The reproductive biology of Nilgiri tahr (Hemitragus hylocrius) was studied in Eravikulam National Park, Kerala, India from August 1979 to September 1981.
The rutting season occurred during the monsoon (July and August) and the main birth season was January to mid-February. Gestation was estimated at 179 days.
Three females which lost their first young in 1981 conceived 18 to 21 days post-partum. One female conceived of her second young while the first was 15 days old. The remainder of the individually identified females gave birth to one young (n=40) or none (n=4).
Infant mortality was highest during the first two weeks after birth. Young born during the monsoon also had a high mortality rate (0.50/week).
Oestrus during the rut was discontinuous in 1980 and 1981. Periods of frequent oestrus corresponded with periods of low rainfall with a mean lag time of 4.9 days.
The ultimate factor determining the birth season in Nilgiri tahr appears to be weather. The main birth season is timed so as to minimize thermal stress.
Several proximal factors may be important in stimulating reproductive activity. These include photoperiod, temperature, the presence of males in mixed groups, and nutrition. The photoperiod response is unusual in that Nilgiri thar evidently respond to the abrupt decrease in apparent day length associated with the onset of the monsoon.
The inverse correlation of rainfall and oestrous activity may have been effected through the sudden decrease in thermal stress on the females or heavy rainfall (5.7 cm/day) masking pheromonal stimulation.
The pattern of lactation anoestrus observed in Nilgiri tahr approximates results obtained from some studies of domestic Caprini.
Nilgiri tahr females are presumably able to increase their lifetime reproduction by giving birth twice in one year. However, this strategy seemed counterproductive in 1981 as survival was low for second offspring.