The production of and sensitivity to cues that delay puberty and prolong subsequent oestrous cycles in female mice are influenced by prior intrauterine position.

Four experiments were conducted with CF-1 house mice (Mus domesticus) to examine the relationship between a female's prior intrauterine position (2M = between 2 male fetuses, 1M = next to one male fetus, 0M = not next to a male fetus) and the timing of puberty and length of subsequent oestrous cycles under a variety of housing conditions. The results of Exp. 1 confirmed that the presence of males was required for females to enter puberty and exhibit regular oestrous cycles, regardless of prior intrauterine position. When housed individually after weaning either with a male in the cage or separated by a wire mesh partition, 0M-females ovulated and mated at a younger age and had shorter post-pubertal oestrous cycles than did 2M-females (1M-females were intermediate between 0M- and 2M-females). Other females were also housed after weaning with a male or separated by a wire mesh partition from a male in a variety of social environments (4 0M-females and 1 2M-female, 4 2M-females and 1 0M-female, 5 1M-females, 3 0M-females and 3 2M-females). The objective of these different housing conditions was to determine whether prior intrauterine position influenced the transmission of and/or sensitivity to cues that delay puberty and prolong subsequent oestrous cycles. Relative to 2M-females, 0M-females both produced more potent cues and were more sensitive to the cues (again, 1M-females were intermediate between 0M- and 2M-females). Specifically, in the presence of other females, puberty was delayed and post-pubertal oestrous cycles were prolonged to the greatest extent in 0M-females. Prior intrauterine position is therefore a source of individual variation in the production of and sensitivity to cues that modulate the timing of puberty and the length of subsequent oestrous cycles in female mice. The findings suggest that prenatally-androgenized 2M-females may have a reproductive advantage over other females at high population densities.