  Blue duiker (impiti), grey duiker, red duiker, klipspringer, oribi, steenbok, suni 
B -  Bushbuck, common reedbuck, mountain reedbuck,(rooi ribbok), grey rhebuck (vaal ribbok) 
C -  Impala, kudu, nyala, springbok, waterbuck 
D -  Blesbok, hartebeest, black wildebeest, blue wildebeest 
E -  Buffalo, eland. 

Group size and social organization
Very selective feeders need to be widely dispersed because their own chosen food items are generally not abundant, small, and widely dispersed. During winter, when the nutritional content of plants is low, it becomes even more difficult for selective feeders to obtain enough food. Food items become progressively more abundant for less selective antelopes, therefore these animals are more common than are selective feeders. So, if antelopes are arranged into classes based on group size, we find that these classes correspond closely with body size and feeding style. The groups are:

A Antelopes which occur singly, or in pairs, or with a maximum of a pair plus a young animal. B Groups in this class consist of a single adult male together with several adult females and the young of the year. Groups consist mainly of 3 to 6 animals, but may be as large as 12 to 15. C In this class, males occur singly. Females are never alone. Group sizes are generally 6 to 20, but occasionally larger aggregations occur. D Territorial males occur singly. Adult females and young form groups from six to several hundred. Animals in this class may remain in large mixed herds for most of the year, splitting into harem herds (i.e. receptive females) and separate non-breeding herds during the rut. E Antelopes in this class form large mixed herds in which a number of breeding males and females may be mixed with non-breeders.

CONCLUSIONS
Abundance and social organisation of antelopes are influenced by their body size and feeding style. The smallest antelopes are the most selective feeders and occur in the smallest, most widely dispersed groups. With increase in body size, there is general decrease in the degree of food selectivity, and an increase in group size and in general abundance as the antelopes need not to be as widely dispersed as are selective feeders.

Reference
Jarman P.J. 1974. The social organization of antelope in relation to their ecology. Behaviour 48:215-267