Palaeontology: American mastodons headed north during interglacial periods

An analysis of the mitochondrial genomes of 35 American mastodons suggests that this extinct species repeatedly migrated into the far northern latitudes of North America in response to interglacial warming during the Pleistocene (2.5 million to 11,700 years ago). The findings, published in Nature Communications, may help researchers to understand the potential ecological responses of present-day species to global warming.

American mastodons (Mammut americanum) inhabited woody and swampy locations in North America, and their remains have been found from the Central American subtropics to the Arctic latitudes of Alaska and Yukon. Cycles of glacial and interglacial periods over the last 800,000 years resulted in periodic expansion of the ice sheet across approximately 50% of the habitable land in North America. However, it is unclear how the mastodon responded to these fluctuations.

Emil Karpinski and colleagues studied samples from the fossil bones and teeth of American mastodons from institutions across North America, and sequenced the complete mitochondrial genomes of 33 specimens (a further 2 published genomes were included in their analysis). They identified five distinct groups (or clades) of mastodons, of which two originated from eastern Beringia (a region that historically joined Russia and America). The authors detected no overlap in the ages of the specimens from the eastern Beringia groups and suggest that the two clades likely resulted from separate expansions into this region. This coincided with interglacial periods when warm climatic conditions supported the establishment of forests and wetlands.

The authors also found that the northern clades had lower levels of genetic diversity than that in groups south of the continental ice sheets. They argue that similar northward population expansions today, owing to climate change, likely consist of a subset of a species. This may leave them vulnerable if more genetically diverse southern populations are eventually lost.