We found high juvenile mortality in both sexes, 23% for males, 30% for females. Offspring of primiparous dams suffered higher juvenile mortality (40%) than those of multiparous dams (17%). Infant survivorship (live-birth to 6 months) was 87.7 % for males and 85.7 % for females. If stillbirths are included, infant survivorship (birth to 6 months) was 66.9 % for males, 70.4 % for females. 75 % of all juvenile deaths (n=16, excluding stillbirths) occurred on the day of birth or the next day. Juvenile and young adult survivorship (6 mo to 12 yrs) was high, on average >99% p.a. Males and females did not differ in survivorship. We found no correlation between juvenile mortality and the inbreeding coefficient. All studies carried out in the wild state that the mortality of calves is approximately 10% and that a large proportion of this mortality is caused by tiger predation. In captivity, juvenile mortality is higher, even though there is no tiger predation. We suspect that a proportion of neonatal deaths escaped observation in the wild.
maximum age in captivity 42 years
Zoo population end 1996. In zoo-born R. unicornis, the sex ratio is male-biased from birth to the age of five years. At sexual maturity, there is still a tendency towards a male-biased sex ratio, whereas in 20-year-old individuals, males and females occur in more similar proportions. In contrast, a female-biased sex ratio was found in the wild population of Royal Chitwan National Park, Nepal. A slight (but non-significant) tendency towards a female-biased sex ratio can also be observed in wild-born animals currently alive in zoos. This partly counterbalances the male-biased sex ratio observed in zoo-born animals. Table 1. Sex ratios in the captive population of R.unicornis. The sex ratios given for the Kaziranga populations are not very reliable because a large proportion (53% and 23% respectivily) of the individuals could not be sexed in these censuses.. Male Female n p (binomial test) zoo-born live births 59.5 % 40.5 % 126 0.040 6 months 60.4 % 39.6 % 106 0.030 5 years 63.2 % 36.8 % 87 0.018 maturity 58.2 % 41.8 % 79 0.177 20 years 54.5 % 45.5 %, 22 0.832 currently alive in zoos wild-born 47.7 % 52..3 % 44 0.880 zoo-born 57.6 % 42.4 % 85 0.193 overall 54.3 % 45.7 % 129 0.379 mature 51.0 % 49.0 % 98 0.920 wild populations (adult animals) Chitwan (1975) 38.1 % 61.9 % 118 0.013 Chitwan (1988) 43.4 % 56.6 % 205 0.069 Kaziranga (1966) 44.7 % 55.3 % 150 0.221 Kaziranga (1972) 51.9 % 48.1 % 391 0.479 Discussion The reason for the male-biased sex ratio of zoo-born R. unicornis is not known. It is possible that this ratio is fixed and coupled with a lower survival rate of male offspring in the wild. Alternatively, it could be a functional adaptation to favourable conditions in the zoos. Since male R. unicornis fight for access to females, females in good condition are expected to produce male offspring, whereas females in worse conditions should produce female offspring. The female-biased sex ratio observed in the wild population of Nepal is probably the result of intraspecific fights among males or of sexspecific poaching.
Lacy (1989) defined the founder equivalent as a concept to quantify the genetic status of a population. The founder equivalent (FE) of a population is the number of equally contributing founders that would be needed to produce the observed genetic diversity. The founder equivalent is usually lower than the actual number of founders, because unequal genetic contributions of founders lower the genetic diversity. The founder equivalent is mainly influenced by the actual number of founders and the evenness of their contribution to the living offspring. In the case of the captive Rhinoceros unicornis population, the founder equivalent is negatively correlated with the sum of the founder representations of the three best represented founders (z = -2.91, n = 37, p = 0.0036). In the captive R. unicornis population, the founder equivalent rose in parallel with the breeding success of additional wild-born animals. After 1974, the founder equivalent dropped because of the death of the only two descendants of the founders #8 and #13, and an increasing over-representation of the three best represented founders (# 5, 7 and 18). After 1980, the founder equivalent slowly recovered, but did not attain its former level until 1995. At present, the founder equivalent (FE = 10.52) is probably still too small to maintain a viable captive population over a longer period. Specialists agree that the genetic contribution of at least 20 individuals is required to maintain a viable population.
The size of the captive population increased steadily between 1950 and 1991, but has remained fairly stable in the nineties. The increase in population size was due partly to the continued introduction of wild-caught animals, and partly to the succesful breeding in many zoos around the world. Of 129 currently living, 85 (66%) were zoo born. However, 18 wild-caught animals were introduced into zoo populations over the last 10 years, the last 1993 in Stuttgart. Of the 33 founders of the current captive population, 24 (including the three best represented ones) were caught in Assam. All in all they have contributed 84.7% of the genes in the captive population. The origin of another 5 founders is uncertain, but probably also in Assam (following capture, four of the five were held at Assam State Zoo, Guwahati, and one was given to Antwerp Zoo as a present from the Assam government). One founder (#157) was caught in eastern India (Bihar State, just south of Nepal), and 3 founders were caught in Nepal.
First reproduction occurred earlier in females (median: 9y 2m, n = 39) than in males (median: 10y 5m, n = 31). The youngest dam giving birth was Studbook #99 at an age of 4y 4m. Studbook #86 was the youngest sire of exactly known age (8y 4m) whose offspring was live-born (#152 was 7y 7m old when his offspring was stillborn two months prematurely). The oldest dam to give birth was #29 at the age of 31y 5m. The oldest sire to become a father was the approximately 42-year-old #10.
Only males have reproduced beyond the age of 32 years. First reproduction occurred earlier in females (median: 9y 2m, n = 39) than in males (median: 10y 5m, n = 31). The youngest dam giving birth was Studbook #99 at an age of 4y 4m. Studbook #86 was the youngest sire of exactly known age (8y 4m) whose offspring was live-born (#152 was 7y 7m old when his offspring was stillborn two months prematurely). The oldest dam to give birth was #29 at the age of 31y 5m. The oldest sire to become a father was the approximately 42-year-old #10. In the present paper, we measured the observed fertility, which is based on all animals recorded in the studbook, including those animals that have never been given the chance to reproduce. If all animals had been allowed to reproduce, the measured fertility would probably be higher. Laurie (1982) reported that in Royal Chitwan National Park the mean age of females at first parturition is about 7.1 years. Thus, on average, females reproduce earlier in the wild than in captivity. In contrast, Dinerstein and Price (1991) reported that 'all but one'of the breeding males observed in Chitwan were older than 15 years, which is much older than the captive breeding males.
