Rhino diversity: the horns of a dilemma

Researchers analysing the genomes of living and extinct rhino species have made several findings about the ancestry and population dynamics of the animals, including the fact that low levels of genetic diversity – thought to be one of the major threats to rhino conservation – have been a feature of rhinoceros populations for millions of years.

Most rhinos went extinct before the Pleistocene, and scientists have long wondered at the evolutionary relationships between the five living species and those long gone. The new research, published in the journal Cell, finds that African and Eurasian lineages split about 16 million years ago.

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“We can now show that the main branch in the rhinoceroses’ tree of life is among geographic regions, Africa versus Eurasia, and not between the rhinos that have one versus two horns,” says co-author Love Dalén, of the Centre for Palaeogenetics and the Swedish Museum of Natural History.

“The second important finding is 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.”

Co-author Mick Westbury, of the University of Copenhagen, Denmark adds that “all eight species generally displayed either a continual but slow decrease in population size over the last two million years, or continuously small population sizes over extended time periods. Continuously low population sizes may indicate that rhinoceroses in general are adapted to low levels of diversity.”

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The findings are consistent with the fact that rhinos have accumulated high quantities of deleterious genetic mutations, and have remained relatively healthy as a population despite their low genetic diversity.

Dalén says the findings are “partly good news, and partly not”. The finding that low genetic diversity is not associated with weakness or poor health is a small win for the rhinos, but modern-day rhino diversity is even lower than it was historically.

“This suggests that recent population declines caused by hunting and habitat destruction have had an impact on the genomes,” says Dalén. “This is not good, since low genetic diversity and high inbreeding may increase the risk of extinction in the present-day species.”

The findings may inform rhino conservation into the future.

“Now we know that the low diversity we see in contemporary individuals may not be indicative of an inability to recover, but instead a natural state of rhinoceros,” says Westbury. “We can better guide recovery programs to focus on increasing population size rather than individual genetic diversity.”

Dalén reports that his team is now working on a more in-depth study of the extinct woolly rhinoceros. Meanwhile, Westbury is involved in comparing the genomes of African black rhinos sampled from before the recent decrease in population size to those of contemporary individuals.

“We hope that this will provide a framework to better understand where translocated populations may have arisen from, direct changes in genetic diversity, and whether any populations may have been lost forever because of humans,” says Westbury.