Spider Monkeys Use Collective Computing When Foraging for Food

They break into subgroups and reorganize as needed.

Baby Spider Monkey
Spider monkeys poll the group to see if they should stay or go. Troy Harrison / Getty Images

Spider monkeys know that the best way to find food is in a group. But when they split up to hunt for fruit, there’s no random pairing off. Researchers have found that they use collective computation when they break up into teams.

Wild spider monkeys dwelling in a protected area near Punta Laguna, Mexico, live in what's known as a "fission-fusion" society. Typically, spider monkeys live in matriarchal societies, meaning the older females lead the rest of the younger monkeys, making most of the major decisions for the rest of the group. But that's not the case here.

When they’re ready to forage for food, the monkeys form teams without a single leader picking who goes into which group, according to a study published in the journal Frontiers in Robotics and AI. It's kind of the opposite of a schoolyard game where there's no coach or no popular kids picking sides for everyone.

Instead, each monkey decides which group to join, how long to stay on that team, and when to move to another group. The result, says researchers, is that the monkeys are collectively computing good team sizes given the availability of food in the forest.

"By forming these subgroups — constantly coming together and splitting — the spider monkeys develop a more thorough knowledge of their environment," lead study author, Gabriel Ramos-Fernandez of the National Autonomous University of Mexico, said in a news release.

"They seem to be pooling information about resources, so that as a group they know their environment better than any individual does on its own."

Using Game Theory

To study the animals’ behavior, researchers spent two years recording the interactions of 47 different spider monkeys for five hours each day. The monkeys are used to being watched by people. For foraging, they usually formed groups of two to 17 monkeys, with those subgroups staying together typically for an hour or two.

"We noted who was where, and with whom, at any given time," Ramos-Fernandez said.

Researchers collaborated with scientists at the Santa Fe Institute in New Mexico, using inductive game theory to figure out how a monkey decided to stay with or leave a group. This is different from traditional game theory where researchers make assumptions about strategies used in game play.

Their analysis found that the monkeys’ decisions to stay or leave an individual team were influenced by the decisions of the other monkeys on the team. They would feel out their teammates about the best size and then make their own decision accordingly.

The results produced teams of many different sizes, which were helpful in finding fruit in the forest. The researchers said that the collectively computed sizes were not always a perfect match based on the fruit that was available.

They suggest a similar analysis could be used to study how other groups or systems work, such as flocks of birds, schools of fish, or financial markets.