Bai Hokou research site in Dzanga Sangha Protected Areas is home a group of habituated agile mangabeys (Cercocerbus agilis). Since November 2004 WWF’s Primate Habituation Programme has been following this group. The group has proved exceptional in size: usually Cercocebus monkeys form groups of approximately 20 individuals, each led by a single dominant male; from early counts, the Bai Hokou group was around 120 individuals. This group size may not be so unusual in the area, as other large groups have been observed in the vicinity. Due to their extraordinary group size, habituation of the Bai Hokou group is ongoing, but in general they tolerate human observers to within 10m, and certain individuals (notably males) to within 3-5m. This allows un-interrupted observation of their behaviour.
In late 2006, astonishingly the group’s number almost doubled (to approximately 230 individuals) and in late 2013 approximately 300 individuals, through a process termed ‘super-grouping’. All studies of Cercocebus mangabeys report temporary sub-grouping, but ‘permanent’ super-groups have rarely been observed. The most notable cases are the mandrills at Lope, Gabon (with groups up over 800 individuals), and the Black and White Colobus in the Nyungwe forest, Rwanda. The ecological causes and consequences of this behaviour are still poorly known but are most likely either related to anti-predation strategies, or abundant, quality food resources. Attempted predation events have been observed at Bai Hokou (in particular by leopards, eagles, and snakes), but due to the group’s spread and the absence of habituation teams during the night, its impact is difficult to assess.
In terms of food abundance, alongside their preference for both ripe and unripe fruit, the group act like vacuum cleaners of the forest floor, including in their diet a diverse array of seeds, shoots, insects, and mushrooms which they frequently store in their cheek pouches. It has been suggested that as the group’s range is focused around the bai (forest clearing) system, that food may be more abundant and hence allow a larger group size. However, the groups total home range extends over kilometers, also including other habitat types in terra-firma forest. Additionally, in the dry season, when fruit is mostly absent, the group ranges further (up to and over 3km/ day), travelling far and wide in search of sufficient food resources for all its members. These seasonal ranging patterns greatly increase their total home range. Although difficult as yet to measure, this is when the group appears most spread, forming flexible sub-groups. This strategy is opposite to that of gorillas, which concentrate on a few staple foods in the dry season, and ranges less.
Another interesting event relating to group size is the occurrence of a major ‘birthing-season’ in July-August, with most of group’s females synchronously giving birth leading to perhaps 70-100 females in the group carrying infants at the same time. This may be a tactic on the part of the females to confuse or hide paternity, a strategy linked to increased male protection and reducing the risks of infanticide. However, it appears that certain females may associate with certain males, and males seem to mate-guard, thus increasing the male’s likelihood of knowing paternity. Alternatively, synchronous births could be a tactic to ‘swamp’ potential predators – no resident leopard could possibly eat so many new-borns. Another associated factor may be the infant-guarding behavior of the adult males. When fighting, males often grab the nearest infant to protect himself from increased aggression, a successful tactic in most cases, but infallible, as males frequently exhibit injuries.
The agile mangabeys at Bai Hokou also exhibit a number of other exceptional and interesting behaviors, which in turn may be related to their very large group size. Soon after researchers began following the group, it became evident that certain individuals, mostly adult males, occasionally ‘hunt’ animal prey. Aside from insectivory, individuals have been observed chasing and catching blue duikers, Peter’s duikers, water chevrotains and a host of rodent species (including squirrels and mice). Such behavior has only been reported in a handful of primate species (notably, baboons and chimpanzees). The importance of meat to their diet has yet to be quantified. The frequency of this behavior in the Bai Hokou group may be a consequence of living in such a large group as duikers are frequently attracted to dropped fruits whilst the mangabeys are feeding. Smaller groups of mangabeys may not encounter duikers as frequently.
This unique long-term study has now begun to throw up some interesting questions. Preliminary research at Bai Hokou has focused on their basic ecology, but with such a large group, behavioral studies remain limited. However, in time more may be learnt on the reasons behind their large group size, and its consequent effects (time spent processing food in cheek pouches, the frequency of hunting behavior, time spent in social interactions, and the timing and frequency of sub-grouping).