“Collective action promoted by key individuals”

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David Wheatcroft and Trevor D. Price (Oct 2018)

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High-gain individuals help form cooperative groups by incentivizing low-gain individuals to join

Simple mechanisms to explain cooperation

A partially hatched brood of Phylloscopus humei (Hume’s leaf warbler), one of the main study species in our article. Mooling Village, Himachal Pradesh, India. 2009.
(Credit: David Wheatcroft)

A farmer rushes out of her house to confront a bear raiding her crops. She is soon followed by her neighbor, and together they succeed in scaring the bear away. The next day, the farmer is at the market when the bear comes again. This time, the bear has a feast while the neighbor watches on. Why might the neighbor join with another to perform an action she would not do on her own, especially given she would also benefit if the bear is driven away from the general vicinity? Complicated explanations, such as that of reciprocal altruism (“I help you now in the expectation you will help me later”) prevail in the literature. A simpler one comes with two conditions. First, the benefits should outweigh the costs for some individuals (i.e., the farmer) who perform the behavior even when alone. Second, once one individual is performing the behavior, the costs for others (i.e., the neighbor) are sufficiently reduced so that they are outweighed by the benefits.

A landscape of the primary fieldsite of Manali, Himachal Pradesh, India, in 2006.
(Credit: David Wheatcroft)

David Wheatcroft and Trevor Price asked if this idea can explain cooperative behaviors in nature. They studied responses of small birds to various enemies over four years in the western Himalayas. Crows prey upon the eggs and chicks of small birds, and cuckoos parasitize their nests. Wheatcroft and Price used taxidermied mounts of crows and cuckoos to show that, when acting alone, the only individuals to strongly attack the mounts were those with nests nearby. As long as nest owners are present, birds without nearby nests also join. By contrast, hawk and owl mounts are always vigorously attacked. Wheatcroft and Price then developed a theory of changing cost:benefit relationships, assuming the principles outlined above apply. They show that the results are as expected if crows and cuckoos are not too costly to attack and costs decline with the number of birds attacking the mount, but benefits always accrue to individuals attacking hawks and owls. Simple mechanisms (i.e., join if you benefit) may regularly explain the formation of cooperative groups.


Explaining why individuals participate in risky group behaviors has been a long-term challenge. We experimentally studied the formation of groups of birds (“mobs”) that aggressively confront predators and avian nest parasites and developed a theoretical model to evaluate the conditions under which mobs arise. We presented taxidermied mounts of predators on adult birds (hawks and owls) and of nest threats (crows and cuckoos) at different distances to nests of Phylloscopus warblers. Even when alone, birds are aggressive towards predators of adult birds, both at and away from their nests. By contrast, birds aggressively confront nest threats alone only when they have a nest nearby. However, strong initial responses by nest owners lead individuals without nearby nests to increase their responses, thereby generating a mob. Building on these findings, we derive the conditions when individuals are incentivized to invest more when joining a high-gain individual compared to when acting alone. Strong responses of high-gain individuals acting alone tend to reduce the investments of other high-gain individuals that subsequently join. However, individuals that benefit sufficiently little from acting alone increase their investments when joining a high-gain individual and can even be sufficiently incentivized to join in when they would otherwise not act alone. Together, these results suggest an important role for key individuals in the generation of some group behaviors.