Macaca thibetana at Mt. Emei, China: II. Birth seasonality.

Birth censuses were conducted every 2 or 3 days for each of six groups of Macaca thibetana along trails at Mt. Emei in southwest China from March 7 to June 15, 1986. Based on direct observations and the timetable of forehead hair growth and behavior, each of 32 infants could be placed in one of sixteen 14-day periods of the 1986 birth season. The mean estimated birth date was March 27 (SD = 39 days); the median estimated birth date was March 14. Sex ratios in newborns and yearlings did not deviate significantly from 1:1. Seasonal birth timing was correlated with the altitude of the range (r = -0.84, P < .05); that is, infants were born earlier in the season at higher altitudes.