Monkey Forest Tales: Why monkey groups do fusion/ fission more often in fragmented areas than in continuous

Unamas - SR Enero 2012 396 (2)

In this post, a story based on some of the observations that we had done over the past years in this project compared with observation done for the same species in a continuous area where I used to work some years before the start of this project.

In continuous area groups of red howler monkeys (Alouatta seniculus), black-capped capuchins (Sapajus apella) and Colombian squirrel monkeys (Saimiri cassiquiarensis albigena) usually move in a cohesive form, i.e. the group dispersion (distance between individuals of the same group) during foraging is short and you can observe most of the group in the same area feeding in the same tree or group of trees.

In fragmented areas were forest size has been reduced due to habitat loss and fragmentation, and where forest had been degraded through selective logging and livestock use of forest, groups of the same species have been observed dispersing more during feeding time, even divided into subgroups and moving separately for up to two days in the case of Colombian squirrel monkeys or just a few hours in the case of red howler monkey.

In Colombian squirrel monkeys, large groups (> more 25 individuals) living in small forest fragments, groups split for up to two days to forage and move independently. Subgroups are composed of females, males, and juveniles, commonly forming subgroups of 10 – 15 individuals. In the study area, this behavior is more common in the months where fruits are less available.

Observations of fusion/ fission patterns had also been observed for red howler monkeys and black-capped capuchins, with groups splitting in solitary individuals or females and juveniles in one subgroup and males and subadults in pairs or alone for up to few hours. In red howler monkeys, these subgroups are formed for just a couple of hours. While in black-capped capuchins we had observed individuals lost from their groups for several hours up to the next day, with intensive calling from the individuals separated from the group.

This pattern of fission/ fusion of groups is a common social structure pattern for other species, like spider monkeys and chimpanzees, but not in the species mentioned above. However, red howler monkeys, black-capped capuchins and Colombian squirrel monkeys live in more cohesive groups where the distance between individuals although variable are not characterized by this type of fission/ fusion structure.

Why this happens? Although not detailed data on the frequency of this behavior, it seems to be more frequent in fragmented areas, especially in forest fragments of less than 50 ha. This pattern can be influenced by resource availability and crowded populations observed in small forest fragments, although more data need to be collected.

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