If you’re being chased by a hungry bear, you’re more likely to escape if you have a friend running alongside you. That way, the bear might kill your friend instead of you.
It could be worse. You could try to trip your friend up or even break their leg, except you’re probably too decent to do that. But a little South American fish has no qualms.
At first glance, twospot astyanax looks pretty innocuous. This small fish swims in groups up to 50 strong, eating a mixture of plankton, plants and debris. Many people in South America keep them as pets.
However, they are a bit of a nuisance. “There’s a big problem with fish entering hydroelectric power station machinery,” says Robert Young of the University of Salford in the UK.
So a few years ago, Young and Vinícius Goulart of the Pontifical Catholic University of Minas Gerais in Belo Horizonte, Brazil, started investigating ways to deter the fish from getting into the machinery. That included a few attempts at scaring the fish away. To their surprise, the pair discovered that the astyanax turned on each other if a predator approached.
“It was a fortuitous discovery,” says Young. He and Goulart have now studied the fish’s treachery in more detail.
Young and Goulart kept twospot astyanax in eight groups of eight, ensuring that all members of each group were roughly the same size. Then they exposed them to three simulated attacks, and a control.
In some trials, the researchers mimicked a predator attack by bringing a resin replica of a hunting fish called a trahira close to the astyanax. In other tests, the fake predator lurked in a plastic tube and “ambushed” passing astyanax. A third test simulated a bird such as a heron trying to peck the astyanax out of the water. Finally, the control experiment involved a plastic box being gently put into the fish tank.
When confronted with the predator attack, the astyanax turned on one fish in the group, which became a target for some of the others to chase. They crashed into it and bit it. The unlucky astyanax fled and hung around on its own for a few seconds before rejoining the group.
The other scenarios, including the control, did not provoke this response. Young says that is because birds and ambush predators do not target weaker members of the shoal: they just go for whichever astyanax is closest. So in these situations, it would not benefit the astyanax to attack one of their number.
“In a life-or-death situation, individuals are selfish,” says Young. “But this is one of the few examples of directly selfish behaviour.”
Safety in numbers
Small fish often protect themselves by swimming in large groups, making it hard for predators to target a specific fish: there is, literally, safety in numbers. In theory, all animals that could become prey have an incentive to attack their fellows when the group is threatened. So why is it only the astyanax that does it?
Young says other species might betray each other in the same way, but perhaps nobody has managed to observe this behaviour yet. “It might be these things are more common than people realise.” However, social animals often punish individuals that cheat the group, and this might deter animals from attacking their fellows.
The astyanax might also be more likely to turn on each other than other prey species, because they live in such small groups. In Young’s experiment, each astyanax had a one-in-eight chance of being targeted by the predator, odds which provide a strong incentive to throw one of their fellows under the bus. But many fish swim in shoals hundreds or thousands strong. For these fish, lost in the crowd, betraying each other may not be worth the effort.
Even when faced with a hungry troll, the three billy goats gruff were never this mercenary.
Journal reference: Animal Behaviour, DOI: 10.1016/j.anbehav.2013.05.041
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