Growth hormone and thyroid stimulating hormone concentrations in captive male orangutans: Implications for understanding developmental arrest

There are two morphs of reproductive male in orangutans. Both morphs span t he age range from adolescent to adult, but "subadult" males are smaller in size and lack secondary sexual features. In this study, urine samples were collected over a 2 year period from 23 captive male orangutans in order to define the endocrinology of this apparent arrest of secondary sexual develo pment. Three males were juveniles, 3 to 5 years of age; seven males showed no secondary sexual trait development and were over 7 years of age; six mal es were in the process of developing secondary sexual features, with the yo ungest male being 6 years of age; and seven males were fully mature adults. Morning samples were analyzed by radioimmunoassay for levels of growth hor mone (GH) and thyroid-stimulating hormone (TSH) and group hormone profiles were compared by analysis of variance. GH is the primary hormone of growth and development and its increase in teenage boys is associated with the ado lescent growth spurt. TSH stimulates the thyroid to produce and secrete hor mones that have metabolic effects and required for normal growth and develo pment. Results show that arrested adolescent male orangutans have a GH leve l about 1/3 that of developing adolescents (P = .0006). TSH levels do not d iffer significantly between arrested and developing adolescents. These data complement other endocrine data showing significantly lower levels of sex steroids and luteinizing hormone (LH) in arrested males than developing mal es [Maggioncalda, 1995a,b; Maggioncalda et al., 1999]. Together with docume nted behavioral differences between reproductive males with and without sec ondary sexual features, these endocrine data support the hypothesis that in male orangutans there are alternative developmental pathways and correspon ding alternative reproductive strategies. (C) 2000 Wiley-Liss, Inc.