Mandrills employ a complex repertoire of visual, vocal and olfactory displays for social communication; they also make use of tactile cues in a variety of contexts, as during mother–infant interactions, when grooming and during copulatory behaviour. The elements of social communication have been well studied in captive and semi-free ranging groups of mandrills, and some useful comparative information is available for the drill. However, where wild mandrills are concerned, only their vocalizations have been studied in any detail, probably because it is much easier to obtain data on the sounds made by wild mandrills than it is to observe their social interactions at close quarters. I shall begin, therefore, by considering the mandrill's vocal repertoire and a seminal paper by Kudo (1987), in which he considered the likely social organization of mandrill groups in the light of his studies of their vocal communication.

Vocal communication

Hiroko Kudo (1987) spent 14 months studying free ranging mandrills in the Campo Reserve in Cameroon, during which time he was able to identify ten vocalizations used for communication. Details of these are listed in Table 5.1. Two further types of vocalizations were heard, but he was not able to record them or to identify their functions. In addition, he noted that all age/sex classes of the mandrill make ‘tooth grinding’ sounds. I have heard only adult males making these sounds, however. As they open and close their jaws, the upper canines rub against the lower teeth (whether the canines or first premolars, I cannot say for certain) in a honing action. Sometimes a male actively works his jaws in this way, and the teeth make a high-pitched squeaking sound as they grind against each other.

Kudo recorded several long-distance calls that were of particular importance for communication, as the mandrills spread out over a considerable area while foraging or when travelling rapidly through the forest in search of fruiting trees. A distinctive low-frequency sound, the 2-phase grunt (2PG), was made only by adult males. The 2PG consists of short, deep groans, repeated every two seconds or so. Males tend to remain near the rear of the group during progressions, and Kudo thought that only one, or a few adult males, gave 2PG vocalizations. He considered that this might indicate ‘the existence of a leader male’ in such groups.