Abstract 10.1002/zoo.1430080503.abs Publication of a studbook for Macaca silenus in 1983 provides detailed demographic information on the captive North American population. These data indicate that females reach sexual maturity at about 4 years of age, whereas onset of reproduction for males occurs at age 5. One-third of all sexually mature females and 57% of sexually mature males have not produced offspring. Maximum fertility of males and females is reached at 6 and 7 years, respectively, but in later life males show less effect of age on reproduction than females. The interval between births for lactating females averages just over 17 months. Lion-tails show no evidence of birth seasonality. Physiological studies in the San Diego Zoo colony revealed that the menstrual cycle is 31 days in length, and is characterized by a peak in estrogen excretion at midcycle, just prior to ovulation. Behavioral studies indicate that males require multiple mounts to achieve ejaculation. Copulation is nearly exclusively a phenomenon of the follicular phase of the cycle.