Observations sur la socio-écologie du Mandrill

Six months were spent in Gabon tracking troops of Mandrills (Mandrillus sphinx) in 1973 and 1974. During two survey-periods of three months each about 2,000 kilometers were covered by foot in the forest and 6,000 by road. Contacts were only established 39 times, totalling 48 hours of actual observation time. Mandrills and drills never occur together in the same areas. Mandrills were always found to live in the lower forest layers, large adult males at the ground level, smaller females and juveniles in the undergrowth and the middle layer. Large rivers cannot be crossed by mandrills, these forest baboons being unable to swimm. The basic social unit is made of a large adult male (one for 25 individuals on average), 5 to 10 females (with or without infants) and about 10 juveniles. During the dry season, as many as 6 or 7 such harems might associate together, forming troops of 200 individuals or more. Excess adult males live a solitary existence in the forest. Mandrill troops have a very wide home range, in the order of tens of square kilometers. The average speed of movement of a group during the day was found to be 1.32 km/h, and the average distance covered 8 km per day. The diet is very varied, mostly vegetarian with possibly a small animal component. Breeding is apparently seasonal, most births occuring from January to April. Besides man, major predators include leopards and the Crowned hawk-eagle (Stephanoetus coronatus). Temporary associations with duikers (Cephalophus callypigus) and calaos (Tropicranus albocristatus) have been observed. Three types of vocalizations were distinguished : a contact call emitted mainly by females and juveniles, an alarm call used by females and subadults, and a rallying call emitted by the leader male. Olfactory marking by the secretion of the sternal glands is performed on tree trunks. When the troop is quietly foraging juvenile and subadult individuals tend to stay in front and around the females, the leader male staying to the rear. When frightened, the adult male leads the troop, followed by the females and the younger individuals. In the rain forest Mandrills exploit the less productive layers, at the ground level or in the undergrowth. Their social structure and the large size of their home range may be considered as adaptive to this poor environment.