Variation in phenotype due to random intrauterine positioning of male and female fetuses in rodents.

Rodents are polytocous mammals, and male and female fetuses can develop in utero contiguous to fetuses of the same or opposite sex. This paper describes experiments demonstrating that random intrauterine positioning of male and female fetuses results in within-sex variation in phenotype in mice and rats. This phenomenon provides a clear example of the degree to which the intrauterine environment can bias development in terms of effects on morphology, physiology and behaviour. I propose that individual differences in reproductively-related characteristics based on prior intrauterine position may play a role both in the regulation of population size in rodents and in the reproductive success of individuals as changes in population size occur.