It’s no secret that rhinos are among the most distinct, and sadly endangered, animals on Earth. Survived by only five (5) remaining species and with only around 27,000 individuals remaining in the wild, the members of the family Rhinocerotidae consist of these animals, each the sole survivor of their genus: the white rhinoceros, Ceratotherium simum; the black rhinoceros, Diceros bicornis; the Indian rhinoceros, Rhinoceros unicornis; the most-endangered Javan rhinoceros, Rhinoceros sondaicus; and the Sumatran rhinoceros, Dicerorhinus sumatrensis.
These beautiful animals have sadly been regulars on the endangered species list for decades now, as humans have been overhunting these beasts to near-extinction for their horns, which some cultures use as part of their traditional medicine. As such, it has proven difficult for scientists to study these animals for research; finding them undisturbed in the wild ends up being quite an effort, not to mention the actual research work that needs to be done to find what you need to find on them.
One such question that, up until recently, has been difficult to answer is the relationship between these five surviving species of rhinos. As one will notice, these giants of the forest are separated by vast tracts of continental land, and even oceans. Aside from this, most of their rhino relatives died out right before the Pleistocene epoch around 2.5 million years ago; fossils of rhinos and rhino relatives have been found scattered across the Earth for decades now, including the famous 5-m (16.4-ft.) high and 10-m (32.8-ft.) long Paraceratherium, known to be one of the largest land mammals to ever walk the Earth; despite this, efforts to truly understand just how these distant rhino relatives are related had been sporadic at best. Now, an international team of researchers has opted to use genetic analysis to finally get to the root of how these animals—the five extant and other three extinct species—are all related. Their study was published in the journal Cell.
The team’s findings reveal new insights on the endangered family of animals. For starters, the African and Eurasian lineages of rhinos split about 16 million years ago. Said Love Dalén, from the Centre for Palaeogenetics at the Swedish Museum of Natural History: “We can now show that the main branch in the rhinoceroses’ tree of life is among geographic regions [of] Africa versus Eurasia, and not between the rhinos that have one versus two horns.” (Both the Indian and Javan rhinoceros have two horns; the other three surviving species have only one.) Dalén also added that “all rhinoceroses—even the extinct ones—have comparatively low genetic diversity. To some extent, this means that the low genetic diversity we see in present-day rhinos, which are all endangered, is partly a consequence of their biology.”
Mick Westbury, from the University of Copenhagen in Denmark, added that “continuously low population sizes may indicate that rhinoceroses in general are adapted to low levels of diversity.” Westbury noted that the five extant and three extinct species they analyzed experienced a constant decrease in population size over the last 2 million years. This, Westbury continued, is apparently consistent with the observed notion that rhinos apparently lack what are called deleterious mutations in their genome, or mutations that would otherwise make the rhino susceptible to certain diseases and disorders that would have impacted their survival. This may explain why rhinos have persisted for so long despite their apparent low levels of genetic diversity within their populations.
Dalén says that the findings are “partly good news, and partly not.” For one, their findings reveal that despite low genetic diversity, rhinos have maintained relatively healthy populations. “However, we also find that present-day rhinos have lower genetic diversity, and higher levels of inbreeding, compared to our historical and prehistoric rhinoceros genomes,” he followed. “This suggests that recent population declines caused by hunting and habitat destruction have had an impact on the genomes. This is not good, since low genetic diversity and high inbreeding may increase the risk of extinction in the present-day species.”
Westbury and the team believe that their findings may have some practical applications for the survival of extant rhinos. He noted that “the low diversity we see in contemporary individuals may not be indicative of an inability to recover, but instead a natural state [for the] rhinoceros. We can better guide recovery programs to focus on increasing population size rather than individual genetic diversity.”
The team hopes that the new findings will be useful for continued study of rhinoceroses and their conservation. Dalén and his team are now in the process of doing an analogous study, but this time to the extinct wooly rhinoceros. Westbury, on the other hand, is involved in more detailed analyses of the genomes of African black rhinoceros.
(For stories about other conservation efforts, read more on the story of Fernanda, a lone Fernandina Giant Tortoise that may champion the revival of her species that was previously thought to be extinct.)
References
- Cell Press. (2021, August 24). Geneticists map the rhinoceros family tree. ScienceDaily. https://www.sciencedaily.com/releases/2021/08/210824121037.htm
- Liu, S., Westbury, M. V., Dussex, N., Mitchell, K. J., Sinding, M.-H. S., Heintzman, P. D., Duchêne, D. A., Kapp, J. D., Seth, J. von, Heiniger, H., Sánchez-Barreiro, F., Margaryan, A., André-Olsen, R., Cahsan, B. D., Meng, G., Yang, C., Chen, L., Valk, T. van der, Moodley, Y., … Gilbert, M. T. P. (2021). Ancient and modern genomes unravel the evolutionary history of the rhinoceros family. Cell, 0(0). https://doi.org/10.1016/j.cell.2021.07.032
- Rhino | species. (n.d.). World Wildlife Fund. Retrieved September 30, 2021, from https://www.worldwildlife.org/species/rhino
