In a fascinating discovery, biologists have learned that moray eel and trout join forces when hunting.
The fine line separating us humans from the “baser” animals has, once again, been transgressed. Whereas human affinity with our close evolutionary cousins the chimpanzees is to be expected, sometimes our supposedly uniquely “intelligent” attributes are recapitulated on surprising branches of the evolutionary bush.
A recently published paper in the journal Current Biology provides some surprising evidence of complex hunting behavior, including careful planning, decision making, and interspecies communication, behaviors previously thought to be restricted to humans and chimps. According to the work of Vail et al. (2014), however, these behaviors have been observed in the coral trout, which apparently communicates with and hunts alongside moray eels.
The coral trout (Plectropomus leopardus), a close relative of the grouper, is an active hunter that lives in and around coral reefs. While the trout is speedy enough to chase down prey items in the open, they are neither small enough nor flexible enough to swim after any potential food that escapes into the nooks and crannies of the reef; this is where their remarkable behavior comes into play.
Moray eels are not powerful swimmers, but they are quick strikers with sinuous bodies capable of winding their way through the twists and turns of a coral maze after their prey. When a coral trout chases a fish into the reef, it will search out a moray eel, and through the use of movements, direct the eel in its hunt for the fish. These include things like headstands and body pointing, indicating to the pursuing moray eel where the prey has hidden.
Of course, when the eel finds the fish, it will try to eat it, and if it does, well, then the coral trout is just out of luck. But, just as often, the moray’s hunt concludes with simply flushing the prey out of its hidey hole in the reef and back out into the open, where the coral trout is waiting for it.
Even cooler than this behavior, however, is the fact that the coral trout are apparently capable of identifying individual moray eels and remembering which was more helpful or more useful in the hunt. This was done through a rather ingenious aquarium set up: the researchers trailed a bit of bait fish on a line into a bit of coral, forcing the coral trout to go into “sheepdog” mode and communicate with one of two moray eels.
However, the eels in the experiment were eels with a difference: they were puppets. One of these puppets behaved admirably, taking directions quickly and driving the bait out of the reef and into the waiting maw of the coral trout. The other puppet, however, was not nearly as responsive or helpful, and the coral trout quickly learned to tell the difference.
In repeating the experiment, the trout would preferentially find and communicate with the helpful eel, ignoring the unhelpful one.
It’s easy to see how such a behavior would evolve quickly in these critters; even a tiny bit of help in the hunt would be incredibly advantageous for the individuals expressing the behavior. Of course, it is unclear what exactly is going on inside the head of the trout when it does this; our evolutionary similarity to chimpanzees, who exhibit similar cooperative hunting, allows us to make hypotheses related to the neurophysiology of our cousins based on this relationship.
Fish, however, have a deep evolutionary history, and our tetrapod ancestors split from that group sometime in the Devonian Period (~390 million years ago). As such, hypotheses explaining the mechanics of fish brains require a bit more work to develop and evaluate.
However, it is a remarkable example of how intelligence and communicative behaviors can be strongly selected for in evolutionary history, and suggests that our common view of “dumb fish,” basically behaving as eating-and-breeding organic machines, needs to be reevaluated.
Vail, A.L., Manica, A., and Bshary, R., 2014, Fish choose appropriately when and with whom to collaborate: Current Biology, v. 24, p. R791-R793