Age‐related hormonal differences in cycling squirrel monkeys (Saimiri boliviensis boliviensis)

This study was planned to evaluate the poor reproductive performance of aged squirrel monkeys in a self‐sustaining breeding colony. Two groups of monkeys aged 6–8 years (Group 1), and >12 years (Group 2) were evaluated. During the midbreeding season, daily blood samples were obtained for a period of 35 days from 10 animals in each group. Blood samples were assayed for serum estradiol (E), progesterone (P), and bioassayable luteinizing hormone (bio‐LH) concentrations and the data were analyzed by ANOVA (analysis of variance). Bio‐LH surges were less frequent in the aged breeders (Group 2). Total P output during periovulatory period (day −;3 to day + 3) and the total P output during the luteal phases of the estrus cycles were significantly lower in aged squirrel monkeys (Group 2, P < 0.003). Although E peaks occurred less frequently in Group 2, they had higher concentrations as compared to Group 1 (P < 0.003). Cumulative periovulatory and luteal phase E levels did not differ between the two groups. The reduction in P output in aged squirrel monkeys may interfere with proper implantation, thus contributing to the lower pregnancy rate observed in this age group. The decreased frequency of bio‐LH surges indicated less frequent ovulations in the aged animals which may further impair reproduction with aging. © 1994 Wiley‐Liss, Inc.