The law of the jungle can be harsh, but the law of the river is even worse judging by the newly observed behaviour of a fish that attacks the weakest member of the shoal when danger is spotted – leaving it stunned to be gobbled up by predators.

It’s well known that fish gather in shoals to reduce their chances of being eaten by predators, but on a project in Brazil, Professor Robert Young from Salford University’s School of Environment & Life Sciences noticed that, when threatened by a predatory fish, a group of two-spot astyanax will turn on one of its members – biting and ramming it.

The unfortunate fish is then left stunned in the water for a few seconds – easy meat for a predator – while its treacherous fellows get away.

Professor Young first noticed the unique behaviour when trying to find ways of keeping the fish out of hydroelectric machinery in Brazil’s rivers.  Intrigued by this extremely rare response to an attack, he investigated further and the results have just been published in the journal Animal Behaviour.

Alongside colleague Vinícius Goulart of the Pontifical Catholic University of Minas Gerais in Belo Horizonte, Brazil, he took eight groups of eight astynax back to the lab for further study.  The fish were exposed to mock attacks – one from a bird, one from an ambush by a fish from a plastic tube, and a slow approach from a model of a predatory trahira fish.

The backstabbing behaviour only showed during the approach from the trahira –leading Professor Young to conclude that the fish only turn on each other when in a situation where one will be selected by a predator for attack.

This behaviour has never been seen before and arises, Professor Young believes, because of the small size of the shoals.  “In an attack each of the fish has a one in eight chance of being attacked, so this behaviour helps with the odds,” he said.  “This hasn’t been observed before, possibly because most social animals tend to punish members of the group which break rules. However, in the case of the astynax, treachery appears to be tolerated and all of the members of the group are happy enough to join in.  It’s rough luck for the victim, but in the wild it helps ensure that the more ruthless members of the shoal can stay alive and breed.”