Selfless guppies don’t fall prey to the volunteer’s dilemma

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.”

Read more: When will fairy wrens help other fairy wrens?

Padget and colleagues put a clay model pike into tanks containing either five, 10, or 20 guppies for seven minutes at a time.

They found that guppies in the large group were most likely to approach and inspect the terrifying model, approaching it an average of 14 times each trial.

Guppies in the medium group were least likely to approach, averaging seven approaches per trial.

They were also the most likely to hide behind plants and gravel in the tank.

Trinidadian guppies
Trinidadian guppies. Credit: Rebecca Padget

“We can’t be sure why guppies in large groups cooperated more,” says Padget.

“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.”

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