Male sexual rest affects litter sex ratio of newborn Norway rats

Associations between male sexual rest and litter sex ratio have been reported in Norway rats, Rattus norvegicus, but the direction of the effect has been inconsistent. Female rats were paired with either a male that had copulated to satiety three weeks previously or a male that had copulated within the previous four days. Females were paired during both a cycling oestrus and a postpartum oestrus. Sex-ratio biases were only observed in litters conceived during a cycling oestrus. Litters sired by rested males had higher sex ratios than litters sired by unrested males. This relationship was mediated by differences in the timing of mating events, including length of post-ejaculatory interval and the time at which effective mating was-completed. Sex ratio was negatively associated with time of mating completion, replicating earlier findings. Only litters that had fewer pups at birth than at implantation showed the relationship between sex ratio and rest-mediated mating behaviour. These litters were not smaller than litters without loss, however. Post-implantation loss was more likely to occur when uterine horns were crowded, suggesting that this type of sex-ratio biasing occurs in the context of adaptive litter reduction. This association between paternal behaviour, paternal condition and offspring sex ratio prompts the hypothesis that females use the timing of mating behaviour to evaluate heritable traits that might affect male mating success, and then adjust offspring sex ratio to maximize offspring reproductive success. (C) 1996 The Association for the Study of Animal Behaviour.