The threat display of the gray reef shark(Carcharhinus menisorrah)results from a conflict between attacking and fleeing. The form of the display has been derived from the act of attacking (biting) prey and is regarded here as inhibited or intention attack of basically the same form as that directed to prey. Head up, and mouth open and moving, is incomplete biting. The position of the pectoral fins, one or both held straight down, provides resistance to body displacement when the head rips laterally through the prey. The arched back may be necessary when simultaneously raising the head to bite and lowering the tail to maintain equilibrium while continuing to swim. Since the behavior appears to communicate threat, selection will have favored an exaggeration of posture and the mode of progression.