Body Mass Records of Zoo-Managed Rhinoceros (Ceratotherium simum, Diceros bicornis, Rhinoceros unicornis) as Compared to Field Data of Free-Ranging Specimens.

The body mass of zoo animals may differ from those in wild populations due to the different environmental and dietary conditions being offered under human-managed care. These differences in body mass may impact health at both individual and population levels. In the case of rhinoceroses, this is relevant because of the distinct feeding requirements of each species and their susceptibility to obesity when inappropriate diets are offered. Here we attempt a comparison between the adult body mass records of the global zoo populations of rhinoceroses (white Ceratotherium simum, black Diceros bicornis, and greater one-horned Rhinoceros unicornis) and the available body mass records of free-ranging specimens in the literature. Body mass data from free-ranging specimens is surprisingly scarce for white and greater one-horned rhinos. Most adult zoo rhinoceroses are within the body mass range described for wild populations except female white rhinoceroses, which are, on average, heavier than free-ranging females. Also, contrary to what has been described for natural habitats, zoo rhinoceroses do not show evidence for seasonal fluctuations in body mass, most likely due to the consistent nutrient supply in zoos. While obesity might be present across female white rhinos, and exacerbate other underlying health issues at the individual level in other taxa, this particular data set does not provide evidence that obesity is a population-level phenomenon in black or greater one-horned rhinos. Our findings should not weaken the efforts towards improving zoo rhinoceros nutrition and body condition monitoring.