It's a common (but evil) survival strategy in zombie movies: Injure somebody else and run like hell while they get eaten. But humans aren't the only bastards who do this. Some shoaling fish also use this selfish tactic when they're being chased by predators, according to new research.

Across the globe, numerous animals live in groups, mostly because it helps them find food and avoid predators. But group living is not without its own costs. There's more competition for the food you find. Plus, being in constant close quarters with other individuals increases your risk of contracting diseases.


Worst of all, group living gets difficult when "selfish" individuals come into the fold. Scientists often see passive, or indirect, selfish behavior in animals — for example, they may see an animal hide behind its neighbor to escape from a predator. "That's common to observe," says Robert Young, a biologist with the University of Salford Manchester in the U.K. "They may be hiding behind someone else, but they're not actively pushing someone forward."

Active selfish behavior — where an individual benefits by injuring or exposing a group member to a predator — has never been recorded. Until now.


A few years ago, Young and his colleague Flávia de Oliveira Mesquita thought they observed such behavior while doing research on behavioral barriers. The scientists were trying to figure out ways to keep fish out of areas of risk, such as turbines, by using different deterrents, including strobe lights and mock predators. To their surprise, they saw group members become aggressive towards one another in the presence of active (chasing) predators.

You see, scientists have long known that numerous fish have specialized cells that release chemical alarm cues when the skin is damaged. These substances elicit a fear reaction and antipredator behavior in nearby individuals, where they become more alert, more aware of their environment and swim faster. But this alarm cue can also attract predators, which may target the injured fish releasing the chemicals. So Young wondered: Were the fish in his study trying to damage each other's skin to draw predatory attention away from themselves?


The two-spot astyanax. Courtesy of Vinícius Goulart.

To find out, Young and zoologist Vinícius Goulart took a look at the antipredatory behavior of the freshwater fish, Astyanax bimaculatus, also known as the two-spot astyanax. These shoal fish are a commercially available, endemic species in Brazil. "They are essentially a prey species and are very commonly preyed upon by other species of fish," Young says. "There was nothing in the literature to suggest there was anything exceptional about them."


The researchers collected 64 two-spot astyanax from fish farms, which ensured the fish had no previous interactions with predators. They divided the fish into eight equal groups and then observed how the fish reacted in response to four stimuli: An active search predator, a sit-and-wait predator, an avian pecking predator and a white plastic container. The groups went through three trials (each five minutes long) for every stimulus.

Experimental setup for sit-and-wait predator trials. Courtesy of Elsevier.

For the active search predator trials, they put a resin replica of a natural predator, trahira (Hoplias malabaricus), into the aquarium and then chased the shoal fish with the mock predator using a thin wire (the puppeteer was hidden from the fish's view). In the sit-and-wait trials, they put a spring-loaded trahira replica into a PVC pipe and made it jump out every time the fish passed near the end of the tube. They used a resin replica of a generic heron bird for the avian pecking trials — the fake predator pecked the water every 15 seconds.


The researchers found that the fish would chase and bite each other when there was an active search predator in the tank — this occurred in every trial in every group. However, it wasn't an all-out group brawl; instead, one fish would usually attack another, or two fish would gang up on one. After getting bitten, the fish would leave the shoal momentarily, only to return a few seconds later. Then it would happen all over again (perhaps with other fish involved). Comparatively, the fish's aggression didn't increase in the other predator trials, though they did show other anxious behaviors, such as swimming faster.

"You will see low levels of aggressive from time to time in normal interactions," Young says. "But we've never seen this kind of intensity before, where the actual aggressive interactions that occur more than a double."

Importantly, the fish exhibited this aggressive behavior from the very first active search predator trial, showing that the behavior is innate, not learned. However, the fish did chase and bite more with successive trials, suggesting, in a sense, that they could become better at evading predators in the wild over time. And tests showed that the fish did indeed release their alarm chemicals after getting bitten (note: the bites didn't draw blood, they only removed scales).


Sections of A. bimaculatus skin, with stained mucous cells (asterisk) and alarm-substance cells (arrow). Courtesy of Elsevier.

But why didn't the fish become aggressive with other types of predators? "The active search predator, from a biological point of view, makes the most sense," Young says, explaining that if a predator is chasing your group, you would want to make another individual more attractive to the hunter. But sit-and-wait predators don't chase, they just grab the fish nearest to them, so biting another individual in that situation would be pointless. Avian predators, on the other hand, won't be able to utilize the chemical cues to find the weakest link in the group, so in this case directed aggression is also useless.


Young is now interested in identifying the individuals involved in the attacks — in his study, he was unable to tell if certain fish were repeated offenders or if other fish were prime targets. He is also curious about whether fish in larger groups also display such aggression. "As the group size gets bigger, maybe they don't feel the need to do this," he says. "This could give us a handle on how individuals might perceive their risk of predation."

Young and Goulart detailed their work recently in the journal Animal Behaviour.

Top image via NOAA/Flick (the fish in this image are not two-spot astyanax)