With friends like these...

The Twospot astyanax (Astyanax bimaculatus) is normally pretty innocuous, swimming in groups up to 50 strong in clear rivers and ponds in South America and happily eating a mixture of plankton, plants and debris. But when scientists tried to find a way to deter the fish from haplessly swimming into the machinery of hydroelectric power stations and causing a nuisance, they discovered that astyanax is actually one of the most treacherous fish around.

Small fish like astyanax normally shoal in large numbers as a defence mechanism against predators, as the safety offered by a large group means that it’s harder to target individual fish. But the principle of “safety in numbers” was turned completely on its head when the two-spot was faced with simulated predators that the scientists had introduced as part of their experiments.

The scientists were shocked to observe that when threatened with the prospect of becoming lunch, the group would deliberately turn on one of their own and attack it, turning the unlucky victim into easy pickings for the predator so that the rest could get away.

This cruel but useful ploy is seemingly unique in the animal world and no other species has been observed to turn selfishness into a survival tactic so effectively. So perhaps there really is something to be learned from the old adage that if you and your friend are being chased by a hungry bear, you don’t need to outrun the bear, you just need to outrun your friend...