Intraspecific group hunting has received considerable attention by researchers interested in cooperative behaviour and animal cognition. Differences between species in the complexity of the hunting with respect to communication, coordination and food sharing have typically been interpreted as a reflection of differences in cognitive abilities. Here we describe for the first time collaborative hunting where individuals play different roles in a fish species, the yellow saddle goatfish Parupeneus cyclostomus. Adults in our study area may live either solitarily or in relatively stable groups formed of similar sized and most likely unrelated individuals. The solitary life style was associated with searching for hidden immobile prey on sandy areas while group living was associated with collaborative hunting of mobile prey in corals. Any member of a group could initiate a hunt by rapid acceleration. Partners did not simply follow the attacker but deviated around coral formation to block the prey’s escape routes. Prey that escaped into a coral crevice was typically encircled with maximal inter‐individual distance and pried on by insertion of the barbels into the crevices. As home ranges largely overlapped and no between‐group aggression existed, we propose that it is the hunting of mobile prey in a complexly structured habitat that selects for collaborative hunting and hence for the evolution of group living in yellow saddle goatfish.

Collaborative hunting, the coordination of animal behaviour in space and time to capture prey, is reported in several vertebrate species. However, previous studies are observational, hampering our ability to identify individual decision rules that result in collaboration. We experimentally investigated collaborative hunting in yellow saddle goatfish () by exposing pairs to a mock prey that fled to an artificial shelter with multiple entrances. The first fish to initiate the chase (the 'initiator') was always closest to the prey and pursued it directly in its path. Conversely, the behaviour of the second goatfish (the 'follower') depended on its spatial position relative to the initiator. When the follower was less than one body length behind the initiator, it also accelerated directly towards the prey in over 95% of cases. However, if the two goatfish were separated by a distance of one body length or more, the follower chose a less direct route to reach the prey in 87% of cases. In this scenario, the follower often reached the prey's more distant refuge first, which might increase its hunting success or block the prey's escape path under natural conditions. Our findings suggest that coordinated hunting behaviour can result from simple, self-serving decisions.

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