How did woolly rhinos go extinct in Ice Age? Study suggests climate change, not overhunting, was likely culprit
The extinct woolly rhinoceros species breathed their last towards the end of the most recent Ice Age. The reasons behind their extinction have been a mystery, although studies have hinted at human involvement. Now, a new study suggests that overhunting is unlikely to have caused their demise. Instead, the finding supports another theory: climate change. The woolly rhinos likely went extinct because they were ill-equipped to tolerate the rapid increase in temperatures, say researchers from Centre for Palaeogenetics, a joint venture between Stockholm University and the Swedish Museum of Natural History.
Woolly rhinoceros began going extinct sometime between 18.5 and 14 thousand years ago. "Therefore, the decline started more than 10 thousand years after the arrival of humans in northeastern Siberia," Love Dalén, professor of evolutionary genetics at the Centre for Palaeogenetics, Sweden, and one of the authors of the study, tells MEA WorldWide (MEAWW).
Earlier studies argued that humans arrived in Siberia fourteen or fifteen thousand years ago, around when the rhinos went extinct. But, in 2012, Dr Dalén found a bone of the extinct animal in Siberia, right next to where humans once settled -- which was dated to roughly 30 thousand years ago. "That made me wonder if these early humans had any measurable impact on the woolly rhino population," he adds.
To reconstruct these past events, Dr Dalén and his team turned to DNA for answers, which they recovered from the remains -- tissue, bone, and hair samples -- of 14 individuals that roamed Siberia. But studying DNA from extinct species is not easy, mainly because researches do not often know the quantity of the recovered genetic material. "The amount of DNA left in a sample varies a lot as it depends on several factors, including the age of the sample, the type of sample (bone, tooth, hair), and temperature of the location," Edana Lord, a graduate student at the Centre for Palaeogenetics, tells MEAWW. "Siberia is much better for ancient DNA studies because it is like a big freezer," she added.
The researches then sequenced the genetic material to study changes in population size of the species. They also estimated inbreeding. If high, it indicates lower differences in DNA between individuals and a likely higher susceptibility to diseases. As a result, it could decrease an organism's ability to survive and reproduce. The analysis showed that their population increased in numbers some 29,000 years ago. "The woolly rhino population size remained constant and that at this time, inbreeding was low," Dr Nicolas Dussex, a postdoctoral researcher at the Centre for Palaeogenetics, says.
Their numbers continued to remain stable even after humans arrived on the scene, contradicting previous theories that suggested the role of overhunting. "That's the interesting thing," says Lord. "The data we looked at only goes up to 18,500 years ago, which is approximately 4,500 years before their extinction, so it implies that they declined sometime in that gap."
The findings suggest that a period of climate warming that started 14.7 thousand years may have mainly driven their extinction. It also "makes sense due to the fact that the woolly rhino was a cold-adapted species," Dalén explained. Further, the period also saw a shift in habitat: an open landscape with fewer trees and more grasses was replaced by woody vegetation, which was dominated by trees and bushes, according to Lord. This change did not suit the woolly rhinoceros, she adds.
The study is limited by small sample size. Furthermore, Dr Dalén added they cannot rule out human involvement with a 100% certainty. "It is, of course, possible that it took more than 10 thousand years until the human population was large enough, or invented a new technology, to start killing the rhinos in more substantial numbers," he explains. "But I think this is a less likely scenario."
The study is published in Current Biology.