We recently embarked on data collection in a new community of chimpanzees. They were new to us, that is, though we had heard a great deal about them. We heard from local friends that this is a large community, consisting of more chimpanzees than other communities nearby. We heard that they come into conflict with embittered human neighbors more often than anywhere else we have been. We also heard that these chimpanzees are “very tough,” killing goats with fierce slaps, surrounding and killing an antelope, and beating a python to death by hitting it against a tree. They were a fearsome group, as the stories went.
While heading to the forest on our first day of data collection there, I couldn’t help but wonder what we would find. Though the tales loomed large, I expected these chimpanzees to be more or less like the others we’ve observed. After all, they live only a few miles from neighboring communities, and by now, we feel like seasoned visitors to chimpanzees in the region. It’s as though we are houseguests who crash on the sofa for a few days, getting a glimpse into the lives of our hosts before moving on to the next place, never overstaying our welcome too long. After so many miles traveled, so many glimpses into chimpanzee lives, we are not easily surprised.
Our first few days of data collection were productive but not extraordinary. The chimpanzees were gathered in a large party, making them relatively simple to locate. We collected many dung samples easily. After a few days, Emily, a volunteer for my project, asked for some help while washing a dung sample for diet analysis. What, she asked, was the strange-looking stuff in this sample? Upon closer inspection, I realized what we were scrutinizing: the hand of a black-and-white colobus monkey (Colobus guereza). The delicate skin, hand bones, fingernails, and hair were all visible. The evidence was gruesome but clear: The chimpanzee who produced the sample had eaten part of a colobus monkey.
For chimpanzees across studied sites in Africa, animal protein—including meat, insects, and eggs—comprises a small proportion of the diet, typically around 8 – 10% overall (Goodall 1986). Red colobus monkeys (Procolobus spp.) are the favored prey animal for chimpanzees where the two species overlap. In our study region, there are no red colobus monkeys. Instead, chimpanzees consume black-and-white colobus monkeys among several other species in the nearby Budongo Forest (Newton-Fisher et al. 2002; Reynolds 2005). Black-and-white colobus hunting has also been reported at the Kasokwa Forest, a forest fragment just to the south of the Budongo Forest (Reynolds 2005). As research volunteers in 2007, Jack and I even observed chimpanzees hunting and eating a black-and-white colobus monkey in Kasokwa.
Despite known instances of hunting at Kasokwa, however, little evidence to date has shown extensive hunting by other chimpanzees in this fragmented forest region. Although my colleague Matt McLennan has analyzed the dietary components of over 2,000 chimpanzee dung samples at nearby Bulindi, he has never found evidence of colobus consumption (pers. comm.). My analyses so far have similarly yielded a dearth of animal protein in the diet.
Now, however, we had our first clear evidence of meat consumption. The next day, we watched as a party of chimpanzees fed in a Pseudospondias microcarpa tree, a common tree that produces fruits enjoyed by chimpanzees among other species. After the chimpanzees left, we searched for dung samples beneath the tree. As we looked, Nick told us he had just seen a female chimpanzee in a nearby Psuedospondias tree. We took a closer look and found the lone female sitting calmly and eating a young black-and-white colobus monkey. We watched in utter fascination for thirty minutes or more as she slowly consumed the carcass, taking intermittent bites of leaves between bits of meat. This “steak with salad” style of meat consumption is a commonly observed dining practice among chimpanzees, and may aid in the processing of raw meat. As we watched her eat, we wondered how she came to have this prized catch. Hunting is usually a group activity and a male-dominated affair among chimpanzees (Stanford et al. 1994). High-ranking males often come away with the best chunks of meat, which they may share with persistent community members who pester them until they give in and share their spoils.
How did this lone female end up with an entire monkey, then? Did she take advantage of an opportunity to snatch a young monkey on her own? If so, were the other members of her party still nearby in the neighboring Pseudospondias tree when she went in for the kill, or did she stay behind after the others left to opportunistically hunt? Alternatively, did another hunter share the meat with her?
The next day, we observed yet another instance of hunting. It was morning, and we had just arrived to hear choruses of excited vocalizations along with the throaty calls of black-and-white colobus monkeys. We strained to watch from a distance as they leapt between trees. Nick had the perfect view to see the culmination of their efforts. Three adult chimpanzees surrounded a colobus. One chimpanzee stayed low in the tree while the other two cornered it from above. Then, one of them reached out and grabbed the monkey. We observed as they divided their prey and proceeded to feast on it.
We now had evidence of three hunting instances in one week. This series of events left us with us with more questions than answers. Did we catch this community during a particularly active period of hunting? Hunting rates in chimpanzees are known to fluctuate based on a variety of factors including party size, food availability, and the presence of estrus females (e.g., Gilby and Wrangham 2007; Mitani and Watts 2001; Stanford et al. 1994). Another question, not mutually exclusive, is whether these chimpanzees are simply more apt to hunt than other nearby communities. Does something about the community composition (e.g., the presence of numerous males) favor a “culture of hunting”? Did the environment somehow promote more hunting opportunities? For example, might the density of black-and-white colobus monkeys be particularly high in this forest fragment, leading to more hunting opportunities? For now, answers elude us. Instead, the questions propel us forward in anticipation of what the next field day might bring among these “very tough” chimpanzees.
Gilby, I. C., Wrangham, R. W. (2007). Risk-prone hunting by chimpanzees (Pan troglodytes schweinfurthii) increases during periods of high diet quality. Behavioral Ecology and Sociobiology, 61, 1771 – 1779.
Goodall, J. (1986). The chimpanzees of Gombe: patterns of behaviour. Cambridge, MA: Harvard University Press.
Mitani, J. C., Watts, D. P. (2001). Why do chimpanzees hunt and share meat? Animal Behaviour, 61, 915 – 924.
Newton-Fisher, N. E., Notman, H., Reynolds, V. (2002.) Hunting of mammalian prey by Budongo Forest chimpanzees. Folia Primatologica 73, 281 – 283.
Reynolds, V. (2005). The chimpanzees of the Budongo Forest: ecology, behaviour, and conservation. New York: Oxford University Press.
Stanford, C. B., Wallis, J., Mpongo, E., Goodall, J. (1994). Hunting decisions in wild chimpanzees. Behaviour, 131, 1 – 18.
This post was originally published at Scientific American.