Sunday, 30 April 2017

Perils of the Big Snooze

There are over 280 recognised species of squirrel, only a little over a half of which spend any significant time up trees; the others include an array of more or less ground-dwelling species, from chipmunks to susliks. They live in almost every terrestrial habitat imaginable, from tropical rainforest to semi-desert. One of the few habitats they don't live in is, unsurprisingly, polar ice cap, but, even then, there's one group of squirrels that at least gets sort of close.

These are the fifteen species of marmot (Marmota spp.), most of which inhabit mountainous regions, often above the tree line. Marmots are the largest of all squirrels, being at least twice the weight of, say, prairie dogs. However, the weight of marmots isn't necessarily the easiest thing to quantify, because it changes so much over the course of the year, which is in turn due to their need to bulk up before entering hibernation. And the reason for hibernation in marmots? Well, that brings us back to the crappy habitat.

Clearly, there isn't a lot of food up a mountain, especially in the dead of winter. But it's worse than that, because, being squirrels, marmots are anatomically adapted to eat nutritious seeds and nuts - their teeth and the structure of their digestive tracts are ideal for this sort of thing. Which is kind of a problem when you live above the tree line where you just aren't going to find much in the way of large seeds. Indeed, what marmots actually eat, for the most part, is grass and herbs, and the former, in particular, is not very nutritious.

In fact, this may be one of the reasons that marmots are so big; having a large gut (and hence needing a large body to put it in) means that they can retain food for longer, extracting more nutrition from it than they would otherwise. In fact, it's been estimated that marmots are about as good at getting nutrition out of grass as are voles, which really do have a gut structure that's adapted for that sort of thing.

But even with this greater digestive efficiency provided by their larger size, marmots still run out of food in the winter, since, unlike some other squirrels, they don't hoard it for later use during the summer. Instead, they hibernate, eating as much food as they can during the autumn and then snoozing for anything up to eight months. Their larger size probably also helps here, since a larger animal needs, proportionately speaking, fewer calories to maintain itself while hibernating. It's worth noting that even the two species of marmot that don't live at high altitude - the steppe marmot and the groundhog - also live in relatively cold, grassy habitats where the same problems apply.

But, when you're large, and have to rely on grass for your nutrients, there's a downside to hibernation, too. Basically, it takes a lot of time to gather as much food as you need to consume in order to survive the winter sleeping in your burrow. And marmots spend a lot of their time doing that; some species spend as much as 80% of their entire lives inside their burrows, and over half of that in hibernation.

All of this has, as one might expect, significant effects on the lives and lifestyles of marmots.

For a start, there's hibernation itself. During hibernation, marmots reduce their body temperature to a minimum of around 6°C (43°F) , thus greatly reducing their energy consumption. Just how much they save depends on how long they spend in this state of torpor, and how much in something more closely approximating regular sleep. For example yellow-bellied marmots (M. flaviventris) spend over three-quarters of their hibernating time in full torpor, saving 83% in calorie consumption, while groundhogs (M. monax) save only 43%, because they do the same for only two-thirds of their hibernating time.

As an aside, one might well ask why groundhogs are less efficient hibernators than yellow-bellied marmots. It's simply because they don't need to be. Yellow-bellied marmots, which inhabit the mountains of the western US, live in a harsher environment than groundhogs, in the wooded margins of the east. For them, efficient hibernation is essential if they aren't going to starve to death before they wake up - something that's their second greatest cause of death, after being eaten. But deep hibernation carries its own risks, such as making the animal more susceptible to disease, so if groundhogs don't need to take that chance, they won't.

But another effect of the combination of long hibernation and large body size is to reduce the frequency of breeding. Smaller squirrels, such as chipmunks or the red and grey tree squirrels of Europe and North America, often breed twice each year, but marmots, forced to spend a considerable portion of their time bulking up in readiness for hibernation, breed annually at best. Breeding typically takes place as soon as possible after they leave their winter burrows, and in many species, even before that. But the stresses of pregnancy and the need to rear the young afterwards mean that female marmots have less time to bulk up their body mass towards the end of the year.

The end result of that is, that, when they emerge from their burrows the following year, those same females are simply not large enough and fit enough to raise young. Instead, they take a year off, concentrating on foraging so that they are large enough to breed the next time. This isn't true of all marmot species, and some do breed annually, but two thirds of them don't. In fact, the very largest marmots, such as the grey marmot (M. baibacina) of the Altai Mountains, typically breed only once every three years. Those that do manage annual breeding include the groundhog, which notably lives in just about the least harsh environment of any marmot species.

This combination of a short feeding season and the need to build up a large body also affects how long it takes young marmots to reach sexual maturity in the first place. Not only do marmots not reach maturity for a couple of years, but in almost all species, the young don't have time to abandon their place of birth and find somewhere new to live in their first year, instead hibernating close to home; the one exception is, again, the groundhog.

This in turn, means that communities of marmots build up, and they become, for squirrels, unusually social animals. Alpine marmots live in groups consisting of multiple adults as well as adolescent yearlings and any pups born that year. Only one pair of these adults breeds, apparently because they're so aggressive and bossy towards the others that the latter are too stressed to breed, even if the dominant pair happens to be taking that year off from rearing young themselves.

It's perhaps worth noting here that a species of squirrel closely related to marmots, the golden-mantled ground squirrel (Callospermophilus lateralis), lives in the exact same mountainous region as some yellow-bellied marmots. Like marmots, they too, hibernate, yet, being smaller, they don't have to use up quite so much of their time searching for adequate food in the autumn. Their young do have enough time to leave home, and it's at least plausible that this is why the ground squirrels, but not the marmots, remain anti-social and avoid getting in each others' way.

Marmots haven't always lived like this. The oldest fossil marmot that we know of, M. vetus, lived in what were, at the time, the warm lowland forests of Nebraska, around 8 million years ago. Not long after this, marmots began to inhabit upland areas, and they only became larger when the world grew colder. The Ice Ages, in particular, seem to have been a good time for them, and the different species seem to have been isolated in their respective mountain ranges since at least that time.

Now, of course, marmots are facing further climate change, and much more rapidly this time than in the past. While many can probably adapt, drier, warmer, weather will affect others by reducing the time available for feeding during the year. When their lifestyle already has to make accommodations for a short feeding season, that could be a real risk. For many, the only viable alternative will be to head further uphill, to higher slopes that remain cold enough for them and damp enough for their food.

But the problem with heading uphill is that, sooner or later, you run out of mountain. Olympic marmots (M. olympus), for example, live only in the highland interior of one peninsula on the western side of the Puget Sound. At the moment, they are numerous, and largely untroubled, but in the longer term, if the local climate changes enough, there are only so many places they can retreat to if they are to avoid the perils of their close neighbour, the critically endangered Vancouver Island marmot (M. vancouverensis).

[Photo by Chad Collins.]

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