ABSTRACT
Pregnant ewes were implanted with 1 g testosterone between days 30 –80, 50 –100, 70 –120 or 90 –140 of gestation. Ewes treated between days 30 –80 and 50-100 showed increased aggressive behaviour and clitoral enlargement, whereas this was not seen in the day 70 –120 or 90 –140 groups. Although the implants released similar amounts of testosterone at all stages of gestation, plasma testosterone concentrations were lower in the day 70 –120 and 90 –140 groups. The increased testosterone clearance in late gestation could be a result of increased placental aromatization. Masculinization of the genital tubercle was complete in female lambs in the day 30 –80 group, partial in the day 50 –100 and 70-120 groups, and absent in the day 90-140 group. These results therefore suggest that the ‘critical period’ for masculinization of the external genitalia is between days 40 and 50 of foetal life. In contrast to this anatomical masculinization, one aspect of behavioural masculinization had a later critical period, since female lambs of 30 –80, 50 –100 and 70 –120 day groups adopted male-like urination postures, whereas the day 90 –140 offspring showed the normal female pattern.
