As a result of their habitat, Drills are much harder to study than savannah baboons, and the details of their natural social organization are still unclear. From what observations have been made, they same to live in multi-male, multi-female groups for at least part of the year. One feature of both species of Mandrillus is that these groups sometimes combine into much larger groups called hordes, which can number many hundred animals. Whether the horde is the true social unit, and the smaller groups are mere subdivisions, is not yet certain. Within the smaller units, a single male seems to monopolise most of the reproduction, and there is a strict dominance hierarchy. This has generated the evolution of one of the most extreme cases of sexual dimorphism known in primates – males can reach well over 30kg, while females are mostly in the 10-12kg range. With this sort of size, adults probably have few predators – leopards will take them of course, and pythons and large raptors as well, but a social animal armed with very large canines is a pretty dangerous target if there is anything else available. Juveniles and infants of course would be far more vulnerable.
One other point to consider is that Drill have been far less studied than their more colourful cousins. The difference in the intensity of the display colours between male Drills and Mandrills suggests there may be subtle differences in their societies – Drill hordes seem to be smaller than those of Mandrills, but this may be a feature of their rarity and the hunting pressure they unfortunately face.
Unfortunately, these hunting pressures are very severe on Drills. They are often caught and eaten as a delicacy, and their terrestrial habit of life makes them vulnerable to hunters. The usual hunting method is to use dogs to chase them up trees, at which point their comparative lack of agility compared to smaller monkeys makes them very susceptible to firearms. Combined with ongoing rainforest destruction, the population, which has a fairly restricted range in any case, is consequently classed as Endangered.
The two Drills we have at Bristol are brothers, eight and nine years old, which makes them full grown. As we have not kept this species before, we will start with developing husbandry and management protocols with them before hopefully adding females to the group. This is the standard way a new species is added to a collection – because females are far more valuable than males to a breeding programme in polygamous species, a zoo starting out will usually have males released to them first. Bristol’s acquiring Drill makes them only the fourth zoo in the UK to hold this species, so if you are visiting please check them out – they are seriously impressive creatures.
(images from wikipedia)