  Trivers (1985) describes an apparent case of reciprocal altruism between non con-specifics. On tropical coral reefs, various species of small fish act as ‘cleaners’ for large fish, removing parasites from their mouths and gills. The interaction is mutually beneficial—the large fish gets cleaned and the cleaner gets fed. However, Trivers notes that the large fish sometimes appear to behave altruistically towards the cleaners. If a large fish is attacked by a predator while it has a cleaner in its mouth, then it waits for the cleaner to leave before fleeing the predator, rather than swallowing the cleaner and fleeing immediately. Trivers explains the larger fish's behaviour in terms of reciprocal altruism. Since the large fish often returns to the same cleaner many times over, it pays to look after the cleaner's welfare, i.e., not to swallow it, even if this increases the chance of being wounded by a predator. So the larger fish allows the cleaner to escape, because there is an expectation of return benefit—getting cleaned again in the future. As in the case of the vampire bats, it is because the large fish and the cleaner interact more than once that the behaviour can evolve.