Variation in circulating and excreted estradiol associated with testicular activity in male marmosets

Concentrations of estradiol (E 2 ) are high in the urine of male marmosets, and links between E 2 and paternal behavior have been proposed in black tufted‐ear marmosets, Callithrix kuhlii . However, it is not clear whether urinary E 2 in male marmosets: 1) represents production of E 2 associated with testicular activity, 2) is associated with adrenal steroid production, or 3) merely reflects peripheral conversion of T to E 2 prior to excretion. We tested the hypothesis that urinary E 2 in male marmosets represents estrogen production‐associated activity in the hypothalamus‐pituitary‐gonad (HPG) axis. We treated adult male marmosets with gonadotropin‐releasing hormone (GnRH), and used saline‐treated males as controls. We collected blood and urine samples from males before and after treatment, and assayed them for testosterone (T), estradiol (E 2 ), and cortisol (CORT). Treatment with GnRH increased circulating T and E 2 , and prevented decreases in levels of urinary T and E 2 . Moreover, changes in plasma and urinary E 2 after treatment were positively correlated with post‐treatment changes in T. Thus, our data are consistent with both plasma and urinary E 2 in male marmosets increasing as a result of testicular stimulation. However, treatment with GnRH did not affect plasma or urinary CORT concentrations of males, suggesting that the E 2 excreted by males is not of adrenal origin. We also compared urinary T, E 2 , and CORT levels between intact and castrated male common marmosets ( Callithrix jacchus ). Urinary concentrations of T and E 2 , but not CORT, were significantly lower in castrated than in intact males, further suggesting that E 2 in male marmosets varies with testicular activity. Am. J. Primatol. 56:27–42, 2002. © 2002 Wiley‐Liss, Inc.