Semi-free ranging mandrill groups in Gabon

Between the years 1990 and 1992, at the Centre International de Recherches Medicales de Franceville (CIRMF) in Gabon, I conducted a longitudinal study of the behaviour of the semi-free ranging mandrill group (Group 1), the seasonal patterns of reproduction of which were discussed in the last chapter. This group was founded in 1983, when 14 young mandrills (six males and eight females) were released into a six-hectare enclosed area of secondary rainforest. By the time the observations described here began, the group contained 45 individuals, and it increased in size to 68 mandrills during the course of the project. All the animals except infants were fitted with numbered ear-tags; thus, it was possible to identify individuals quite easily when conducting behavioural observations.

The behavioural work benefitted from the valuable assistance of two students from the University of Zurich (Thomas Bossi and Edi Frei). Our detailed observations of the menstrual cycle and of sexual and associated patterns of behaviour extended throughout two annual mating and birth seasons. Blood samples were collected from all the infants born during the study period, as well as from their mothers and the various males in the enclosure (see Plate 24). Subsequently, Dr Jean Wickings conducted a DNA fingerprinting paternity analysis of these infants, so that, for the first time, it was possible to examine how the behaviour of male mandrills translated into reproductive success, in terms of infants sired with individual females. The dominance ranks of all the animals were known, as well as many details of their visual displays, vocalizations and grooming interactions (these were described in Chapter 5). Thus, it was possible to analyse how rank and other social variables might influence patterns of sexual behaviour and reproductive success in mandrills living under relatively natural conditions.

I shall discuss the results of this initial study in some detail, because fine-grained behavioural data were collected at a time when Group 1 remained relatively undisturbed and its social organization was stable. As was described in the previous chapter, mandrills were subsequently removed from Group1 (in 1994), in order to set up a second group in an adjacent enclosure. Subsequent transfers of additional adult males from Group 1, to enlarge Group 2, inevitably caused further social disruption.