 Reference Base  Past and future breeding of the Indian rhinoceros in capt... |
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| Location: |
Captive |
| Subject: |
Ecology - Population |
| Species: |
Indian Rhino |
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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.
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