THE ANNUAL REPRODUCTION CYCLE OF A FREE-LIVING POPULATION OF LONG-TAILED MACAQUES (MACACA FASCICULARIS) IN SINGAPORE

Abstract

A two-year field study was conducted on seasonal variation in the reproductive behaviour of a free-living population of Long-tailed macaques in the Singapore Botanical Gardens (latitude 1N), Visits were made at least twice a week in the morning. A fixed route was followed. All the monkeys in sight were counted and counts were made of the different categories of young and sexual incidents seen. The results indicate an annual cycle of two discrete mating seasons, the first from about December to March and the other from July to October. These occurred simultaneously in all the four troops studied, Each mating season had at least one mating peak. Discrete birth seasons resulted from these mating seasons but one period of relatively great sexual activity in May 1966 failed to produce any young. The fertile mating seasons coincided with the North-east and South-west monsoon seasons; the onset of each appears to be correlated with increasing day length and low daily maximum temperatures. The periods between mating seasons were characterised by high daily maximum temperatures; they also coincided with periods of high relative humidity. The precise nature of the stimulus, or stimuli, triggering synchronous mating remain obscure. These stimuli, however, apparently affect both sexes simultaneously. Increased sniffing of the genitalia of adult females by adult males was as good an indication of the occurrence of mating seasons as copulation. Mounting of females without intromission of the penis occurred during all month of the year and did not follow fluctuations in copulation.