Reproduction

The Annual Cycle of Reproduction  ( From Cliff's study)

The reproductive cycle of a species is closely attuned to the ecological conditions under which it lives.   The reproductive cycle of Nilgiri tahr is therefore of interest in light of the climatic and  biotic cycles in which it lives.  This information can be compared to Caughley's (1971) work on the closely related Himalayan tahr inhabiting a decidedly temperate regime in New Zealand. Jerdon (1874) stated that the Nilgiri tahr "is said to produce  two young at birth", and this was apparently the prevailing opinion when Fletcher (1911) wrote that he was more inclined to judge one to be the usual number.  Prater (1965) maintained that sometimes two, but more commonly one young are produced.  However, more recent observations in both the wild (Davidar, 1978; Schaller, 1978; and this study) and captivity (Wilson, '1980; Waterhouse pers. comm) indicate that one young per female is the rule.  Davidar (1978) reports one instance of twin fetuses and states that "occasionally, a mother with two young at heel have been observed."  However, I encountered no indications of twinning during the course of my study. So,  although twins may occasionally occur,  the usual number of young is one. 

Detailed information on reproduction during the study was obtained during the birth and rutting seasons of 1981.  Flighty animals, adverse weather conditions, and the lack of individually   recognizable females precluded a comparable accounting  during 1879 and 1980.  The distribution of births was well charted in the winter of 1981, and the occurrences of estrus was observed during the monsoon of that followed.  Unfortunately, the study ended before the births which resulted from the 1981 rut.  However, by good fortune, one of the first females to be collared at the end of the 1980 rut was one of the last females to be bred, and both the breeding and  birth dates were documented.  She was bred on 03 September 1980, and gave birth on 01 March 1981,  an  gestation of 179 days.   This gestation period compares favorably with the one  period of 180 days reported by Winbigler (1977),  and the 180 days  estimated for Himalayan tahr (Caughley,  1971).  This gestation period can be  used to estimate dates of conceptions leading to the 1981 births, as a means of comparing the 1980 and 1981 rutting seasons. In 1981, the first young of the year was observed on 10 January,  and this was considered as the start of the birth season.  From then  until births declined in mid-February, I attempted to find all females in the Vaguvarrai intensive study subpopulation daily Because of the large number of collared females, it was possible to keep track of the subsequent births on a daily basis.  There was an initial peak, with Additional births trailing off until the last of the season on 01 April.  However, there was also a conspicuous gap in the parturitions between 23 January and 31 January. 

This distribution of births spread over a period of 80 days shows a remarkable similarity to the skewed distribution of births calculated by Caughley (1971) for Himalayan tahr,  especially if the Nilgiri tahr distribution curve were smoothed by probit analysis as Caughley's was.  Estrous, as indicated by copulation, tolerance of mounting, and  high frequency of courtship displays was recorded for 28 females.   Only one female was recorded in estrus twice, at an interval of 49 days (15 July-02 September).   Since nearly all sexually mature females gave birth in the main (winter) birth season of 1981, and assuming that this was the case in 1982 (after the study ended), the logical conclusion is that the conception rate (per estrus) was very high (over 90%).  

There were also two distinct periods of estrous activity, separated by a gap of 32 days. In 1981 there was also a series of births which occurred during the monsoon.  Of these births, two were to collared females whose young had died early in the season.  Birw's was first seen on 12 January 1981, already dead. She gave birth to her second young that year 197 days later.  Rrw's first young was born on 15 January 1981 and was missing 14 days later.  She gave birth to her second young 184 days after this. These records indicate that females which lost their young early in the birth season, come into estrus again quite soon, and conceived  again.  The clustering of the other monsoon births would seem to indicate that the other females showed a similar pattern.  Of the individually recognized females which gave birth in the monsoon, two showed definite subsequent signs of estrus 36 and 24 days later.  In New Zealand, Himalayan tahr give birth from November-January,  with the mean day of birth calculated as 26 November (Caughley, 1971).  In the Himalayas, the birth season falls from mid-April—mid-  July (Schaller,  1973).   These spring-early summer birth seasons are typical of temperate ungulate species, and are     presumably an adaptation which favors survival of offspring born at the time of warming temperatures and increasing forage production.