Contiguity to male foetuses affects morphology and behaviour of female mice

Abstract

DURING studies on the role of gonadal hormones in aggressive behaviour of mice, we observed large variations in both the morphology and behaviour of females. At birth, variation was evident with respect to the principal external sex-differentiating marker—distance between the anus and the genitals, with some females being difficult to distinguish from males. Moreover, a significant proportion of females, upon reaching adulthood, were more responsive to androgen than others, exhibiting male-like aggressive behaviour more quickly following its administration. These observations led to the speculation that a certain proportion of females had been masculinised naturally during prenatal development. We present here data which show that such masculinisation does occur and that it is mediated by proximity to male foetuses.