For hundreds of thousands of years Mastodons were North America's largest animals, shaping the ecology from the Arctic to Mexico. At any one time, however, much of this range would have been unsuited to them. A new study reveals the way these relatives of elephants and mammoths migrated across the continent in response to changing conditions, and provides a warning for large modern animals living in similar places today.
Most mastodon fossils cannot be dated precisely, being too old for carbon dating, so we know little about the timing of their occupation of specific locations. However, by extracting mitochondrial DNA from the bones, teeth, and tusks of 35 mastodon fossils, scientists have reconstructed their relationships and revealed probable gaps in their occupations of the far north of the continent.
Professor Hendrik Poinar of Canada's McMaster University found the genomes came from five distinct mastodon populations or clades. Representatives of two clades were found in Alaska and the Yukon – areas so cold even mastodons probably could not have lived there during glacial periods. The two clades are so distinct, genetically and in suspected age, Poinar and co-authors conclude in Nature Communications they represented independent occupations. One clade would have lived in the area in a warm era before being driven to extinction as the temperatures cooled, with a new group moving in when temperatures rose again.
The presence of mastodons from several clades in closely located locations in Alberta confirms suspicions of a migratory corridor used in times of climatic change. "The genetic data show a strong signal of migration, moving back and forth across the continent, driven, it appears entirely by climate," Poinar said in a statement.
The mastodons were probably not the only ones making such journeys. Western camels and giant beavers also expanded into Alaska and the Yukon during interglacial periods, but probably died out as temperatures fell, only for new members of the same species to move in again when the ice retreated.
Both northern mastodon clades lacked genetic diversity compared to those that lived further south during colder eras. This would have made these northern warm-era clades less adaptable than their counterparts, possibly contributing to their extinction.
"Today, you might think that it's great to see animals like brown bears in northern Canada and the Arctic islands, well beyond their historical range. They are obviously benefiting, just like these mastodons did for a time, as a result of natural climate change," co-author Professor Ross MacPhee of the American Museum of Natural History said. "But that benefit can be very limited.”
"If you lose genetic diversity, you are losing ability to respond to new conditions,” explained co-author Grant Zaxula, “In this case, they were not up there long enough to adapt to northern conditions when they cycled back to cold." Modern species may suffer the same fate.