“It really does look like they were contemporaneous,” says Anthony Martin from Emory University, who specializes in tracks and other so-called trace fossils. “This is a common problem we have with dinosaur tracks: We have something that looks like following behavior, but could have been offset by days or weeks. Here, the humans maybe had the sloth in sight.”
Ground sloths were not slow-moving slackers like the sloths we know today. They were well-armed and potentially dangerous animals, which ranged from bear-sized to elephant-sized. Those that lived in New Mexico were on the smaller end, but they were still substantial beasts with meter-long strides. A human would have had to stretch to walk in its footsteps. What possessed them?
Bennett thinks the pursuer was trying to provoke the sloth—and if he’s right, it clearly worked. At the end of the overlapping tracks, the team found a very different series of sloth prints, indicative of pivoting feet and scraping claws. The animal was rearing up onto its hind legs, and swinging its claws around.
Meanwhile, another set of human footprints approaches from the opposite direction. These are daintier, with impressions made by raised toes. It seems that while the sloth was flailing, someone else tip-toed up to it from the back. That’s a hunt, Bennett says. “The strategy was all about stalking to distract and irritate the animal, and get it to turn its back on someone approaching from the blind side.”
That’s just one possible interpretation, but it’s consistent with other tracks from the region. Sloth trackways usually go in a straight or gently curving line. But when human tracks are around, the sloth paths change direction sharply. The sloths seemed to be carrying out evasive maneuvers, and Bennett’s team also found many other sets of circular tracks indicative of upright, pivoting animals. They call these “flailing circles”—a term that makes me feel more connected to ground sloths than I have ever felt before.
Still, this scenario doesn’t explain why the pursuing human was stepping in the sloth’s footprints. There’s something almost playful about that, and I ask Bennett whether the tracks could have been made by, y’know, a bunch of teenage kids harassing the sloths for kicks.
“It’s really difficult to rule that out,” he says. “But I think that’s highly unlikely. These were fearsome animals. They had claws like Wolverine. I wouldn’t have wanted to go head-to-head with one. It would be a very silly risk to take.” I feel like this underestimates the willingness of teenagers to take silly risks, but I also take his point.
“I’m prone to agree with their interpretation about a possible hunt,” says Richard Fariña from the University of the Republic in Uruguay; it fits with the idea that humans drove the megafauna of the Americas to extinction. Ricardo Melchor from the National University of La Pampa isn’t fully convinced, either. Hunting is the simplest explanation, but “the human footprints are not well preserved,” he says. And why would people hunt a sloth near a lake—a flat landscape “where the animals can escape easily?”