Complex origins of variation in the sexual behaviour of male Trinidadian guppies,Poecilia reticulata: interactions between social environment, heredity, body size and age

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Field observations have shown that there are inter-population differences in the sexual behaviour of male guppies in Trinidad. The greatest differences are between guppies that co-exist with different predators. Here, the sexual behaviour of male Trinidadian guppies was studied to determine to what extent these differences in behaviour evolved in response to selection pressure by the predators, to what extent they are an environmentally induced response to aspects of guppy biology that covary with the predators and to what extent these factors interact. To do this, male offspring of guppies from different predator localities were reared in the laboratory under conditions designed to mimic natural variation in wild populations. Two aspects of young male guppies' social environment were manipulated: (1) population demography and (2) origin (predator locality) of conspecifics. Heredity (origin of the males' parents) was responsible for only a small proportion of the variation in sexual behaviour; social environment had a much greater influence. Also, inter-population variation was found in the degree to which a male's behaviour was affected by demographic conditions and in the relationship between a male's body size and his rate of courtship. Male sexual behaviour also varied with male age and with the origin of the female being courted. Various components of male sexual behaviour (e.g. courtship, mating attempts) were influenced to different degrees by the factors examined. Therefore, inter-population differences in male sexual behaviour result from complex interactions between heritable factors, social environment, male age and male size.