 Reference Base  Research on the effects of temporary horn removal on blac... |
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| Location: |
Africa - Southern Africa - Namibia |
| Subject: |
Ecology - Interspecific Relations |
| Species: |
Black Rhino |
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Hyena preying on calves of dehorned Diceros bicornis
A contrast in the experimental design of Berger & Cunningham (1994a) is the presence and absence of large carnivores in three parts of a large study area in northwestern Namibia, variously described as ca. 7000 km? (Berger & Cunningham. 1994a) or 10,000 km? (Berger & Cunningham, in press) or 4,500 km? (Berger, in press). There are enormous logistical and methodological problems in determining large mammal densities in large areas of broken terrain, especially if an aircraft is not used, as, for example in the study of Berger and Cunningham. The two researchers have not explained their methods of monitoring mammal densities, but these could not have been more than sightings of animals and their tracks at waterholes and along the few roads in the region. Such methods may yield valid estimates of ungulate densities under controlled conditions, but we know of no situation in and savanna or desert regions where such methods have been considered useful for estimating densities of large African carnivores. Experienced MET ranger and research staff consider it most unlikely that more than 30 spotted hyenas or more than 10 lions occur in the entire 40.000 km? of the 'Damaraland' part of the Kunene region, as also suggested by the low reported incidence of predation on livestock in this region (unpublished data, MET). Berger (in press), concedes an estimate of 10 hyaenas in 4,500 km? in 'central Kaokoveld'.
The brown hyaena Hyaena brunnea (which almost never preys on large mammals [Mills. 1990; Skinner & Smithers, 1990]), occurs throughout the Kunene region of Namibia, and partly overlaps in track dimensions with spotted hyaenas Crocuta crocuta. Much of the region has coarse substrates not suitable for accurate distinction between the tracks of related species. Intriguingly, Berger & Cunningham (1993, 1994a) report no hyaena presence from area DC at all, while both species have regularly been recorded there (Skinner & Smithers, 1990; unpublished MET records; pers.obs.) Spotted hyaenas were found at the carcass of the sub-adult rhino which died in 1992 (Table 1) in the DC area which, according to Berger & Cunningham, had no large predators at all. In a later manuscript, Berger and Cunningham (in press) mention that the number of spotted hyaenas had not been determined in their study area, and yet they state elsewhere with conviction that some parts of their huge study area had no hyaenas while other parts did.
We conclude that their survey methods were inadequate, given the importance attached to apparent differences in predator density as a factor in rhino calf survival. There is, in fact, no evidence for predation on rhino calves in area DC, where both species of hyaena occur sympatrically with rhinos which were dehorned in 1991.
Predation on rhinos
By accepting that black rhino calves are vulnerable to spotted hyaena and lion predation, one cannot simply conclude that any missing black rhino calves in the Kunene region are therefore killed by predators. No carcasses of any calf born in 1992 or later have been located. There is no evidence to suggest that either predation was the cause of death of three rhino calves when their carcasses were never recovered, or that calf predation was related to a horn reduction exercise three years earlier!
Interesting questions may nevertheless be asked about predation on rhinos, such as 'How much horn does a rhino mother need to deal with a predator?', or 'Is there an optimal size or shape for a rhino mother's horn?'. As evident from the table, Berger (1993a) and Berger et al. (1993), the anterior horns of the three dehorned mothers had already regrown to between 19 and 26cm by the time their calves had allegedly disappeared. This length of horn overlaps with the natural distribution of adult horn lengths (Berger et al.. 1993). Rhino horn shapes and lengths show extreme variation under natural conditions and the horns are the main features used in individual recognition of rhinos by researchers in southern Africa. This suggests that horn dimensions per se are not that important for the protection of calves. It seems likely that if maternal horn length and shape are important for the protection of the calf, these parameters would have evolved towards an optimum shape and length rather than varying to the degree seen in all populations.
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