Male mating behaviour and sperm production characteristics under varying sperm competition risk in guppies

Since natural populations of guppies, Poecilia reticulata, often differ fro m one another in social structure, the intensity of sperm: competition is l ikely to vary between localities. Guppies are promiscuous, with female choi ce for colourful males playing a central role in the mating system. In addi tion, male guppies use forced copulations to circumvent female choice. Both methods of copulation are used interchangeably by individual males, but th e degree to which either is used may depend on the social environment into which males are born. Here we show that male mating behaviour varies accord ing to the rearing sex ratio: when reared in male-biased groups, males perf ormed more forced copulations and fewer courtship displays but showed the o pposite pattern of behaviour when reared in female-biased groups. Our predi ction, based on sperm competition theory, that stripped sperm number would reflect social structure was not supported by our results. Instead, the ove rall level of sexual activity (gonopodial thrusts + sigmoid displays) was a better predictor of sperm number in the different groups of males. Rearing density, where sex ratio was controlled, did not significantly affect male mating behaviour or sperm traits. Males reared under the different sex rat ios continued to show their characteristic behaviour patterns when placed i n equal sex ratio tanks. We conclude, therefore, that males adopt mating st rategies to suit their social environment, and that these strategies remain fixed, for short periods at least, if population structure changes. (C) 19 99 The Association for the Study of Animal Behaviour.