Association between taxonomic relatedness and interspecific mortality in captive ungulates

Interspecific aggressive interactions are known among a variety of animals including ungulates. Nevertheless, most studies on interspecific interactions in ungulates involve case reports without testing any specific hypotheses. We tested two mutually exclusive hypotheses; that mortality rate in interspecific interactions of captive African ungulates would be higher if combatants are taxonomically (i) more closely related species or (ii) more distantly related species. In addition, we examined if mortality in these interactions was affected by the age, sex and weight difference of combatants. In total, we analyzed 101 interspecific aggressive interactions among 25 species of African ungulates kept in mixed species exhibits in Dvůr Králové Zoo over a period of 20 years. In 18 cases, one of the combatants died. We found that probability of death was higher when the target of aggression was a young animal. Since the majority of fatal attacks towards young were performed by equids, the only known explanation is that the strong defence instinct of equids may cause them to mistake young antelope for small predators. When analysing only fights between adults we found that more aggressive interactions were recorded between taxonomically more distantly related species, however the interactions between taxonomically more closely related species led to the death of combatants more frequently. A possible reason for these highly escalated conflicts among closely related species may be higher competition over resources as the resource needs of closely related species are likely to be similar. In conclusion, we recommend that closely related species should not be put together in mixed species exhibits to prevent serious injuries or fatal attacks.