By Drew Enigk and Kris Sabbi
Chimpanzees occasionally supplement their diet of fruit and herbaceous vegetation with meat, which they most frequently obtain by hunting red colobus monkeys. Rates of hunting and successful capture of prey vary across communities. For example, within Kibale National Park, chimpanzees in the Ngogo community have decimated local red colobus populations, while chimpanzees in Kanyawara have typically hunted less often. Since Eslom took over the alpha position in January, however, the Kanyawara chimpanzees have targeted monkeys more frequently, and caught many more per hunt.
Eslom instigates a large fraction of hunts in Kanyawara, taking after his father, Johnny, who was one of the community’s most active hunters before his death in 2014. A newly published study by Ian Gilby and colleagues uses over 70 years of collective behavioral data from three study sites, including Kanyawara, to examine the effects of “impact hunters” like Johnny on community hunting rates. The impact hunter hypothesis proposes that certain individuals hunt more readily than others, and that by seizing the initiative during encounters, they cause monkeys to scatter, creating opportunities for others to catch prey. The presence of impact hunters during monkey encounters is positively associated with the probability of a hunt occurring.
Several of the hunts in which Eslom was involved after his alpha takeover in early 2015 yielded more than five red colobus captures, with Eslom sometimes responsible for >50% of the kills. This trend continued into the summer. In early June, Eslom single-handedly captured a grey-cheeked mangabey and two black-and-white colobus monkeys. On June 24, the Kanyawara chimpanzees took down a record number of colobus in a single hunt!
Before the chaos started, the chimpanzees were lounging and playing on a hillside. Suddenly, chimpanzee hunting calls cut through the air, indicating that monkeys had been spotted and targeted. Suddenly, everyone was at full attention, sprinting toward the call.
Eslom was already up a tree beside Kanyawara’s second-best hunter, an adolescent male named Tuber. Other males displayed beneath them, dragging branches as they ran from one side of the group to the other. While female and juvenile monkeys retreated to the top of the tree, the male colobus formed a defensive line beneath them, squawking and screaming at their enemies below. Eslom and Tuber individually rushed at the monkeys, but each was forced back. More and more chimpanzees joined the offensive. With each attack, the colobus banded together, repelling one chimp at a time. Finally, a hole opened up in the monkeys’ defenses, and the chimpanzees rushed in toward the females.
The pace of the hunt escalated quickly, with panicked monkeys scattering from their perches. Soon they misjudged their leaps, falling from thin branches into the eager hands of the chimpanzees. They were seized so quickly that we researchers could barely keep up with the action.
Our voices rang out across the scene: “Eslom caught one!” “Tuke—Tuke has one!” “—And Tuber!” “Tacugama!!” Four males, four monkeys. By the fifth kill, even the young chimpanzees were catching monkeys! Adult males are the usual hunters among chimpanzees, so it was especially surprising that three of the kills were made by 6-11 year-olds, and that two of these young hunters were females. When the slaughter was over, nine red colobus lay dead, the most successful hunt ever recorded at Kanyawara, and particularly noteworthy considering the involvement of the young chimpanzees. (The record from Ngogo, with many more males hunting, is 13.)
Eslom killed two monkeys in this historic hunt, and a large party of chimpanzees spent the rest of the day sharing the spoils. Now that Eslom has attained alpha status and entered his early 20s – prime hunting years for male chimpanzees – he appears to have established himself as the new impact hunter at Kanyawara.