One of the things that I love more of this project is that every time I go to the field, I came back with more questions than answers. One of those questions that still I cannot answer and that sometimes make me wonder of what happens in at least part of my study area is why black capped capuchins move in big groups (> 10 individuals) in very fragmented areas?

Black-capped capuchins usually live in groups of 5 – 8 individuals in this area, with up to 2 adult males, several females (usually, 2 – 3) and their juveniles and infants. However, over the past 15 years I have seen groups of black-capped capuchins of up to 22 individuals moving together as a cohesive group.

Although this species of capuchins has been observed in groups of up to 19 individuals in continuous forest (Izawa 1990, a group of black capped capuchins that was feed with plantain and bananas for behavioral studies).

In fragmented areas where there is less food and more predators (including domestic dogs), black-capped capuchins groups usually are small (5 – 8 individuals). So, why are we observing these big groups sporadically.

At least for one of my observations I’m sure that big group (22 individuals) was the result of several groups (2 – 3 groups) moving together. This observation was made in a live fence on June of 2011, around noon.

All animals were moving from one live fence (a line of native trees left by the landowners to separate different pasture plots) to another by a 10 m gap, going down to the ground and up again on the other live fence. I was able not only to count them, but also to determine the composition of this big group (4 adult male, 4 adult female, 10 Juveniles, 4 Infants).

The area in which this observation happens is part of three black capped capuchin groups. However, I couldn’t determine the reason why these groups were moving together. There wasn’t any evidence of a very large availability of fruits that make this big group to remain together or any evidence from that month or the previous and following months that indicated the presence of any particular big predator.

The other observation was made in a forest fragment of 79 ha. In this case it was only one group with a high proportion of juveniles and infants (2 adult males, 5 adult females, 1 subadult male, 5 juveniles and 5 infants). And only this group seems to be living in this fragment despite its area.

There are two main theories of why monkeys live in big versus small groups. First as a strategy to reduce predation risk: if you are an individual in a big group then there are less probabilities that a predator focuses on you and catch you. Additionally, if you live in a big group there is more individuals looking for predators and therefore the time, I spend looking for predators (vigilance behavior) is reduced because I had help from other group members (Fragaszy et a. 2004).

Second, living in big groups can give you an advantage on your feeding strategy, because you have more individuals to help you find and defense those food resources (Fragaszy et al 2004). However, a research had found that for this particular species of capuchin monkeys in continuous areas, this is not true (Janson 1988).

Then why black-capped capuchins are found in this big groups in fragmented areas? We have observed 18 groups of different forest fragments with more than 9 individuals present in the study area. The answer to this question is not clear but probably both theories are playing a role here. Additionally, we don’t know if there are any factors influencing how well the capuchins in this area can disperse. We know that they can disperse using wire fences, living fences, and crossing pastures. Local people had reported this, and I had observed all three methods of dispersing in the area over the years.

We also observed predation attacks by domestic dogs, common caracaras and tayras towards juveniles from different groups in the study area. Most of them in living fences and small forest fragment. Moreover, we know that at this area there are cougar and jaguar, as well as other felines such as ocelots. We also know there are big prey birds such as crested eagle in the area.

So, it’s possible that in some cases young individuals decided to stay in their natal groups longer as a measure to reduce predation, although this could mean a reduced reproductive success for them and more competition for food (18 groups of different forest fragments with more than 9 individuals present in the study area).

References

Fragaszy D.M., Visalberghi E. and L.M. Fedigan 2004. The Complete Capuchin: the biology of the Genus Cebus. Cambridge University Press. Cambridge, UK.

Izawa K., 1990.  Social changes within a group of wild black-capped capuchins (Cebus apella) in Colombia (II). Field Studies of New World Monkeys, La Macarena, Colombia 3: 1- 5.

Janson 1988. Food competition in brown capuchin monkeys (Cebus apella): quantitative effects of group size and tree productivity. Behaviour 105: 53-76.

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