One May morning in 2009, a group of white-faced capuchins were foraging on a hillside on Barro Colorado Island, Panama. Two researchers from the Smithsonian Tropical Research Institute, Anyuri González and Lucia Tórrez, were watching the group from nearby, but when one of the monkeys started shrieking, they realized they weren’t the only voyeurs.
The screams were an alarm call indicating a predator on the ground. The group’s alpha male took off in the direction of the noise and was joined several other adult members of the group, and they all began making facial and vocal threats and shaking and throwing branches toward a dense tangle of liana vines.
There was something in there, and the monkeys didn’t like it.
While many animals go to great lengths to hide or flee from predators, some animals take the opposite tack and confront their attackers. Some species of primates, birds, fish and squirrels will harass and assault potential predators with mobbing behavior that involves loud vocalizations and physical attacks at close range.
The mobbing of a predator by capuchins is a real sight. Led by adult and subadult males, the monkeys will make loud threat and alarm calls while breaking large branches from trees and dropping them on the predator like bombs.
Researchers have been mistaken as predators themselves and received blows – at least one admitted to being knocked unconscious – from branches broken and dropped over their heads. Scientists working in capuchin territory quickly learn to to stop taking data and get out of the way when they see a monkey scanning the canopy directly above them, a sign that its looking for a branch that it will be able to break.
Some monkeys will get closer to predators if the aerial assault doesn’t work. In 1988, biological anthropologist Susan Boinski watched as a group of capuchins in Costa Rica killed a terciopelo, or fer-de-lance, after pinning it to the ground with a heavy branch and then approaching on the ground to beat it with sticks. One monkey rained down a flurry of 55 strikes to the snake’s body and head with a stick that it clumsily wielded like a club. Boinsky later approached the snake and found a mangled mess of bleeding wounds, exposed tissue and broken bones.
A Death in the Family
As González and Tórrez watched, a sense of alarm spread through the capuchin group. Juvenile monkeys and an adult female with a clinging infant gathered some distance away from the liana tangle, while the rest of the adults joined the alpha male on the frontline.
The capuchins’ alarm calls intensified and González, who was downwind from the liana tangle, noticed a distinct smell. A cat. She could see the right side of whatever was in the liana, and based on its size and fur color and patterns, decided it was not one the ocelots normally seen on the island, but a jaguar visiting from the mainland.
The cat growled loudly, making the capuchins visibly nervous. They glanced repeatedly at one another and one of the adult females retreated to where the juveniles were waiting. Together, they moved further away from the liana. The jaguar growled again and moved around in its hiding spot, and most of the remaining capuchins also retreated.
As far as González and Tórrez could see, only six adult and subadult males remained near the liana tangle, and their vocalizations had decreased in both frequency and intensity. They looked back and forth at one another and then moved silently away from the jaguar, following their groupmates.
Only one monkey stayed behind, and continued to make alarm calls. González and Tórrez weren’t able to see the monkey, and as they attempted to get close enough to verify which one it was, the cat made a very loud, very aggressive growl, and the researchers decided to retreat a little themselves.
From a safer distance, they could still hear the lone capuchin making alarm calls. Then, there was the sound of sudden movement and the rustling of branches. The alarm calls stopped and they heard two weak moans.
After that, there was only silence.
González and Tórrez encountered the capuchin group later that morning. The group was quiet, and had returned to foraging for insects. They took a headcount and found that one of the subadult males, named LK, was missing. Every other group member was accounted for but LK was not seen again over the course of their study.
At What Cost?
Approaching and mobbing predators appears to invite some obvious risks, as LK’s death shows. The true costs of mobbing are hard to suss out, though, because reports of primates being injured or killed while mobbing are very rare. The LK incident is, as far as González, Tórrez and their colleagues know, the first documented fatality arising from a mobbing event.
With limited observations, researchers have hypothesized several risks. The risk of injury and/or death seems to depend somewhat on the predator. Many snakes are cryptic hunters, and, once discovered, will freeze and can be mobbed with relatively little risk. Other predators, like large cats, continue to pose a significant threat to monkeys even after they’ve been discovered, and some will even chase their prey into the trees, giving them no quarter.
There are also other less grisly losses to consider, like the effort that goes into fighting back and opportunity costs of taking time from other activities. Some monkeys have been known to remain at the site of a spotted, attacked or even injured or dead predator for up to two hours after mobbing. That’s time and energy that could be spent foraging or mating sacrificed to a risky behavior, and even a “win” for mobbers might compromise the animals’ health and reproductive success, making it a Pyrrhic victory.
Figuring out the costs of mobbing, in turn, would help explain its benefits. If the price that primates are willing to pay to harass their predators are as large and varied as they seem, the rewards should be enough to make the risk worthwhile.
The obvious benefit of mobbing for social primates is that chasing off or killing predators helps protect their social group, relatives, mates and offspring. It’s also possible that mobbing events act as a learning opportunity for younger monkeys. Juveniles are able to study the animals that provoke alarm calls and defensive behavior and compare them to animals that don’t, and learn to know danger when they see it.
Another benefit to mobbing may be that, because it is so costly, it acts as an honest signal of an individual monkey’s quality as a mate. Attacking a predator gives a monkey the opportunity to display their physical condition, agility, and speed. Mobbing might be a way for females to evaluate the ability and willingness of males to protect them and their future offspring against threats.
Unfortunately, with so little data from the field or the lab, we don’t know how often mobbing ends lethally, or the likeliness of a fatal attack when a primate does vs. doesn’t mob, so it’s hard to weigh the costs and benefits that play on a monkey’s decision to stand its ground or retreat, maybe to fight another day.
S. Boinski (1988). Use of a Club by a Wild White-Faced Capuchin (Cebus capucinus) To Attack a Venomous Snake (Bothrops asper) American Journal of Primatology, 14 (2), 177-179 DOI: 10.1002/ajp.1350140208
Tórrez, L., Robles, N., González, A., & Crofoot, M. (2012). Risky Business? Lethal Attack by a Jaguar Sheds Light on the Costs of Predator Mobbing for Capuchins (Cebus capucinus) International Journal of Primatology, 33 (2), 440-446 DOI: 10.1007/s10764-012-9588-1
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