The giant moray eel can grow to three metres in length and bites its prey with two sets of jaws—the obvious ones and a second set in its throat that can be launched forward like Hollywood’s Alien. It’s not a creature to be trifled with. But the coral grouper not only seeks out giant morays, but actively rouses them by vigorously shaking its body. The move is a call to arms that tells the moray to join the grouper in a hunt.
The two fish cooperate to flush out their prey. The grouper’s bursts of speed make it deadly in open water, while the moray’s sinuous body can flush out prey in cracks and crevices. When they hunt at the same time, prey fish have nowhere to flee.
Redouan Bshary from the University of Neuchatel in Switzerland discovered this partnership in 2006. He usually studies cleaner wrasse, which pick parasites off larger fish. When tracking groupers to see if they patronised several cleaners, Bshary noticed one of them rousing a moray. They also team up with the humphead wrasse—a huge human-sized fish whose extensible jaws can suck prey out from cracks. It too complements the grouper’s open-water skills.
Bshary was fastidious in recording his grouper observations—all 187 hours of them. He also took several films and when Alexander Vail from the University of Cambridge watched them, he noticed something Bshary had missed. The groupers always summoned the wrasses and morays with a vigorous shimmy, but they also used a second, much rarer signal—a headstand, combined with head-shaking. Vail thinks it was a signal, one that said: “The prey’s in here, guys!”
When doing their headstands, the groupers always swam over the location of hidden prey that they had failed to catch. They only used the move when a moray or wrasse was nearby, continued to do so until one arrived, and stopped as soon as one did.
Most morays and all wrasses headed towards the grouper’s location when they saw the signal, causing the prey to break their cover. (The fact that the prey didn’t abandon their hiding spots beforehand shows that the headstand itself isn’t a hunting tactic.) And when the morays ignored the headstand, the groupers actually swum after their partner and either performed their “recruitment shimmy” or forcibly tried to push the eels in the right direction.
To Vail, the headstand has all the hallmarks of an attention-grabbing “referential gesture”, like a human pointing at an object. It’s directed at a ‘listener’, draws attention to an object of interest, triggers a response, and has no other purpose beyond being a signal. It also seems like something the grouper intends to do, rather than a random movement that coincidentally summons a moray—after all, if the signal doesn’t work, the grouper gives up and attracts the eel through other means.
The team also found that another reef fish—the coral trout—uses the same signal to team up with octopuses! The partners have the same set of complementary skills as the grouper and moray—the trout chases exposed prey and the octopus grabs hidden ones.
If this really is a referential gesture, that’s an important discovery. Such gestures are part and parcel of human life, but the only animals that seem to use them are intelligent ones, like chimps and other great apes, ravens, dolphins, and domestic dogs. In fact, the discovery of gestures in ravens was taken as further evidence of their impressive mental abilities.
But Vail and Bshary believe that intelligence is a red herring. The grouper and trout use gestures, and while they may be more intelligent than we give them credit for, it’s very unlikely that they rival apes. Instead, they evolved to use gestures simply because they benefit from coordinated cooperation with other species. Their gestures are driven by needs not smarts. They remind us once again that complex behaviour doesn’t necessarily imply complex minds.
Reference: Vail, Manica & Bshary. 2013. Referential gestures in fish collaborative hunting. Nature Communications. http://dx.doi.org/10.1038/ncomms2781