Changes in testosterone and behavior during adolescence in the male rhesus monkey.

Five adolescent male rhesus were followed for 3 years, from age 2 through age 4, with the collection of monthly testosterone levels, along with observation of behavior. Testosterone levels showed initial rises by age 3, but did not show the adult male seasonal pattern until 4. Play behavior fell significantly from age 2 to age 3, while sex behavior showed seasonal increases in both Year 2 and Year prior to the rise in plasma testosterone. The number of adult males in the social group may influence the timing of the rise in testosterone during adolescence. Injury may also serve to delay the increase in testosterone in males during their third year of development.