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The European rabbit, both in its wild and domesticated forms, has been a pioneer species in the study of mammalian chemical communication, and illustrates well the difficulty of understanding the functional significance of these often complex signals. Here we investigate the performance of one of the rabbit's most conspicuous chemical signaling behaviors, chin marking (chinning), and the hypothesis that this expresses social dominance. In tests of 21 chinchilla-strain sexually mature males we predicted 1) that animals would show marked and stable individual differences in the frequency of chinning, 2) that these differences would correlate with behaviors associated with dominance such as intrasexual mounting, and 3) that individual differences in the frequency of chinning and dominance-related behaviors would correlate with individual differences in a commonly used physiological indicator of dominance, concentration of serum testosterone. Supporting these predictions and consistent with previous reports, animals showed large and stable individual differences in the frequency of chinning which correlated with the behavioral indicators of dominance and less strongly, with serum testosterone. As our animals had been kept in single cages and without direct contact with other males since weaning, these findings raise the question as to how and when during development such differences among individuals arise. We are currently investigating the possible relation between pups' intrauterine position, postnatal competition among littermates for milk and thermally advantageous positions in the litter huddle, and later differences in indicators of dominance such as those reported here.

Social hierarchy plays an important role in the lives of gregarious species. We investigated how social hierarchy between male Mongolian gerbils (Meriones unguiculatus) housed together before sexual maturity affected their scent-marking behavior, endocrine status and physical traits as adults. The social rank between paired males was assessed based on sexual activity in adulthood at 18 and 20 weeks of age. In most cases, the male that showed higher sexual activity at 18 weeks also exhibited greater sexual performance at 20 weeks, suggesting a precedence relationship between paired males regarding reproductive opportunity. Sexually active males scent-marked more often than their cage mate, particularly after a sexual experience. No significant differences in physical traits (e.g., the weight of the ventral gland, testes and adrenal glands) or endocrine status (e.g., fecal testosterone and corticosterone levels) were found between the two males. These findings suggest that social rank between familiar males formed during cohabitation, and rank was closely correlated with scent-marking activity, but not with endocrine status.