Timothy Neal Coulson, Professor of Zoology and Joint Head of Department of Biology, University of Oxford
The woolly rhino, Coelodonta antiquitatis, would have been an impressive sight to the ancient people who painted images of them on cave walls and carved figurines of them out of bone, antler, ivory and wood.
The sadly now extinct rhino lived on the steppes and tundra of Europe and Asia, living alongside people for thousands of years. And a new study of woolly rhino DNA, extracted from the stomach of a wolf challenges a long held belief about species at risk of extinction.
The species, which evolved in the middle of the Pleistocene era, approximately half a million years ago, weighed up to three tonnes. It was similar in size to the two largest rhino species alive today, the white rhino of southern and eastern Africa and the one-horned rhino of India.
The woolly rhino was well adapted to live in ice age conditions. It had a thick layer of fat below the skin, a warm, woolly fleece and small ears and tail to minimise heat loss. It also had a shoulder hump to store fat, to help it survive through periods of food scarcity, and a horn that, in exceptional cases, could grow to 1.6 metres in length.
Abrasions on horns have led biologists to suspect that the rhino used its front horn (the species had two horns, like most species of rhino alive today) to sweep aside snow so it could access the grass and shrubs on which it fed.
At their peak, woolly rhinos could be found from the Iberian peninsula in the west to northeastern Siberia in the east. If it was cold, and there was grass to eat, they seemed to do well. But by around 14,000 years ago, they were gone.
Woolly rhinos were a victim of a changing climate, which made their habitat steadily vanish. The mammoth steppes they lived on were replaced by first a shrubbier habitat and eventually forest. They were also occasionally hunted by people, and that didn’t help them. A lack of good habitat, with a helping hand from the most efficient predator to have ever evolved, signed their death knell.
When a species experiences a long period of decline before eventually disappearing, scientists expect to detect signs its impending doom in its genome. As populations shrink, genetic diversity is lost from a population and inbreeding increases. This means that the last animals to be born are likely to have parents who were closely related.
As a species heads towards extinction, animals in the final few cohorts typically become ever more inbred. Because the woolly rhino’s extinction was thought to be a long, drawn-out affair, scientists assumed that individuals living 15,000 years ago would start to show genetic signatures of inbreeding. The findings of a recent paper from a team by led by Solveig Guðjónsdóttir are consequently quite a surprise.
The woolly rhino sample came from the frozen remains of an ice age wolf discovered in permafrost near the village of Tumat in north-eastern Siberia. When the ancient wolf was autopsied, the researchers identified a small fragment of preserved tissue in its stomach.
The team Guðjónsdóttir led skilfully sequenced the remains of a 14,400-year woolly rhino found in the stomach of the wolf pup. Both the wolf and rhino died just a few centuries before the woolly giant disappeared.
A healthy adult woolly rhino would have been too big for a pack of wolves to take down and kill, so it seems probable that the remains were either scavenged, or from a baby. Regardless of the source of the meal, analysis of the genome revealed that the woolly rhino was not inbred.
The genetic diversity of an individual can also be used to estimate the population size of breeding individuals using a statistical method called Pairwise Sequentially Markovian Coalescent modelling (PSMC). PSMC models compare differences between genome sequences on the two strands of DNA each individual has, one from each parent.
The model uses this information to estimate the distribution of times since each bit of the sequence shared a common ancestor. The greater the difference between the two strands of DNA, the greater the genetic difference between the parents, and the larger the population size would have been.
As part of the study, the researchers analysed two older woolly rhino genomes that had already been published and compared them to the new specimen. Their analysis showed that although the population of woolly rhinos had declined since its peak, it was still sufficiently large to maintain genetic diversity.
Guðjónsdóttir’s paper is important for two reasons. First, it is a wonderful demonstration of how DNA retrieved from the most unlikely of sources can tells us about population declines from millennia ago.
Second, it shows we might need a little bit more research into how population declines of long extinct animals might influence the statistics that geneticists frequently use, and we might need to revisit our current understanding. The woolly rhinos range certainly contracted as the world warmed, and its population size shrank, but it might not have died out as genetically impoverished relic.
Maybe the woolly rhino held onto its genomic diversity for much longer than we think it should have. So, we should keep checking the stomach contents of long-dead predators found in the permafrost, however unpleasant that task might sound.
Timothy Neal Coulson does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
