Sunday 3 June 2018

Broken Bones and Missing Toes

Woodland jumping mouse
Life can be hard, especially out in the wild. Injuries can be common, and among the most painful are broken bones. As humans, we can mitigate bone fractures using splints, braces and so on, but wild animals have no such luxury. Nonetheless, bones do heal by themselves, a process that starts with the formation of a tough fibrous scar at the injury site that at least helps to keep things fixed partially in place. Over time, the scar is mineralised to form weak, but functional, bony material, and then eventually rebuilt with as much of the original structural integrity as possible.

Without splints and braces, this is likely to be an imperfect fix, even assuming that the injury doesn't prevent the animal from feeding or otherwise kill it before the process completes. If the animal does survive, the signs of the injury are always going to be there in its skeleton, and are quite likely to cause at least some ongoing problems. But, survive they do, and bone healing wouldn't have evolved in the first place if it never worked in the wild.

Examining the skeletons of deceased animals can therefore provide us with information about not just how they died, but how they lived. What sorts of injuries do they suffer, and what sorts are they likely to survive for long enough that the injuries heal? All we need to do this is a skeleton, so it's perfectly possible to do even for the fossils of long-vanished species, and we do see such injuries on fossil bones from time to time.

Fossils, of course, are rare. If we want a large sample size, especially one that we can compare with the known behaviour of the animal in question, we are going to have to look at much more recently deceased creatures. Although, in principle, there's no reason you couldn't do it with just about any mammal (or, indeed, other vertebrate) it's easiest with small, land-dwelling creatures that are particularly common.

The US Forestry Service conducts a number of long-term studies to monitor the wildlife in their forests, resulting in the collection of a large number of small mammals that can (among many other things) be checked for these sorts of healed injuries. Over three thousand individuals collected at the Bartlett Experimental Forest in New Hampshire were examined by researchers to compare how the injuries suffered by the seven most common species there differed from one another.

The study site is part of the much larger White Mountain National Forest in the north of the state, and is home to around two dozen small mammal species, not counting bats. The most common include three kinds of mouse, three shrew species, and a vole, all of which are widespread (in many cases being found across most of the continent) and not even remotely endangered.

In all, about 6% of the animals studied had a skeletal injury of some kind. Two of these appeared to be due to the results of bone infections, rather than due to physical trauma, and the authors also note that one of the mice had two big toes on one of its hind feet, a deformity it had presumably been born with. This still leaves us with 174 individuals with injuries that they had evidently survived with for long enough to be caught in a trap alongside healthier animals.

But where this is significant isn't how likely an animal was to survive a broken bone, but which sorts of injury it had the best chance of surviving. One might expect that injuries would be fairly evenly distributed across the skeleton, at least in general terms, but, aside from the fact that fractured skulls were relatively rare across all animals (perhaps unsurprisingly), this was only true for the voles and for the cinereus (Sorex cenereus) and smoky shrews (S. fumeus). In the other animals, most injuries were to the ribs or tail, with limb injuries being less common.

This is probably down to the way that these animals move about. The southern red-backed vole (Myodes gapperi), along with the the two regular shrew species, walks about on all fours in a fairly normal manner. As a result, even with one leg broken, it's probably not too badly inconvenienced. Sure, it will be in pain, and have to hobble about a bit, but it's at least capable of hunting or searching for food. In fact, six of the animals had such severe fractures that, even, after healing, one of their arms was significantly shorter than the other, while one of the voles was entirely missing the lower half of one of its back legs. All had evidently survived for at least a month after receiving the injury, which is a fair while for such short-lived animals.

But, with the other four species, this isn't the case. Northern short-tailed shrews (Blarina brevicauda), for instance, spend a lot of time burrowing, searching for earthworms to eat, and, in these animals, although the hind legs were as likely to be injured as they were in the other two shrews, the front legs were rarely broken. In their case, even one broken arm would probably prevent them from digging for food, so they would die before the injury had any chance to heal.

The two mouse species, the North American (Peromyscus maniculatus) and white-footed (P. leucopus) deermice, had, overall, the highest rates of injury, with 20% of individuals showing healed broken bones somewhere. But the injuries were almost always to the ribs or tail, and rarely to the limbs. Both species regularly climb trees, something that surely raises your chance of injury when you fall, and that is particularly hard to do with one of your legs broken. This is something that has previously been found in other small climbing rodents, although, interestingly, not squirrels, perhaps because, being heavier, they hit the ground harder when they do fall, skewing the results.

The remaining species was the woodland jumping mouse (Napaeozapus insignis), and you can probably guess from the name why having even one broken limb is going to be a real problem for them.

But the kinds of injuries that the animals sustained tell us about more than their odds of surviving them. An animal with a broken or missing toe, for example, probably isn't going to die as a result, although it might make it more difficult to run from a predator, say. And, even if it does, there's no reason to suppose that it would be a bigger problem for one species than other - they all use their feet for something.

Yet not one of the mice or voles in the study had any such injuries, while they were fairly common in shrews. In most cases, these weren't simple breaks, either, but partial or complete amputations. If such injuries are no more likely to be fatal, the conclusion must be that shrews are more likely to lose their toes in the first place. Why might that be?

Because they're vicious little buggers, basically.

Shrews, of both sexes, are highly aggressive animals, and often fight with one another, biting and wrestling. Since they sometimes threaten one another by sticking a paw out, that they sometimes get their toes bitten off is, perhaps, not a huge surprise. It may be significant that such injuries were more common on males; while both sexes are equally aggressive, the males may be more likely to compete for mates or other resources.

The study also noticed that, whereas mice and voles with broken ribs typically broke their second rib, shrews had such injuries more spread out along their chest. This suggests that when rodents suffer such injuries, it's because they've banged into something while running forward, but that shrews are at least as likely to get them while wrestling.

If you regularly climb trees, falling out them is an occupational hazard. But losing toes just seems like carelessness.

[Photo by D Gordon E. Robertson, from Wikimedia Commons.]

1 comment:

  1. Similar to your shrew example, golden bandicoots in north-western Australia have long tails when born but are almost invariably tail-less when adults. Apparently this is also because of their habit of tearing each other apart in fights.