When biologists study cooperation in animals, they usually offer just a single task at a time. But what happens when animals can choose between several opportunities to work together? Biologists at Utrecht University discovered that this can make a remarkable difference. Their findings raise a broader question: do many behavioural experiments offer only a limited view of how animals actually behave in the wild?
Behavioural scientists often study cooperation using a simple experiment. Two animals must pull ropes simultaneously to obtain a reward. Usually, only one such apparatus is placed in front of the enclosure.
This resembles situations in nature in which a group of animals cooperates to capture a single prey item. But for many primates, such situations are relatively rare. Much more often, monkeys spend their time foraging for food in trees and shrubs, where several food sources are available at the same time.
Changing social dynamics
"When monkeys gather to obtain food, they usually have more than one opportunity in nature," says primatologist Liesbeth Sterck , who led the study. "A fruit tree does not contain just one place where fruit is available, but many. That allows animals to avoid one another, observe one another, or actively seek each other out. But when there's only a single opportunity to obtain food, the social dynamics are very different."
According to Sterck, this means researchers should be cautious when drawing conclusions from experiments that offer only one place to cooperate. Such experiments mainly show how animals behave when there is only a single opportunity available. Once several opportunities are introduced, partner preferences can change.
From one dominant duo to an entire group
Sterck's team discovered this while studying a group of long-tailed macaques. Instead of offering the monkeys just one cooperation device, they sometimes provided three or even five.
When only one device was available, it was almost entirely occupied by two adolescent males. They accounted for the vast majority of cooperative interactions in the group to obtain a food reward.
But when three or five devices became available, the social dynamics changed dramatically. Cooperation became much more evenly distributed throughout the group. The two males remained active, but they no longer dominated the task. They also began cooperating with other group members.
"When there is only one place to cooperate, you create a very specific situation," says Sterck. "That does not mean previous studies using this setup were wrong. But it does mean that our interpretation of the animals' behaviour may depend strongly on how many opportunities they are given."
The peppernut principle
Sterck uses a typically Dutch example to explain the effect: the Sinterklaas celebration, during which small spiced biscuits known as "pepernoten" are thrown into crowds of children.
"During Sinterklaas celebrations, these treats are scattered over a large area, and not offered to the children in one single spot," she says. "That reduces competition and prevents arguments among children. We see something similar in our study. When there are several places where rewards can be obtained, the behaviour of the group changes."
Hunting in the wild
Biologist Jeroen Zewald , who conducted the study as a biology student at Utrecht University, compares the findings to hunting in the wild.
"Imagine hunting deer together when there is only a single animal available," he says. "You want a partner who is good at hunting but who will not keep the entire reward afterwards. If there are prey animals everywhere, it matters much less whom you choose to cooperate with. The monkeys showed exactly this difference."
The findings underline how strongly social behaviour depends on the surrounding environment. Animals themselves do not solely determine whom they cooperate with; the opportunities available to them matter as well.
Implications beyond animal cooperation
The researchers believe the findings may have consequences far beyond studies of cooperation.
Many other behavioural experiments, including studies of social learning and animal culture, also rely on a single location or a single apparatus. This may limit the opportunities animals have to observe one another, learn from one another, or participate in the task.
If we want to understand how animals cooperate in the wild, we may also need to pay more attention to how many opportunities they have to do so, the researchers conclude.
The study was published today in the journal Animal Behaviour.
