This, you probably won't be surprised to learn, is as nothing compared to some of the fossil species.
These include such animals as the giant beavers (Castoroides spp.) of North America, which were approximately human sized. But larger still were the dinomyids, which lived in South America from roughly the mid Miocene onwards. By no means all dinomyids were gigantic, but some of them certainly were. So far as we know the largest rodent ever to have lived was Josephoartigasia monesi, which lived during the Pliocene of Uruguay about 4 to 2 million years ago. It looked a bit like a capybara, with a stocky body and wide, bulky snout - except that it was about the size of a cow, and may have weighed as much as a ton.
Which is big, for a rodent.
But being really large isn't the only peculiarity of the dinomyid skeleton. Two features in particular stand out as oddities. One is that the second and third neck vertebrae are fused into a single bone. This must surely have made it harder for the animal to turn its neck flexibly, and is the sort of thing seen in animals that don't really want to do that - notably mole-rats, which need to keep their heads stiffly forward to avoid hurting their necks as they push through soil, and also animals that need to stay streamlined, such as dolphins.
It's also seen in North American porcupines, and, since, like dinomyids, they aren't noted for either their swimming prowess or their subterranean lifestyle, this was long thought to indicate that the two kinds of rodent were closely related, this odd feature having evolved for no particularly obvious reason, and then just remained because it wasn't really a problem.
We now think that dinomyids, while closer to porcupines than to, say, mice, were likely most closely related to chinchillas, endangered rat-sized rodents of the Andes sometimes farmed for their fur or raised as pets. Which, since chinchillas don't have fused neck-bones, means that this feature must have evolved twice, perhaps making it seem less likely that it is due to sheer random chance.
The other unusual feature is just a little further up the skeleton. Along the length of an animal's backbone, the various vertebrae articulate with one another with a set of paired facets at either end. The forward (or upward, if you walk upright) facets of the first vertebra articulate with a pair of similarly shaped protrusions on the base of the skull, termed the occipital condyles.
That's the normal arrangement, anyway. Because, in dinomyid skulls there are two pairs of protrusions: the condyles proper, and a second pair flanking them that, for lack of a better term, we call 'paracondyles'. And these are entirely unique among mammals.
So what the heck were they for? Back in 1916, Carlos Ameghino, younger brother of the much more famous palaeontologist Florentino Ameghino, published an analysis of the dinomyid species Tetrastylus intermedius in which he proposed that the paracondyles helped anchor heavy neck muscles that were needed to support the animal's unusually large head. That was over a hundred years ago, and seems to have been broadly accepted ever since.
But was he right, and can we use more modern methodology to check?
Well, yes we can, thanks to a recent re-analysis of Tetrastylus, along with three other extinct dinomyids and a large number of living rodent species. Among other things, this compared the size of the heads of these animals with that of their bodies, and revealed something rather surprising. It turns out that, even though Tetrastylus was slightly larger than a modern beaver, its head was about the same size. Indeed, when compared to all other rodents examined, it wasn't especially large, at all - proportional to the animal, it's about the same as that of a marmot, and, in absolute terms, it's much smaller than that of a capybara.
On the other hand, the shape of the back of the skull certainly suggests that it did anchor large muscles, and, taken together with the fusion of the two bones further back, this suggests that Tetrastylus did, indeed, have difficulty moving its head too much. So the authors of the new study suggest that there must have been some behavioural reason for this, not something related to size alone.
You might expect me, at this point, to say that, since dinomyids are all extinct, this is all speculation, and we can't really be sure what these animals were like in the flesh, let alone how they behaved. But I won't. Because they aren't.
The dinomyids were at their height during the mid to late Miocene and the Pliocene, when the very largest forms existed. The Ice Ages, and perhaps the collision of North and South America, with its subequent arrival of new northern species in the south, wiped most of them out. But just one dinomyid species survived, and it's still alive today: the pacarana (Dinomys branickii).
Pacaranas live in northwestern South America, mainly in eastern Peru and western Brazil, where they inhabit the heavily forested foothills of the Andes. They are nothing like the size of their giant relatives, although, with the largest individuals being 79 cm (31 inches) long, and weighing up to 15 kg (33 lbs) they aren't exactly small by rodent standards.
And pacaranas do, as it happens, have particularly large heads for their bodies, which may be why Ameghino thought that Tetrastylus did, too. His argument, however, is further dented by the fact that, while the odd paracondyles do exist on pacarana skulls, they are quite a bit smaller and less well-developed than on the proportionately smaller-headed extinct species.
Unfortunately, it appears that nobody has ever really looked at pacarana neck muscles to see how they are actually arranged, which would surely help clear up some of the mystery. But the fact that they live in forests may be relevant, since, while pacarana behaviour has not been intensively studied, there are grounds to suppose that their young, at least, are reasonably comfortable climbing about in the trees.
This semi-arboreal lifestyle also makes sense in light of their skeletal similarity to tree porcupines. In addition to the fused bones in the neck, tree porcupines also have enlarged occipital condyles (although not the additional 'paracondyles' of dinomyids), and, because they have been studied, we know that these serve as anchors for particular strong muscles that hold the head steady.
On the basis of this, it had previously been proposed that the first dinomyids were themselves tree-dwelling. Clearly such structures aren't essential for climbing mammals, or squirrels and monkeys would have them, too, but that doesn't mean that they aren't useful for particular versions of the arboreal lifestyle. Particular features that seem to be true of both tree porcupines and living pacaranas include a relatively large size (for rodents), a habit of slow and stealthy movement and the tendency to pick up and manipulate food with their fore-paws. Tree porcupines, and possibly some of the earlier dinomyids (although this is difficult to tell for sure), also have prehensile tails in a way that squirrels don't.
Large and muscular arms, with a corresponding strengthening of the shoulder region, where the other end of the neck muscles are anchored, are something that would obviously be advantageous for tree-climbing animals. With other parts of the body more flexible than usual, there might also be less reason to move the head about, allowing the neck to become stiffer and better able to absorb the impact of, say, leaping onto a branch from a distance.
The proposal, then, is that dinomyids originally climbed trees, explaining their skeletal similarity to tree porcupines. When they became larger, and moved down to the ground to behave as large browsing animals, they kept the stiff neck and large/duplicated muscle attachments, perhaps partly because there wasn't much reason not to, but perhaps also because, as has been proposed for opossums, it might have been helpful in defence against predators.
This, of course, does take us firmly into the realm of speculation. The giant dinomyids surely didn't climb trees, even as young individuals, although it's reasonable to suppose that their ancestors might have done. We can say that the older theory, that they needed strong necks to support large heads, just doesn't seem to be true, but we probably need more evidence of the anatomy of the very earliest members of the group to confirm or refute this new hypothesis about what the real reason was.
[Illustration by Gustavo Lecuona, from Wikimedia Commons.]