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Natural Variation in Horn Size and Social Dominance and Their Importance to the Conservation of Black Rhinoceros

The importance of behavioral ecology to evolutionary theory, zoo biology, and animal husbandry is well recognized (Caro 1994; Krebs & Davies 1984), but, despite the prevalence of global biodiversity issues, its contribution to in situ conservation has been limited (Soule 1986; Ulfstrand 1996; Clenmmons & Buchholz 1997). Populations of the world's elephants and rhinoceros have been decimated by poaching; in Africa, rhino conservation measures increasingly rely on protection in heavily guarded reserves (Brett 1990). Extreme measures have included dehorning and shooting poachers. Although the case has been made that social status in black rhinos (Diceros bicornis) is unaffected by horn removal (Lindeque 1990), it is less than convincing because neither the internor intrasexual consequences of asymmetries in horn size on dominance have been studied. To understand horn function, models of sexual selection have been applied to dimorphic ungulates (Jarman 1983; Packer 1986; Clutton-Brock 1988), but predictions do not fit black rhinoceros because (1) the sexes are similar in horn and body size, (2) secondary sex ratios are equal, and (3) mortal fighting, the highest recorded for any mammal, results in about 50% of the males and 30% of the females dying from combat-related wounds (Berger 1994). We report field data on 52 black rhinoceros of known horn and body size and 442 dyadic encounters from three sites in northern Namibia. Our results suggest that intersexual dominance is female-biased, that intrasexual dominance is related to horn mass in males but not in females, and that where asymmetries between horned participants exceed 10 cm, larger horns confer