Historical survivorship and demographic structure of zoo-housed hippopotamuses Hippopotamidae

Abstract

Zoos have made efforts to improve the welfare and survival of their animals through ever-evolving husbandry techniques. In principle, this accumulation of expertise and skills should lead to higher survival rates for zoo animals over time, as demonstrated in several zoo animal species in recent years. Hippopotamuses (hippos) are part of the charismatic fauna commonly present in zoos, and their conservation status in natural habitats increases the potential and value that ex situ populations have to offer for their conservation. In this study, historical adult and juvenile survivorship of the two hippo species while kept in zoos was evaluated from 1900 onwards. The survivorship of adult common hippos Hippopotamus amphibius has remained unchanged across this timespan while the survivorship of adult pygmy hippos Choeropsis liberiensis has seen some progress in the last two decades. For both species, juvenile survival has remarkably improved. The survivorship of common hippos in zoos is superior to that observed in the only documented survivorship data from a free-ranging population. For the common hippo, the absence of improvements in recent decades could either indicate that the species has already reached its optimum lifespan, or that a key husbandry practice has yet to be identified to further enhance survivorship rates. The improvements seen in pygmy hippo may be the result of increased collective efforts in the last two decades to better understand the biology and care of this species, as evidenced by scientific interest and husbandry guidelines. In the population of both species, the proportion of juveniles has decreased and that of adults has increased, with the current barrel-shaped population pyramid in the common hippos creating the risk of jeopardising the long-term maintenance of the population. Especially for common hippos, we suggest that a change in the management approach might be necessary to guarantee population sustainability, with the inclusion of a breed-and-cull strategy.