Urine-marking by female mice throughout their reproductive cycle

It is commonly believed that the scent-marking activity of female mammals is elevated when they are sexually receptive, yet urine-marking by female mice does not vary in relation to their estrous cycles. We examined the marking behavior of female mice during their reproductive cycle with attention to their postpartum period of sexual receptivity. In the first experiment, females were allowed to mark prior to mating, during gestation, on the evening of parturition, during lactation, and after weaning their pups. They were exposed to a male or female mouse or nothing at each testing period in a counter-balanced fashion. Overall, the subjects marked most frequently in the presence of a male stimulus; their marking rates did not differ when they were exposed to another female or to nothing. The females' marking remained relatively steady through gestation, declined dramatically at parturition, then gradually returned to the levels observed prior to mating. In the second experiment, multiparous females were allowed to mark while they were simultaneously lactating and pregnant. Compared to unmated females, the subjects' marking frequency was low at parturition, declined further to its lowest level about 1 week later, then regained the preceding low level prior to weaning the initial litter and delivering the next. Together these results demonstrate that urine-marking by female mice is diminished just after parturition. Thus, in contrast to some other species, the marking behavior of female mice apparently is not enhanced around the time they are sexually receptive. Because female mice generally mark most frequently when males are nearby, we postulate that a major function of this behavior is to evoke endocrine and behavioral responses in proximate males rather than to attract distant males.