A Tale of Three Ground Sloths

Today, sloths are slow-moving tree-dwelling animals found only in South and Central America. For most of their evolutionary history, however, the majority of sloths have been ground-dwellers, in many cases too large to have climbed a tree even if they had wanted to. In comparison, tree sloths are a relatively recent evolutionary development. 

Ground sloths, however, are not a single type of animal in the strict biological sense. This is because the two-toed and three-toed sloths that we have today are not especially close relatives, last sharing a common ancestor at least 28 million years ago. In fact, tree sloths evolved twice, in separate branches of the sloth family tree, and some ground sloths were more closely related to one or the other. On this basis, we used to divide the ground sloths into two families, but the fact that some of lived recently enough that we can recover and analyse DNA from their fossils gave us some surprising detail and a key 2019 study suggested that we should consider there to be no fewer than six different families of ground sloth.

So whatever we can say about one sort of ground sloth isn't necessarily true of all the others, even on quite a broad scale.

During the last Ice Age, the most widespread and common sloth in North America was probably Jefferson's ground sloth (Megalonyx jeffersonii). This belongs to the ground sloth family that seems to be the most closely related to the modern three-toed sloths and which is, in fact, named for it - the Megalonychidae. Like other sloths, they first appeared in South America, with one genus, Pliometanastes, appearing in the North just as the two continents first joined up. This probably gave rise to Megalonyx, which evolved through a series of increasingly large species to culminate in Jefferson's ground sloth around 300,000 years ago.

This was just about the first fossil mammal to be discovered in North America, and was first described in a 1797 presentation to the American Philosophical Society by a certain Thomas Jefferson, shortly before he went on to be more famous for other things. Jefferson assumed that the creature would still be alive somewhere, since he didn't believe that God would create animals only to let them go extinct again. In retrospect, despite having asked Lewis and Clark to keep an eye out for one, he proved mistaken on that point - we now know that they died out around 9,000 BC. Nonetheless, his role was considered pivotal in kickstarting American palaeontology, and French zoologist Anselme Desmarest honoured Jefferson when we gave the animal its official scientific name. 

We have fossils of these creatures from almost every state of the contiguous US outside of New England, plus some from Mexico, western Canada, and even Alaska. The wide range of the animal suggests that it was more adaptable than living tree sloths, at the very least being more tolerant of the cold. Its presence in the more northerly regions coincides with the warmer gaps between the Ice Ages, so it's not as if it inhabited snowy wastelands, but its preferred (though not only) habitat seems to have been mixed woodland with a high proportion of spruce.

At least 180 different sites have revealed fossils of the species, although few have revealed more than one example. There is hardly anything unusual about this when it comes to fossil animals, but sites that do include multiple fossils of the same species can provide additional information when we do find them, especially if it appears that the animals in question all died at the same time. Given that sort of lucky find, we can make inferences about how they lived that would otherwise be unattainable.

For example, one fossil site in Alabama that has been estimated to have contained the remains of at least sixteen Jefferson's ground sloths, deposited over a lengthy period of time, was a cave where there clearly wouldn't have been much to eat. This raises the possibility that the sloths had retreated there to seek shelter from a cold winter or that, considering some of the fossils belonged to juveniles, that mother sloths used the site as a maternity den.

Another such site is the bank of a creek in southwestern Iowa. Part of a fossil was first identified here in 2001, following a flood in the 1990s that had altered the path of the creek, cutting into one of the banks. The following excavation revealed three partial skeletons (one of them just the shoulder blades) embedded in fine blue clay. Dating techniques revealed that the clay around the fossils had last been exposed to sunlight around 100,000 years ago. 

This dates to slap in the middle of the last interglacial - the warmer time between the penultimate and last Ice Ages. Supportig this, the clay also contained fossilised remnants of plants such as reedmace, stinging nettles, amaranth, pennycress, swamp verbena, and lobelia, all of which suggest a marshy habitat along a stream bank, while pollen came from oak, cedar, hickory, and pine among other plants - none of which are very different from what you'd find in Iowa woodlands today. 

We can also gain further information by looking at the condition and orientation of the bones themselves. The way that they were arranged suggests that the animals died where they were found, rather than their bones being washed downstream, and were slowly buried under accumulated mud as the stream periodically flooded. They must have been exposed for some time, since some of the bones were gnawed on by rodents and damage to the skull of the largest individual suggests that something trod on it when it was still fresh, rather than it being caused by later rock compaction around it. That some of the limb bones, and almost all the bones from the smallest individual, are missing may also support this - a scavenger probably carried them off to eat elsewhere.

But it's the presence of three individuals that's particularly significant. If the bones had been washed downstream, that might not prove much - perhaps the site could have been a river bend where such things tend to wash up year after year. But, instead, it's likely that they all died together, and here what's noteworthy is that they were such different sizes.

The largest is, in fact, one of the largest Jefferson's ground sloths so far found. It's hard to know exactly how big something was just by measuring the bones in an incomplete skeleton, but the researchers who dug it up estimated that it would have weighed 1286 kg (1.4 US tons), well over the 1,000 kg estimated as typical for the species. It's unclear whether males were larger than females in this particular family of ground sloths, although this has been suggested for some of the others. So could this be a male?

Obviously, we have no way of knowing that for sure. But the other two individuals were immature, since not only were they much smaller, but the structure of their bones showed that they hadn't stopped growing. That might suggest a mother with her offspring, and the researchers also say that the shape of the part of the skull that anchors the jaw muscles is more indicative of a female than a male. If that's right, maybe Jefferson's showed the opposite pattern to the usual mammalian one, with the females being the larger sex. That's unusual in large mammalian herbivores, but not unknown, since it's also true of, for example, tapirs.

However, living sloths only ever have one offspring at a time, and here it's notable that while the second sloth was about half the size of the putative mother, the other was only around a third of the size. If the larger one was the older of the two, by how much was this true? Well, if they were anything like similarly-sized mammals today, Jefferson's ground sloth would have given birth about once every three years or so and reached sexual maturity at six or seven years. So, perhaps three years older, or a little more.

The scenario we end up with is that the smallest individual was very young, and the older one must logically have been weaned by the time its younger sibling arrived. And yet, unlike any modern sloth, it hung around, likely receiving continued parental care of some kind.

Since there are so many different kinds of ground sloth, we can't make the same inference of others, perhaps not even those that are close relatives. But that it's so different from modern sloths means that we may not be able to make the opposite inference, either. Modern tree sloths are solitary animals but at least some of the predecessors may have had some kind of family life, with the mother caring for her young long after we might expect them to have left home.

[Photo by "Daderot", from Wikimedia Commons.]