Agonistic Display in the Gray Reef Shark, Carcharhinus menisorrah, and Its Relationship to Attacks on Man

An agonistic display directed primarily toward divers was observed on 23 occasions in Carcharhinus menisorrah at Eniwetok Atoll, Marshall Islands, during January, 1971. This display was filmed in normal speed and slow motion and found to consist of two locomotor elements: [1] laterally exaggerated swimming and [2] rolling and/or spiral looping, and four postural elements: [1] lifting of the snout, [2] dropping of the pectoral fins, [3] arching of the back and [4] lateral bending of the body. This behavior was found to occur under approach-withdrawal conflict situations, and rapid diver approach was shown to be a releasing stimulus. It was a graded phenomenon with the most intense displays occurring when there was maximum escape-route restriction and when diver aggression was initiated while the shark was approaching rather than lateral or departing. Under the circumstances in which it was observed, this display probably expressed defensive threat. It appeared ritualized in nature and is likely to be of value in normal social encounters. This display has been related to attacks on man and may indicate a motivational basis other than feeding for such attacks.