More than 10,000 years ago, a band of hunter-gatherers chased a group of giant ground sloths along the shores of an ancient lake, and their footprints still preserve the story. West of the sparkling white gypsum dunes of White Sands National Monument in New Mexico, at a site called Alkali Flat, layers of mud and sand left behind by a long-vanished lake hold more than 100 human and giant ground sloth tracks—rare evidence of Pleistocene hunters stalking, and sometimes cornering, giant sloths.

Most of them are so-called “ghost tracks,” visible only under very specific conditions.

“Most of the time they are invisible. There is a lot of salt in ground, and when it rains, the salt dissolves. And, crucially, as it dries out, the fill dries at a different rate and the difference between the fill (footprint) and the surrounding sediment makes the track visible for a brief time while it dries out,” explained paleoecologist Sally Reynolds of the UK’s Bournemouth University, a co-author on the paper. The team, led by the US National Parks Service’s David Bustos, used aerial photography to spot the tracks and then selected a few groups for careful excavation and study.

The tracks reveal that ancient humans once hunted a group of giant sloths along the shores of Lake Otero. Several human footprints are clearly superimposed inside the long, kidney-shaped impressions of sloth feet, pointing in the same direction, as if the human was deliberately stepping in the sloth tracks. That would require real effort; the average human stride, according to Reynolds and her colleagues, is about 0.6 m, but a giant sloth’s stride was anywhere from 0.8 to 1.1 m, so the human tracker would have had to take bigger steps.

Based on the way the sediment beneath the prints had been deformed and the lack of even a thin sediment layer between the surfaces of the prints, the sloth tracks were still relatively fresh when the hunter followed.

Faster than they look

The sloths, it seems, weren’t too keen on meeting up. Sloth trackways veer sharply away in several places where they meet groups of human tracks, as if the sloths were trying to evade the humans. It’s not hard to imagine why; the humans would probably have been well-armed. Reynolds and her colleagues say the tracks are probably between 15,560 and 10,000 years old (the way the winds mix organic soils from different layers here makes it difficult to be much more precise), which means the hunters of Alkali Flat probably carried spears tipped with the large fluted stone points of the Clovis culture.

Those tracks tell us something about how Clovis people hunted; based on the mix of sizes, the party appears to have included both adults and children.

In some places, groups of prints preserve ancient confrontations between humans and sloths. Several sets of prints mark what Reynolds and her colleagues call “flailing circles,” or places where a giant sloth appears to have reared up on its hindlimbs to its full height and lashed out with its long, clawed forelimbs in an effort to defend itself. These spots are marked by slightly deformed hindlimb tracks, surrounded by a circle of irregular impressions and long claw marks or scrapes in the sand.

“In one case, a line of human toe impressions leads to the circle center, suggesting that someone approached on raised toes,” wrote Reynolds and her colleagues. In another, a single adult human track appears in the center of a flailing circle.

Bustos et al 2018

Bustos et al. 2018

Bustos et al. 2018



Bustos et al. 2018

Wikipedia Loves Art participant "Kamraman"

Bustos et al. 2018

Bustos et al. 2018

Approaching a giant sloth in that situation would have been a dangerous undertaking, even for a hunter armed with a stone-tipped spear. We think of sloths as slow, quiet, almost comical animals, but Pleistocene giant ground sloths were nothing like the sluggish DMV employee in Zootopia. Picture an animal weighing in at around three tons, measuring 6m from head to tail, with large, sharp claws—then imagine facing down a group of them.

“Sloths would have been formidable prey,” wrote Reynolds and her colleagues. “Their strong arms and sharp claws gave them a lethal reach and clear advantage in close-quarter encounters.”

The one that got away?

What we don’t know is how any of the confrontations recorded in the dried mud of Alkali Flat turned out. The cycles of wet and dry conditions on the playa, plus the high pH of the water and sediment (it’s not called Alkali Flat for nothing) would have quickly degraded any bones left behind, whether they were sloth or human. Modern hunter-gatherers go home empty-handed more often than not; the Hadza people of Tanzania fail to bag their prey about 94 percent of the time, according to Reynolds and her colleagues. But Pleistocene hunters in North America must have been successful often enough, since they’re currently the lead suspects in driving most of the continent’s megafauna, including giant sloths, into extinction.

Based on the varying sizes of the sloth tracks—anywhere from 30 to 56cm long and 10 to 35cm wide—there were multiple giant sloths moving around the shores of Lake Otero, trying to evade their probably well-armed human neighbors. It’s impossible to say which species of giant sloth left the tracks; fossils from two different genera have been found in the area, but their sizes were so similar that it’s hard to link the prints to one or the other.

Their presence, though, could tell paleontologists more about how giant ground sloths behaved and socialized. Today’s sloths tend to be loners, but Reynolds says these living species probably aren’t good guides to the behavior of their much larger, long-extinct cousins.

“We know very little about extinct sloth behavior, but the footprints at White Sands can help us understand group size, and possibly even other dimensions of this extinct species' behavior, which is exciting!” she told Ars.

A wealth of other tracks are waiting to be studied at White Sands, including mammoths; ancient canines and felines; extinct bovid species related to modern antelopes, buffalo, cows, and goats; and ancient relatives of today’s camels.

“The site continues to provide new and fascinating insights into the lives of people and animals at the end of the Pleistocene,” said Reynolds.

Science Advances, 2018. DOI: 10.1126/sciadv.aar7621 (About DOIs).