Today a Guy Carrying a Sack of Sloth Teeth Sat Next to Me

So this happened? Earlier today a man (pictured above) who studies the occlusal surfaces of sloth teeth plunked down beside me, at the desk normally occupied by our own Derek Mead, bag of teeth in tow. I had so many questions.

This chat has been edited lightly for length and clarity.

BAA: Who are you, what are you doing here, and what's in the bag?
Ryan Haupt: I'm a paleoecologist working on my Ph.D. at the University of Wyoming. I'm here because I was flying through on my way back from the Dominican Republic, where there used to be sloths, but now they're aren't because people ate all of them. In the bag is the work I was doing down there, molding fossil sloth teeth to see if we can figure out what they were eating based on the microscopic textures left on the teeth by their food. So I have a bag full of sloth teeth replicas in waiting.

How'd you get into any of this in the first place?
I first got into studying sloths because my master's advisor and I were trying to figure out a project. She was getting into the method of looking at microscopic tooth textures. She sort of said, "everyone assumes this technique won't work for sloths because they're super weird, why don't you see if they're right?" So that ended up being my master's, which has just lead to more and more questions about these weird and wonderful animals.

The Caribbean thing happened because my buddy Robert is trying to describe some new species down there. Some of the curators he's working with asked him if he could also tell them anything about the sloth's diet in the past.

He said, "No, but I got a guy." And that's how I ended up in the DR for a week molding teeth.

Brilliant. Sloths are very chill creatures. You said humans ate sloths to extinction in that part of the world?
As far as we can tell. There's a lot of debate about the Pleistocene extinction in North America, but in the Caribbean it's a little clearer: sloths were around after the continental extinction, but as soon as humans arrived they were gone. They actually survived in the Caribbean until about 4,000 years ago, so there were ground sloths chilling when the pyramids in Egypt were already finished.

One of dozens of sloth tooth molds in Ryan's bag.

I wasn't aware sloth was considered sustenance, at least not to the degree you're saying it may've been. These were like, normal-sized sloth? Or giant ground sloth?
Modern sloths certainly don't seem like they'd make a good meal. Up to 30 percent of their body mass is their stomach, so they're essentially just sacks of fermenting plants with arms and legs for climbing to where there are more plants to keep the bag full.

Ground sloths came in a lot of different sizes. We see the largest ones, the elephant-sized Megatherium and Eremotherium, on the continents, but because of the way island biogreography works it's not uncommon (which is a double-negative scientists adore using) to see large animals get smaller when living on an island.

"Sacks of fermenting plants with arms and legs." What a great description. Sloths are tight!
We don't have a great record of sloths evolving into their smaller modern tree-living forms, but the smallest ground sloths we do see came from the Caribbean, so it's possible these were some of the first sloths to take to the trees, even if they're not the ancestors of the ones we see today.

As you can see from the molds in the bag, the teeth weren't huge. I was working with two genera, Parocnus and Acratocnus, the former was a bit bigger, but not by a huge amount. Small bear to large dog sized animals.

Yes. Back to the teeth. How many teeth you got?
73

Not you. Sloth teeth.
Ha ha.

A pile of bags, each with their own individual sloth tooth mold in them. Labels on and inside the bag are critical. Photo: Ryan Haupt

Why do you have them?
A lot of the work I did was just cataloguing what the collections down there contained. There were a lot of loose teeth that needed to be bagged and sorted before I could get to work. I'm going to take these molds back to my lab and replicate the teeth using epoxy. From there, the epoxy casts can be scanned at 100x magnification on the chewing facet to get a sense of what the sloths were eating.

Sloths are super weird. My collaborator jokes that they're "one cold day from going back to being reptiles." One of their weird quirks is their teeth. Sloths closest living relatives are anteaters, which have no teeth at all.

Reptiles?
Sloth teeth are super simplified and lack enamel, so they're softer than our teeth, but they grow continuously throughout their entire life, like the incisors of a rodent, but for their entire mouth. Like reptiles because their metabolism is super slow, their body temperature is very low, and they just seem to go out of their way to defy mammalian norms at all times. [The grinding teeth] are an adaptation to no longer having enamel.

(Quick side note: Tell me you've seen the clip of the swimming sloth from Planet Earth II? I sometimes think we're all sloth just trying to keep our heads above water.)
(Oh yeah, I've been to that region of Panama where the sloths do that, not hard to see how they ended up all throughout the Caribbean.)

Mandible (lower jaw) of an Acratocnus ye from a cave in the Dominican Republic. The cotton swab is used to clean the teeth with acetone prior to molding, because at the fine scale dirt and dust can mess up the microwear scan and obscure the tooth's true texture. Photo: Ryan Haupt

If you had to compare sloth teeth with human teeth, it sounds like they're almost like human teeth at the end of a long life. Lacking enamel, ground down.
Except human teeth don't keep growing to replace what's been ground away. But yes, we do see loss of enamel in older mammals, including humans. Enamel is a very expensive tissue to replace.

Teeth are super important to mammals, a lot of mammalian evolution can be looked at through teeth. There are entire species that are known from a single tooth.

Teeth stress me out.
I totally have that teeth falling out nightmare on the regular.

Upper left caniniform tooth (scaled in centimeters) from a Parocnus serus, also from a cave in the Dominican Republic. The outermost layer of cementum has worn off so you can see those beautifully preserved bands of ever-growing dentine underneath. Just lovely. Photo: Ryan Haupt

What's up with that? I do too. Except I'll go through a stretch of regular falling-out teeth dreams then they'll go under for a while. A stress thing maybe. Anyway. So you're en route back to Wyoming. When are you leaving? And what'd you do with Derek?
I'm leaving in a few hours, hoping the snow both here in Brooklyn and up in the mountains doesn't get in the way.

Derek and I go way back, we studied abroad in Costa Rica together, years before I ever even thought I would end up a sloth researcher. We have a really good group of folks down there, I keep in touch with some of them who also became scientists, and cool science supporters like Derek. Even met my wife on that trip.

A sloth love story.
It did take a long time for her to come around on me…

Sloth pace. All things in time.
"Tranquilo" as Derek and I's Costa Rican mentor, Frank, would tell us.

I love it.
DR was great by the way. Friendly people curious about my work, and awesome street meats. I'll definitely be back.

Well on that note.
Yeah, it was great chatting with you about all this. I don't have any results yet, but it's cool to tell people about how science works as a process.

And if people want to follow along and get updates while I work, they can follow me on Twitter @haupt or check out my website, ryanhaupt.com, where I link to all my publications and stuff.

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