POPULATION DIFFERENCES IN THE SCHOOLING BEHAVIOR OF THE TRINIDAD GUPPY, POECILIA-RETICULATA - ADAPTATION OR CONSTRAINT

Populations of the guppy Poecilia reticulata in Trinidad vary markedly in their tendency to school. In many cases this variation in behaviou r can be attributed to variation in the predation regime: guppies that co-occur with the pike cichlid, Crenicichla alta, spend more time sch ooling and form larger schools than their counterparts from low-risk h abitats. However, the association between schooling tendency and preda tion risk is not ubiquitous. In this paper we document the behaviour o f guppies from populations in two Trinidad drainages. Guppies from the (Lower) Aripo River (in the Caroni drainage) display well-coordinated schooling behaviour irrespective of whether they are observed in the wild or raised under standard conditions in the laboratory. By compari son, Oropuche River guppies (from the Oropuche drainage) show only a w eak schooling tendency. The contrast between the two populations is ap parent even in newborn guppies. As pike cichlids are abundant at both sites it seems unlikely that reduced predation risk can account for th e weaker schooling of the Oropuche River fish. The behavioural differe nces in the two drainages are paralleled by considerable genetic diver gence and we therefore consider the possibility of phylogenetic constr aints on the evolution of schooling.