Referential gestures in fish collaborative hunting.

In humans, referential gestures intentionally draw the attention of a partner to an object of mutual interest, and are considered a key element in language development. Outside humans, referential gestures have only been attributed to great apes and, most recently, ravens. This was interpreted as further evidence for the comparable cognitive abilities of primates and corvids. Here we describe a signal that coral reef fishes, the grouper Plectropomus pessuliferus marisrubri and coral trout Plectropomus leopardus, use to indicate hidden prey to cooperative hunting partners, including giant moray eels Gymnothorax javanicus, Napoleon wrasses Chelinus undulatus and octopuses Octopus cyanea. We provide evidence that the signal possesses the five attributes proposed to infer a referential gesture: it is directed towards an object, mechanically ineffective, directed towards a potential recipient, receives a voluntary response and demonstrates hallmarks of intentionality. Thus, referential gesture use is not restricted to large-brained vertebrates.