CYCLES, SEXUALITY, AND CONCEPTION IN FREE-RANGING LANGURS (PRESBYTIS-ENTELLUS)

Reproductive parameters and sexual behavior of free-ranging Hanuman langurs (Presbytis entellus) were studied over a 13-year period at Jodhpur, India in a population that breeds throughout the year. Long-term monitoring of individually identified females living in one-male-multi-female troops revealed a mean cycle length of 24.1 days (n = 113), a mean gestation of 200.3 days (n = 31), menarche at 29.0 months (n = 10), first conception at 35.0 months (n = 12), and a mean birth interval of 16.7 months (n = 114). The loss of unweaned infants accelerated the resumption of menstruation, receptivity, and attractivity; shortened the period between estrus and next conception; and reduced the median birth interval by 20.5%, thus supporting the sexual selection hypothesis of male infanticide. The decline of fertility with age (as measured by the rates of conception per estrus period), a regular postreproductive survival for up to 9 years (which can account for about 25% of a female's life span) and indications for the existence of a "true menopause" support the view that a postreproductive period in langurs is the result of natural selection. There is very limited evidence for situation-dependent receptivity, because only 7.5% of all estrous days (n = 1,037) fell outside of the mid-cycle period with at the same time strongly reduced proceptivity and attractivity. Moreover, postconception estrus behavior showed a stereotypical pattern during 31 pregnancies and did not reflect a situation-dependent response to the immigration of new and potentially infanticidal males. It is likely that females competed for sperm of the harem holder, because the probability of conception increased significantly with an increase in the number of copulations, and when the number of females copulating on the same day decreased.