Prior to the present study, no systematic field data were available on the behaviour and ecology of Erythrocebus patas. This monkey, widely distributed over the grass and woodland savannah regions of West and East Africa, appears to be unique in some of its physical and social adaptations to a terrestrial mode of life. In three main study periods, 640 hours of observation were carried out on groups in the Murchison Falls National Park. Small samples of data on baboon and vervet groups in the same area were obtained. Numerical sizes of patas groups ranged from 9 to 31, mean 15. In no group was there more than one full-grown male. Groups were followed for many successive day-ranges, distances travelled varying from group to group, season to season, with a maximum of about 12,000 m and a minimum of 500 m. Home range size of one group was 5200 hectares. Groups tended to use a different area of the home range each night, and individuals dispersed far apart in taking up night resting positions in trees. The day activity pattern comprised two main feeding periods, with a rest period of one to three hours in the hottest time. Drinking at water holes or other sources was infrequent.
Infants less than three months old occurred only up to June, and were commonly seen in March and April. Infants aged 3 to 12 months, and juveniles, engage frequently in long, active bouts of play-chasing, wrestling, and bush-bouncing–almost all play is on the ground. Grooming amongst them is common, its social pattern being similar to baboons. Submissive gestures are very rarely seen, baboon-like presenting and being mounted not occurring. Threat-attack is mainly by adult females, very rarely by the adult male. Vocalizations audible at about 100 m occur very infrequently (about two to four a day, compared with 25 to 50 for baboon groups).
The adult male patas plays the part, for the group, of watchdog. Whenever the group is disturbed, or approaches new areas, he may reconnoitre from a high point several hundred metres away from the group. When disturbed by the observer, he has a noisy, bouncing display on bushes and trees, and probably a form of diversionary display by running away from his group and from the observer. In the group, he is spatially peripheral, except when day resting, mating and grooming, and the adult females habitually initiate directions and times of group movement. Their function is essentially that of looking after themselves, their infants, and the juveniles, while that of the adult male is very strikingly that of watching for predators or other patas groups or baboons, etc. Isolated adult males and one all-male party of four have been observed. Spacing between groups is such that inter-group encounters are exceedingly rare (two occurrences only). Reaction consisted of chasing by the larger group, rapid avoidance by the smaller. A contralto bark is uttered repeatedly, in short series, by the adult male of a group only, it seems, on encounters with extra-group patas.
The patas adaptations of swift locomotion, silence, concealment, and dispersal, contrast very clearly with those of the much noisier, larger, more aggressive baboons in whose groups there are usually several adult males.
Further research on the species is required from West Africa, and captivity studies must be used to determine the precise nature of the social organization amongst the adults and the processes of socialization.