The development of seasonal variation in gonadal hormones and body weight in the maturing squirrel monkey

Captive-born squirrel monkeys of three different subtypes (Colombian, Bolivian, Peruvian), ranging in age from 1 to 4.5 years for males and 1 to 3.5 years for females, were examined at 3-month intervals throughout the year for seasonal changes in levels of plasma testosterone and progesterone and in body weight. Cyclical activity of testosterone in males began between 2 and 3 years of age but became more defined by the time the animals were 3–4 years old. Peak values occurred in February and May and corresponded to the the time of year in which adults normally mate. Elevations of progesterone occurred initially at approximately 2.5 years of age in February and May, and mainly infernales of the Colombian subtype. However,unlike levels of testosterone in males of this age, which declined in August and November (following the typical breeding season), levels of progesterone in females continued to increase during these months. Seasonal changes in body weight were also found in males but these changes were less evident than, and did not correlate with, hormonal changes.