Period of maximal susceptibility to behavioral modification by testosterone in the golden hamster

Male hamsters castrated on the day of birth (Day 1) and female hamsters were treated with the free form of testosterone (100 μg/day) on Days 1 and 2, 3 and 4, 5 and 6, 7 and 8, or 9 and 10 postnatally. Following androgen treatment in adulthood, animals treated on Days 1 and 2 or 3 and 4 showed significantly higher mounting and intromission frequencies than animals treated later in life. Sexual receptivity measures following ovarian hormone treatment showed no differences among the male groups, whereas females treated on Days 1 and 2 or 3 and 4 were significantly lower in sexual receptivity measures than females in other treatment groups. Histology of the adult ovaries indicated no modification of normal function in any treatment group. In a subsequent experiment, Day 1 castrated male and intact female hamsters were treated with the free form of testosterone on Days 1–5 (40 or 100 μg/day), 6–10 (40 or 100 μg/day), or Days 1–10 (50 μg/day). Masculine behavior measures were significantly higher in males treated Days 1–10 than in other groups. Among the females, masculine behavior was highest in those treated Days 1–5 postnatally. Sexual receptivity in both males and females was significantly depressed by testosterone treatment Days 1–10 postnatally. Ovarian histology also revealed alterations in gonadal function in females treated Days 1–5 and 1–10 postnatally. Compared with previously published findings, these data suggest that testosterone can be as effective in inducing behavioral masculinization and defeminization as testosterone propionate, provided that treatment extends over a prolonged period during early postnatal development.