Effects of prior intrauterine position and housing on oestrous cycle length in adolescent mice.

Female mice that had developed in utero between 2 male (2M females) or 2 female (0M females) fetuses were housed individually at 32 days of age in the presence of a male. The 0M females had a significantly shorter cycle. When the females were housed in groups of 5 in the presence of a male, cycle length in 0M females was significantly longer than that of 2M females for the first cycle recorded, but this relationship reversed completely by the third and fourth cycles. These results are compatible with a hypothesis that former intrauterine proximity to male fetuses affects the intrinsic timing of the oestrous cycle and the capacity to emit oestrus-suppressing cues and/or the sensitivity to such cues.