Novelty seeking in periadolescent mice: sex differences and influence of intrauterine position.

In rodents, beside basic sex differences, a certain degree of within-gender phenotypic variation can also be provided in utero by hormones from adjacent fetuses. We investigated novelty-seeking behavior in two groups of male and female mice from know intrauterine position: 2M (between males) and 0M (between females). Subjects were assessed during periadolescence (postnatal days 33--43), an ontogenetic phase, which is characterized by an elevated expression of this novelty-seeking behavior. Periadolescent mice underwent a familiarization session for 3 consecutive training days with one side of a two-chamber apparatus. On testing day 4, the opening of a partition, which allowed mice to freely move from the familiar compartment to a novel one, produced an increased behavioral arousal in all animals. Marked sex differences were found, with females being in general more active than males, whereas the latter showed significantly higher levels of novelty seeking than females. Uterine position failed to affect the profile of novelty preference in females, whereas within the male group 2M subjects expressed a marked profile of novelty seeking. The differential titers of sex hormones reported to characterize the 0M and 2M condition early in fetal development are suggested to account for the individual variability in the seeking for novelty within the male group during puberty.