Copulation, dominance, and plasma androgen levels in adult Rhesus males born and reared in the laboratory

Abstract

The subjects consisted of adult male rhesus macaques (N = 22) that had been born and reared in the laboratory and had failed to ejaculate in standardized tests of sexual behavior in adulthood. As infants they had been given regular but limited social experience, and as juveniles and adults they had been housed in individual cages. Saphenous vein blood (3 ml) was taken on each of 5 days; the plasma was removed and stored for later assay for testosterone (T) and dihydrotestosterone (DHT). The males were then released into a 1-acre outdoor enclosure with 8--14 adult females. Behavior was observed daily for 2 hr, and blood was taken from the males once a week for hormone assays. During the first 8 weeks, four males ejaculated, and they were removed from the enclosure. During the second 8 weeks, four additional males ejaculated, and they were also removed from the enclosure. The remaining 14 males were never observed to achieve intromission or to ejaculate during the 28 weeks in the enclosure, but 11 masturbated to ejaculation. There was no evidence that individuals with high plasma T levels were more sexually active or more dominant than males with lower T levels. Mean levels of plasma T were within the range observed in wild-born rhesus males, but the level of sexual performance was not. At the end of the 12th week, all of the females initially in the enclosure were removed and 13 different adult females were introduced. Three females became pregnant.