The sexual, social, and agonistic behaviour-patterns of the Chacma baboon, P. ursinus, were the subject of field study chiefly in the Cape of Good Hope Nature Reserve where intensive close-range observation of a single group was possible. The mating system in this group was one in which the male dominant in terms of such criteria as frequency of aggressiveness and of social “attractiveness” (as shown by the number of animals presenting to him, and participating in grooming with him) tended to have a brief, exclusive relationship, with the female in full ocstrus. Other males would also mate with her, on the same day as the dominant male, before the stage of exclusion was reached. Data are presented on the frequency of occurrence of components of sexual behaviour. Presenting, grooming, mounting, and mating all tend to occur most frequently in the early part of the day. Social gestures, other than presenting and mounting, include a variety of greetings behaviour-patterns which occurred repeatedly between a tame young male baboon released near the group and the young males and females of this group.

Agonistic behaviour was analysed particularly in terms of frequency and intensity of occurrence within the group. Aggressive opisodes by the dominant male occurred, on average, about once in six hours of day-time, except on the first occasion when the tame baboon accompanied the group on the whole of its day-range. The dominant male was then repeatedly aggressive throughout the day. A high positive correlation was found between the frequency of mating per day by the dominant male and the frequency of aggression by females against females. Analysis suggests that the peak of mating in the dominant male results in a proportional increase in tension amongst the females which is directed partly at the oestrous female, partly at other females. Aggressive episodes, sometimes very noisy and rough-looking, never resulted in visible injury to the victim, and mostly consisted of threats without physical contact. Supposed conflict-expressions, such as yawning, are briefly described.

Further progress in such field studies may come chiefly from systematic quantification of results, obtained at close range on identifiable individuals and from experimental variations of the social situation such as the introduction of strange baboons on two occasions described in the present paper.