We report the results of the first release project of 36 captive-bred mandrills into the Lékédi Park, Gabon. A mortality rate of 33% was recorded in the first year post-release, with dependent infants the most affected age class, as a result of environmental stress and malnutrition. A programme of provisioning ensured that individuals remained in a good physical condition. During the second year the death rate decreased to 4%, and 6-month survival rate of infants was 100%. Over time the mandrills extended their spatial use of the park, although their exploration remained limited. Our results demonstrate that provisioning and the lack of knowledge of ecological characteristics of the new, complex seasonal habitat were the likely causes of this situation. After 2 years, reproduction and survival appear stable. While habitat preservation and in situ species protection are the best conservation options, release projects may constitute a viable short-term solution for particular species. In this context, this first release of captive-bred mandrills could play a role in the future conservation of this Vulnerable species.

Mandrill Mandrillus sphinx hordes in the Lopé Reserve, Gabon, the approximate centre of the mandrill species range, were studied over 3 years from 1996 to 1999. Part of the study site included gallery forests within savanna areas, allowing observation of entire hordes, hitherto impossible in dense forest habitat. Horde size and composition (sex and age classes) were documented using exact records on video film whenever a horde or subgroup crossed an open space. Mean horde size was 620, and hordes of up to 845 individuals were documented, probably the largest stable group size found in any wild, unprovisioned primate population. Hordes were cohesive throughout the study period and did not seem to be aggregations of smaller units. Mandrill societies seem to be quite different from the baboon societies, to which they have been compared to date. Mature, breeding-age males were not resident members of hordes, but entered at the onset of seasonal cycles in the females (as deduced by the presence of sexual tumescence) and emigrated once female sexual cycles ceased. The number of breeding males present in the horde at any one time is best explained by the number of sexually attractive females. It is postulated that the extreme coloration of males and strong sexual dimorphism in mandrills may have evolved through an enhanced need for competitive signals in a situation where no long-term social bonds between breeding partners exist.

In order to identify antigens associated with protection and those associated with active infection, the humoral immune response of 6 Mandrillus sphinx immunized with 150 irradiated L3 and challenged with 100 normal L3 of Loa loa or 6 animals infected with 100 L3 were compared. The plasma of these animals was analysed by Western blot using adult, Mf and L3 antigens. Several antigens with molecular weights varying from 120 kDa to 13 kDa were recognized by the plasma of all animals. It was shown that early recognition of microfilarial antigens with molecular weights of 97, 68, 45 and 33 kDa correlated with the amicrofilaraemic state. A total of 83% of animals with circulating microfilariae had antibodies against the microfilariae 21 kDa antigen. Furthermore, the antibodies against the 21 kDa appeared 1 month before detection of microfilariae in the peripheral blood of 80% of these animals, and declined when animals became amicrofilaraemic. In contrast, when L3 antigen was used, a molecule with a relative molecular weight of 20 kDa was recognized by antibodies of the only animal which remained amicrofilaraemic for 1 year after immunization with irradiated L3. These results suggest that the microfilarial molecule of 21 kDa may be useful as a marker of Loa loa patent infection, whereas the 97, 68, 45 and 33 kDa molecules of microfilariae and the L3 molecule of 20 kDa may be associated with resistance against Loa loa.