Today beavers, elk and the like are spreading further and further north as a result of climate change. Similarly, the mastodon apparently immigrated to previously icy areas of North America during global warming in the past. Analysis of fossil DNA from proboscis provided evidence of this repeated spread. It also became apparent that the Arctic mastodon populations had a comparatively low genetic diversity, which is considered a risk factor for the long-term survival of a species. Similar developments could also occur in the spread of animal populations in the context of today’s climate change, say the scientists.
In the north it will be milder, in the south it will be hotter and drier: In some regions of the world, man-made climate change is shifting conditions particularly quickly and significantly. It is already becoming apparent that some animal species will follow – there is a retreat, but also an advance. Some species can spread to the north in particular, as the warming there creates new opportunities for them to live. “Animals like elk and beaver are currently expanding their range by ten to hundreds of kilometers per century,” says Emil Karpinksi of McMaster University in Hamilton.
He and his colleagues have now dealt with an animal that, however, left the stage of life around 11,000 years ago. In the age of the Pleistocene, which began 2.5 million ago, the American mastodon was a widespread representative of the proboscis in North America, alongside the mammoth. In contrast to the grazing mammoth, the giants, weighing up to nine tons, lived more on tree and bush vegetation. They successfully survived the climactic capers of the Pleistocene, which shaped the earth especially in the last 800,000 years. The prevailing glacier periods were interrupted by warm phases in which the ice sheets retreated strongly. It is unclear why the mastodon died out around 11,000 years ago along with other representatives of the Ice Age megafauna. However, there are indications that they succumbed to the combination of climatic changes and the heavy hunting by the immigrant people.
Climate-driven retreat and advance
As Karpinksi and his colleagues report, an interesting aspect of the mastodon is its apparently once astonishingly wide distribution from north to south: Remains of these animals were discovered in central Mexico, but also in what is now Alaska and the Yukon. However, until now it was unclear when they occurred in these areas and which propagation waves could have occurred in the context of climate fluctuations. In addition, in contrast to the mammoth, little was known about the genome of the mastodon. In order to gain new insights, Karpinski and his colleagues elicited samples of fossil bones and teeth from 35 mastodons from different areas of their former area of distribution. They finally succeeded in sequencing the complete mitochondrial genomes of the specimens.
As the researchers report, five different lineages of the mastodons emerged in the genetic comparisons. These included two different animals from the far north that once lived in the area of the land connection between Siberia and Alaska. Certain clues in the genome made it possible to draw conclusions about when these lines arose. According to this, the two groups of northern mastodons did not live in parallel, but came from two different spreading events in the context of different warm phases. This result suggests that the mastodons probably migrated several times from their southern distribution areas to the north when there were opportunities for them to live due to the spread of forest and bush landscapes. “The data suggested that there was movement back and forth,” says co-author Hendrik Poinar.
Little variety among the northern pioneers
The genetic analyzes of the “pioneer” populations that made it to the north also revealed another interesting aspect: the genetic diversity of these animals was very low compared to the populations in the ancestral southern distribution areas of the mastodon. “It’s always a danger signal to animal species,” says co-author Grant Zazula of the Canadian Museum of Nature in Ottawa. “When groups of animals lose genetic diversity, they lose the ability to adapt to changes in conditions. In this case, the pioneering mastodons were apparently not adaptable enough to keep up in the north when it got colder again, ”said the paleontologist.
According to the researchers, information about the reactions of extinct animal species may also shed light on developments in the context of today’s climate change. It could be, for example, that population expansions to the north emerge from a small subset of a species, similar to that of the mastodons back then. This could end up making them vulnerable if the more genetically more diverse southern populations are lost. “Studies like ours can provide clues as to how arctic ecosystems will react to future warming scenarios,” concludes Zazula.
Source: McMaster University, American Museum of Natural History, specialist article: Nature Communications, doi: 10.1038 / s41467-020-17893-z