Kitchener, A.C., 1997. Ageing the Sumatran Rhinoceros: preliminary results. International Zoo News 44 (1): 24-34, pls. 1-2, figs. 1-3, tables 1-2
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World |
| Subject: |
Ecology - Population |
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Sumatran Rhino |
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Examined 6 female skulls in Natural History Museum London and National Museums of Scotland in Edinburgh, trying to find a method to age adult skulls. A number of skull features were noted, which seemed to be correlated with increasing age and wear of the (pre)molars in the maxillae and mandibles:
1. Eruption of the upper incisors.
2. Fusion of the premaxillae.
3. Ossification of the nasal septum.
4. Increasing skull surface rugosity.
5. Loss of the lower incisors.
The minimum crown height at the mid-point of the buccal aspect of all premolars and molars was measured using dial callipers. Given the very small sample size, the results must be taken with some degree of caution, but they do indicate, in general, a reduction in crown height of the molars with increasing age. The premolars show a different pattern of wear with an apparent increase in crown height followed by a decrease. This reflects the replacement of the deciduous premolars by the erupting permanent premolars before their eventual wear once fully erupted.
The relatively smooth rate of wear of the molars of female Sumatran rhino provides a potential method for absolute ageing, if they can be calibrated with known-age individuals. The oldest animal is the type specimen of the hairy-eared rhinoceros (D. s. lasiotis), captured in 1868, who lived in captivity (mostly at London Zoo) for 32 years 8 months. She arrived at London Zoo in 1872 when she was estimated to be at least six years old, so that she was unlikely to have been more than 35 years old at her death in 1900.
The main problem here is that no other animal has a known age. However, it may be possible to calibrate the tooth-wear curves in two other ways. Van Strien estimated that they did not reach sexual maturity until perhaps seven or eight years. If it is assumed that this more or less coincides with reaching full adult size, it is possible to estimate that tooth-wear rate for some of the premolars and all molars varies between 0.45 and 0.62 mm/year, if it is assumed that BMNH 1868.4.5.1 is a young adult of 7 ? years.
It has been suggested that it is inappropriate to use the teeth of captive animals in ageing studies. However, Spinage (1973) argues that rates of tooth wear are unlikely to differ significantly between captivity and the wild without fairly major dental problems. The tooth-wear curves suggest that the rate of wear in captivity may be slightly less than in the wild, and that wild animals would be unlikely to live as long as the London Zoo female before their teeth would completely wear out.
In many respects the basic skull morphology of the Port Lympne female matches that of the London Zoo female. Both have a heavily rugose skull surface with deep ridges on the mandible and the occipital region of the cranium for the attachment of jaw musculature. They also have fused premaxillae, a heavily ossified nasal septum, very worn molars and premolars, and significant resorption of alveolar bone. The only noticeable difference is that whereas only one lower incisor has been lost by the London Zoo female, both are missing in the Port Lympne animal. Based on estimates of tooth-wear rate and similarity in crown heights, the age of the Port Lympne female would vary between 28 and 47 years, but most probably about 28-37 years. This suggests that the Port Lympne female was as old as the London Zoo female and was quite likely to be too old to breed.
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Ageing the Sumatran Rhinoceros: preliminary results
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