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Tiny fish called Trinidadian guppies have surprised scientists when faced with the so-called "volunteer's dilemma" [exeter.ac.uk]:

Tiny fish called Trinidadian guppies have surprised scientists when faced with the so-called "volunteer's dilemma".

The idea of the dilemma is that individuals are less likely to cooperate if they are in a large group.

Various studies have demonstrated this in humans – but guppies appear to buck the trend.

"When faced with a possible predator, guppies have to balance risks," said Rebecca Padget, from Exeter's Centre for Research in Animal Behaviour.

"At least one guppy needs to approach, to find out if there is a threat.

"An individual that does this could get eaten. However, if none of the guppies take this risk, the whole group is in danger.

"In this 'volunteer's dilemma', mathematical models suggest that individuals in larger groups should be less willing to cooperate.

"In a larger group, there's more chance another guppy will take the risk."

To test this, the researchers placed a clay model of a pike cichlid (a natural predator of guppies) in a tank containing small (5), medium (10) and large (20) groups of guppies.

[...] "We can't be sure why guppies in large groups cooperated more," Padget said.

"We know guppies have different personalities, so it could be that larger groups are more likely to contain more cooperative individuals – and others then follow their lead."

Journal Reference:
Padget Rebecca F. B., Fawcett Tim W. and Darden Safi K. 2023 Guppies in large groups cooperate more frequently in an experimental test of the group size paradox, Proc. R. Soc. B 290: 20230790 https://doi.org/10.1098/rspb.2023.0790 [doi.org]

Original Submission