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| Location: |
Africa - Southern Africa - Namibia |
| Subject: |
Ecology - Interspecific Relations |
| Species: |
Black Rhino |
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Reply to Lindeque & Erb (1995)
Previously, we suggested that when dehorned mothers were sympatric with spotted hyaenas (Crocuta crocuta) in the Namib Desert, fewer calves were recruited than in the absence of hyaenas. We also pointed out that drought was likely to have exacerbated these effects and that our sample of 10 calves was small (Berger & Cunningham, 1994a,b). Our findings have been challenged by government and non-government officials in Namibia (Loutit & Montgomery, 1994a,b; Lindeque & Erb, 1995).
Abundance of spotted hyaenas
Lindeque and Erb (1995) suggest that our study design was flawed because spotted hyaenas occur throughout the study region. Support for their claim is unbalanced. They fail to cite Skinner and van Aarde (1981) who surveyed the Namib Desert for brown (Hyaena brunnea) and spotted hyaenas and reported 'we still have no idea what numbers occur in the area or .... range'. Instead, they cite Skinner & Smithers (1990) although these authors provide range maps only and not data of the resolution needed to distinguish among our three respective study regions. Additionally,. the use of unpublished records to bolster their argument is questionable because it is impossible to decide how credible the records are.
Lindeque and Erb (1995) reported spotted hyaenas at a rhino carcass in the Doros Crater (DC) area. They were fortunate in their observation because, on average, spotted hyaenas in the northern Kalahari spent less than six minutes on a carcass (Cooper, 1990), yet the Namib Desert rhino in question had been dead for about three weeks when discovered (Morkel,1992). Lindeque and Erb (1995) also imply that it is difficult to distinguish between the tracks of spotted and brown hyaenas. However, both Damara herdsmen in the Namib Desert and !Xo trackers in the Kalahari can distinguish between the species because of 'the relative difference in size between the front and back feet. In the brown hyaena the back feet are much smaller than the front feet. while in the spotted hyaena the difference is not nearly so marked' (Mills, pers.comm.; Liebenberg. 1990).
We evaluated hyaena abundance using standard methods employed in southern Africa, using counts of tracks crossing roads (Mills et al., 1984). We recorded every possible hyaena spoor on roads and elsewhere. If Lindeque and Erb (1995) are correct that both species of hyaenas are widespread, then our inclusion of all hyaena signs would inflate the number of hyaenas irrespective of species. We also used more direct methods to distinguish between brown and spotted hyaenas, Vocalizations of spotted hyaenas were recorded nightly as either existing or absent. This approach is conservative because it discounts the possibility that more than one animal may be present or calling. We also recorded how many brown and spotted hyaenas were seen per day spent in the field (and by accounting for km/ transect; see Table 1 and below for details) but, as above, groups were recorded as single observations.
At no site were brown hyaenas seen. Spotted hyaenas were noted at only two of our three study regions (see Figure). Irrespective of species, there was not a single track, vocalization, or sighting in the DC area although transects there totaled more than 1,675 km (Table 1). Similarly, there was no evidence of lions in the DC area. We therefore designated the site as predator-free. DC differed from the other two areas (Vocalizations: G-Test for Independence, G adj=1.01, p<0.001; a test for Homogeneity of Variance reveals that neither site with hyaenas differs from each other but both differ from the DC area; p<0.00 1). The frequency with which tracks were detected also varied among sites (Figure 1) (Kruskal Wallis Test, H = 10.89, p<0.004) with the DC area differing from the other two area (p<0.01).
Why spotted hyaenas were not seen at the Springbok River (SR) site although tracks and Vocalizations were noted is easy to explain. We made no effort to observe them there. When hyaena-like calls emanated on multiple occasions next to our SR camps, we made no effort to verify that they were indeed made by hyaenas. We believe that we can discriminate the calls of Spotted hyaenas from those of other mammals. Nevertheless, our other data, shown in the Figure, make clear that hyaena presence varied regionally and consistently during the period of our study and that one area was free of hyaenas and lions.
Horn size, missing and maimed calves, and evolution
Lindeque and Erb (1995) argue that 'horn dimensions per se are not that important for the protection of calves...(because) ... these parameters would have evolved toward an optimal shape and length rather than varying to the degree seen in all populations' and suggest rhino horns show 'extreme' variation under natural conditions. We have presented data elsewhere from four populations in which Lindeque was a co-author (Berger et al., 1993) showing that coefficients of variation in horn size range from 31 to 62%. However, since horn length is significantly related to age in both sexes (Berger & Cunningham, 1995), it makes little sense to argue about the functional significance and optimal design of horns without controlling for age. It is incorrect to imply that just because a trait is variable the possibility of selection is relaxed (Barnard, 1991). With respect to the size of anterior horns of mothers, the fact remains that in areas with spotted hyaenas, mothers with surviving calves had anterior horns that were significantly longer ( = 40 cm, N=-4) than mothers with regrowing horns whose calves disappeared ( = 23cm; N=3) (Wilcoxon Test, Wx=22; p<0.029).
It is also important to ask what, if any, evidence from other sites may suggest that horns are associated with calf protection. Lindeque (1990) made such an attempt and suggested a null hypothesis, that negative biological effects are not expected from dehorning, a decision 'taken in the absence of strong evidence of likely detrimental effects' (Lindeque, 1990). Is the a priori assumption that predation may not affect horned rhinos or their calves reasonable? We believe a more thorough search of the literature and discussion with other researchers would have raised the alternative possibility
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Is rhino dehorning scientifically prudent?
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