On the importance of evolution to developmental psychobiology

Sex differences and the development of social behavior in a marsupial, the gray short-tailed opossum (Monodelphis domestica)

The effects of sex and age on social behavior were examined in gray short-tailed opossums (Monodelphis domestica), small didelphid marsupials. Each animal received five behavior test batteries spanning prepubertal to postpubertal ages. Each test battery consisted of two tests with animals of the same age, one with a male and one with a female. Precopulatory behavior toward females, intermale fighting requiring test interruptions as well as scent marking behavior were seen at higher levels in males than in females and were seen more frequently around and after puberty than before puberty. Females showed more threat behavior than males in mixed-sex and in same-sex interactions. This sex difference was apparent after puberty in tests with male partners and prior to as well as around puberty in tests with female partners. Because climbing over and boxing with another animal were seen more frequently prior to than after puberty, these behaviors may be elements of play fighting (i.e. attack and defense without submission and threat). These findings are discussed with respect to the role of gonadal hormones in the organization and activation of behavior and with reference to their comparative significance in mammals.