Wave of mutilation: scavenging hermit crabs use social information to locate carcasses

Carcasses are patchily distributed and often short-lived resources, placing scavenging animals under pressure to locate them before they rot or are depleted by competitors. Scavengers may search for carcasses directly, or indirectly, using social information. Aggregations of feeding animals and their conspicuous competitive behaviour may be more readily detectable to searching scavengers than the carcass itself. Moreover, the actions of attendant scavengers upon the carcass, breaking it apart and releasing odour or chemical cues, may further enhance its detectability to others foraging nearby. Here we test this idea. In the first of two experiments performed in the field, we found that hermit crabs (Pagurus bernhardus) were attracted to shelled mussels (Mytilus edulis) that other hermit crabs were already feeding on. They showed no strong tendency to approach aggregations of conspecifics in the absence of food, nor conspecifics that were confined close to mussels but prevented from feeding on them. We speculated that through breaking up the carcass, the feeding hermit crabs released chemical cues and drifting particles of mussel tissue that further attracted other hermit crabs. We tested this in a second experiment, finding that finely chopped mussels attracted significantly more hermit crabs than did intact mussels. We suggest that scavenger feeding action upon carcasses makes these more detectable to others by releasing odour and particle plumes, a form of inadvertently produced social information.