Modern sloths are curious beasts. Generally fairly small, tree-dwelling critters, they’re notorious for their slowness. But they come from a grand tradition of great size. Until the big extinction of large mammals that occurred about 10,000 years ago, there roamed across the land giant ground sloths that would have made most people run in terror.
These giant sloths coexisted with great beasts like mammoths and woolly rhinos and saber-toothed tigers. They didn’t live in the trees; they were far too big. Instead, they moved about on the ground, using their huge claws to rake leaves from trees to eat.
All this is romantic, but seriously, if giant sloths were as slow as their modern cohorts, wouldn’t they have just been gobbled up by the saber-tooth tigers and the dire wolves?
Well, that’s a good question. How can it be answered?
Modern sloths are slow because they have low metabolic rates. Their diets consist of foods of poor nutritive value, so they balance this by sticking high in the trees and taking their time to get around. The low metabolic rate is reflected by having a low body temperature. Most mammals (like us, or horses and cattle) keep their bodies at 37-39°C. Modern sloths (and other low-metabolic-rate mammals) keep theirs at around 32°C.
So all we need to do is measure the body temperature of a giant sloth! Oh, wait. They’re extinct. Dang.
Geochemistry to the rescue!
Almost all of my research revolves around the geochemical analysis of fossilized teeth in mammals, to make inferences about their biology, and the environments in which they lived. To do this, I measure the relative amounts of stable isotopes (not the radioactive ones!) of carbon and oxygen from tooth enamel. The methods I use are (relatively) straightforward, and have been used actively for decades. The relative amounts of the different isotopes of oxygen and carbon can be related to temperature – and here’s our foot in the door to get at body temperature.
It can be complicated though, especially for oxygen, and until recently we couldn’t easily distinguish temperature changes from things like changes in the amount of precipitation. We also could only look at changes in environmental temperature, rather than body temperature. (Sigh.)
That changed a few years back with the development of a new method of temperature determination called “clumped isotope” paleothermometry or just delta-47 (Δ47). As it happens, the heavy isotopes of carbon and oxygen can exist together (clump) in a single molecule of carbon dioxide, CO2 (which is what we measure with the mass spectrometer). This carbon dioxide comes from carbonate (CO3) which comes from the tooth enamel. How often the heavy carbon and heavy oxygen clump in a molecule is directly related to the temperature at which the molecule formed. In the case of mammals, this is the temperature of the mammal’s body.
So all we have to do is count how many carbon dioxide molecules have both the heavy carbon and the heavy oxygen (= clumped isotopes) and we can measure body temperature!
It sounds simple, it’s really not, but only because there aren’t that many molecules with the clumps, so we need a lot of material and tons of analytical time to get it done. This makes it expensive and it’s hard to get materials because you basically have to destroy most of a tooth. Museums don’t like to lend you specimens that you’re going to destroy. I don’t blame them, really.
We’ve been fortunate, however. One museum has recognized the importance of this study: We really do need to know the metabolic rates of giant sloths if we want to understand their biology and behavior. We were lent teeth from two species of giant sloth, as well as teeth from a horse and a bison from the same cave locality that the sloths came from. We know body temperature in horses and bison, so we can use those results for comparison.
We’re also lucky that the clumped isotope method is so new, that the few labs that are capable of running these analyses are eager to try different things. Right now, we’re not having to pay for the analyses, though we do plan to see if we can get funding to pay for more analyses later.
Cool! Let’s do it!
But wait. There’s another problem. You see, sloths don’t have tooth enamel.
We use enamel from fossil teeth because it’s really hard and resistant to alteration during the process of fossilization. If the material we want to measure the isotopes from has been altered, we may be measuring something besides the body temperature signal – and that could be anything!
Sloth teeth are made entirely of dentine (which we have in our teeth, too, underneath the enamel). Sloths have two layers of dentine, a harder outer layer equivalent to enamel and a softer inner layer like our dentine. We’ve decided to measure the clumped isotopes from both the inner and outer dentine layers (assuming that the outer one is less likely to be altered, because it is much harder). We’re also measuring the clumped isotopes from the enamel and dentine of the horse and bison. This is how we’re going to determine if there is any alteration of the dentine in the sloth. If the sloth outer dentine gives the same temperature as the dentine in horse and bison, we have to be suspicious that it represents some alteration value and not really body temperature (and then all this work is for naught!).
Where we are.
Well, the preliminary data are in. They weren’t what I expected, but I’m not a sloth expert, so I’ll wait for my colleagues to chime in.
In the meantime, it’s time to start writing an abstract on the subject for the Society of Vertebrate Paleontology Annual Meeting, which this year will be in Raleigh, North Carolina. I think it’s gonna be pretty exciting!!!