Evidence for extended action of gonadal hormones on the organization of sexually dimorphic behavior and morphology in gray short-tailed opossums (Monodelphis domestica)

Male and female gray short-tailed opossums were gonadectomized (GDX), or tr eated with the estrogen receptor antagonist tamoxifen citrate (TX), or corn oil (OIL) (control) during the 5th postnatal week, a time period equivalen t to the 3rd postnatal week in rats and associated with high levels of circ ulating gonadal hormones and neural aromatase activity in this marsupial sp ecies. In adulthood following gonadectomy (for animals not previously gonad ectomized) and replacement therapy with estradiol or testosterone, GDX male s showed less male-typical scent marking and had shorter phalluses than OIL and TX males. Following replacement therapy with estradiol, GDX females we re more likely to fight with and less likely to mate with stimulus males th an TX females; OIL females were intermediate in these measures. Along with previous findings, these results suggest that gonadal hormones act over an extended postnatal period to organize sexually dimorphic behavior and morph ology in male gray opossums and may have some effect on the organization of aggressive behavior in females of this species. (C) 2001 Academic Press.