Variation in Schooling and Aggression Amongst Guppy (Poecilia Reticulata) Populations in Trinidad

Schooling behaviour is an effective defence against predation but, since it depends on coordinated behaviour, it may restrict individual competition for limited resources. We tested the hypothesis that levels of individual aggression will be reduced in fish with a high schooling tendency by comparing the behaviour of guppies, Poecilia reticulata, from eight Trinidad populations. Schooling tendency was assayed in the wild. Fish were then transferred to the laboratory where aggression was measured when groups of eight males foraged on a small, but profitable, food patch. An inverse relationship between schooling tendency and intensity of aggression emerged. Guppies from populations without fish predators displayed a wide range of aggressive behaviours including active patch defence by the most dominant individual. Conversely, populations experiencing high predation, and with well-developed schooling behaviour, showed little aggression. Female guppies (three population tested) also varied in level of aggression and aggression was present in laboratory-bred as well as wild-caught fish. Individual aggression increased with group size but was independent of tank size. These results point towards a trade-off between antipredator behaviour and resource defence and confirm that schooling has associated costs as well as benefits.