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Share life in the slow lane with sloths

When Greg McDonald embarked on a career in vertebrate paleontology at Idaho State University, the first of a series of far-sighted professors helped steer him to giant ground sloths.

When Greg McDonald embarked on a career in vertebrate paleontology at Idaho State University, the first of a series of far-sighted professors helped steer him to giant ground sloths.

It was, however, a lonely specialty for a sociable young student. Mammoths, mastodons and saber-tooth cats were the celebrities of the Pleistocene. Less-glamorous ground sloths, despite their fascinating histories, didn’t attract much attention.

That’s changed over the four decades since McDonald, who earned a PhD at the University of Toronto, set out in pursuit of sloths. Now when he attends paleontology conferences, he manages to find other sloth scientists to chat with. He enjoys discussing his love for giant sloths, including the Far North’s Megalonyx jeffersonii - aka Jefferson’s ground sloth - with specialists and with the rest of us.

McDonald, senior curator of natural history in the U.S. National Park Service’s museum management program, will be sharing sloth science with an audience at the Yukon Beringia Interpretive Centre this Saturday. He is part of the Long Ago Yukon Skype Speaker series.

“There are many different types of giant sloths,” says McDonald by phone from his US National Park Service office in Fort Collins, CO. “They range from elephant size down to cat size and they are well represented in the fossil record.”

Megalonyx, though about as large as a cow, rather than an elephant, is among the stars of what scientists call the Great American Faunal Interchange. During that process, various South American animals, such as porcupines, trekked into North America. Some northern creatures, like llamas, tapirs and peccaries, went south.

The interchange took place in three stages over eight million years. The early ancestor of the Megalonyx was part of stage one. McDonald calls him “the sloth that led the charge.”

For some species the heavily forested region around the Isthmus of Panama acted like a filter, says McDonald. Food selection, rainfall or temperature prevented some creatures from completing their journeys north or south. Other species, more comfortable in tropical rainforests, were content to remain in a forest that ended in southern Mexico.

“My former professor Dave Webb did a lot of work on this and he noticed animals that were very well-adapted to grassland environments ... just were not able to make it through the Isthmus of Panama.”

Animals adapted to forests, such as Megalonyx, did much better. These sloths eventually lived near Old Crow in the Yukon, as well as in parts of Alaska and N.W.T., during the Sangamonian interglacial, a warm spell late in the Pleistocene.

The interglacial began roughly 135,000 to 140,000 years ago. “One could say that there was about a 100,000-year interval when conditions were right to allow the sloth to live that far north,” says McDonald. As the interglacial ended, the northern landscape became more arid and the northern population of Megalonyx vanished.

Our Yukon sloth may not be the largest sloth overall, but it certainly earned the name Megalonyx, which means “great claw.” Its blades were well designed to allow the animal to pull branches down to its mouth. They would also have been a deterrent to all but the most desperate predators.

Sloths were likely solitary, says McDonald. They didn’t travel in herds. A family unit probably consisted of a mother and a couple youngsters. When young, they would have been vulnerable, much as baby elephants are today, but mom’s claws ensured that the price of a meal would be too high for most carnivores. “Sloths probably lived a long, happy life.”

McDonald has never been to the old sloth stomping grounds near Old Crow, though he has visited the territory to speak. He has also conducted extensive field work in the U.S., the Caribbean and much of South America.

Like many other scientists, he has benefitted from the research and collecting done by northern colleagues like the legendary Dick Harington. McDonald and Harington, along with G. De Iuliss, co-wrote the “The Ground Sloth Megalonyx from Pleistocene Deposits of Old Crow Basin, Yukon,” which appeared in the September issue of Arctic in 2000.

Contemporary paleontologists also owe a debt to the 18th- and 19th-century naturalists who roamed the planet collecting fossils, says McDonald. Their finds are awaiting further study - or rediscovery - in museum drawers in Europe and all over the Americas.

“There’s a saying: ‘Some of the best collecting these days is in museum collections,’” he says. “I like to think that there’s a lot of material that I’ve collected over the years that’s gone into museum collections. Though I’ll never benefit from it, I’m very pleased to see people incorporate it into their research.”

“I’m sure Charles Darwin didn’t have me in mind when he was putting specimens in the British Museum,” McDonald adds.

North to the Yukon (Slowly): the Jefferson Ground Sloth from Old Crow gets underway at the Beringia Interpretive Centre at 1 p.m., Saturday, February 28.