The ‘volunteer’s dilemma’ – doing something to help a group, at some personal cost – has been fascinating behaviourists for decades.

In humans, it’s theorised that the bigger the group, the less likely a person is to volunteer.

But this is not the case for guppies, according to new research in Proceedings of the Royal Society B.

Using a clay “predator” to provoke threats to groups of Trinidadian guppies (Poecilia reticulata), the researchers found the fish were more likely to risk themselves to help larger groups.

“When faced with a possible predator, guppies have to balance risks,” says Rebecca Padget, a postgraduate student at the Centre for Research in Animal Behaviour in the University of Exeter, UK.

“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.”