A Good Winter's Sleep

Mammals have an advantage over reptiles in that they don't need the weather to be warm in order to stay active. This makes it easier for them to live in parts of the world that have cold winters, but even then, the scarcity of food at such times of the year means that they often need some additional survival strategy. Some, of course, simply migrate somewhere warmer during the winter - which typically means moving downhill from summer grounds on mountainsides, the long-distance migration of birds being less of an option. Others, such as polar bears, are just good at surviving cold weather anyway, and may not need to do anything significantly different in winter.

But, leaving those possibilities aside, three basic options for surviving the winter present themselves. They could do something behavioural, such as storing food during the summer and coming back to their hidden caches later in the year when food is short. Or they could change physically, such as by building up fat over the summer or having an extra-thick winter coat that falls out in the spring. (And these are not, of course, mutually exclusive).

The third option is to dodge the issue by hibernating.

Hibernating, however, brings a number of challenges of its own. Perhaps the most obvious are the physiological ones; how to survive for months on end without eating and drinking, or poisoning yourself with the build-up of waste products such as urea. But there are also behavioural challenges, such as picking when and where you should go to sleep.

The place that an animal hibernates in is termed a hibernaculum, and different animals will have different things they are looking for when they select one. In general, though, it needs to be reasonably secure so that nothing is going to come and eat you while you're asleep, and it also needs to be warm enough that you aren't going to freeze to death. 

The latter part of that isn't necessarily as simple a decision as it might appear at first. This ties in to the other question about the behavioural aspects of hibernation: when to start doing it. Many animals want to start hibernating before it gets cold, so that they aren't already stressed and hungry when they begin. So, the hibernaculum has to be somewhere that's not going to be too hot and uncomfortable in the period after you've gone to sleep, but before the snows come (or whatever). Furthermore, if you're going to use the same hibernaculum year after year, it's got to work as well in mild winters as it does in harsh ones - especially since the animal often won't know in advance which will happen in a given year.

And here, of course, we come across one of the great challenges of the modern world: climate change. If winters become milder, and the spring starts earlier, ideally you need to either move somewhere else where the climate is more suited to you, or you need to change the timing, and possibly location, of your hibernation.

Some animals are better at this than others. For instance, it has been reported that yellow-bellied marmots (Marmota flaviventris) now hibernate for shorter periods than they used to, giving them more time to be active, find food, and raise their young during the year, thus boosting their overall population. Columbian ground squirrels (Urocitellus columbianus), on the other hand, have suffered reduced population growth, apparently because snows come later in the year, forcing them to emerge at a later date than they otherwise would.

So, while one would expect milder winters to be a good thing for most animals, it isn't necessarily so. (And, of course, this is ignoring any effect from hot or excessively dry summers, which might cause problems from the opposite direction). One hibernating animal that might find mild winters an issue, for instance, is the northern Idaho ground squirrel (Urocitellus brunneus).

This is already an endangered species. It has been listed as such internationally since the 1980s, when it was thought to consist of two subspecies. These are now regarded as entirely separate species, and the southern one (U. endemicus), while still threatened, is not in such dire straits as the northern one, which retains the formal scientific name - and the endangered status.

This particular ground squirrel lives, as we might expect, in Idaho. More to the point, though, it doesn't live anywhere else, and it isn't exactly widespread even in that state. Historically, it used to live across the eastern edge of the Wallowa Mountains, occupying an area of just 1,600 km² (620 square miles) which is probably less than any other species of ground squirrel. But that's the historical range.

Idaho ground squirrels live in forests of Ponderosa pine and Douglas fir, both of which have actually done rather well in the area of late, with (ironically, considering what's going on further south in California) relatively few wildfires. This allows the forest to become denser, with trees growing on areas of ground that would previously have been naturally cleared.

Which you'd think was a good thing, and probably is for a number of animals. Unfortunately, the Idaho ground squirrel lives in open meadows - treeless patches within the forest, typically formed by wildfires, where it likes to eat flowers, low-growing herbs and other plants that don't really want a forest canopy cutting out the light. So for it, this is bad news and the possibility of recreational housing developments in the near future isn't helping much. 

Indeed, the species is now restricted to just four patches of land, isolated from one another and with an estimated total area of a mere 20 km² (7½ square miles). The population is probably around half what it was just 20 years ago, with perhaps around 1,000 breeding adults... divided between those four bits of land, so that individual breeding populations are even smaller. Despite which, in recent years, there seems to be an indication that they are recovering, due in part to habitat management efforts.

But they certainly aren't recovering rapidly, and the species is sufficiently on the brink that it wouldn't take much to undo the progress so far. Hibernation is crucial to it, because it spends as much as eight months of the year hibernating, doing all its feeding and breeding between March and July. So are the changes in weather systems - as typified by the recent "heat dome" over the Pacific Northwest - good or bad for this endangered species?

To try and answer that we need to have an understanding of how this particular species behaves when it hibernates. To do this, scientists fitted GPS radio collars to 99 ground squirrels and followed their activity over a five-year period. When we do this it turns out that, as is typical for ground squirrels, it is the males who emerge from their burrows first. 

They do this so that they can establish territories and be ready and waiting when the females emerge and can mate with them as soon as possible. That's important for this species in particular, because they have to go back to sleep in four months, and any young will have to not only be born, but fully weaned, before that happens. Indeed, the females are only sexually receptive for a few hours each afternoon on a couple of days a year, so timing is crucial.

Years, or specific locations, with late snowfall resulted in the squirrels leaving their hibernation burrows later, so they're evidently waiting for it to have gone before heading out in search of food and sexual opportunities. But the only factor that seems to affect when they enter the hibernacula in the first place is how large they are - larger, presumably better fed, squirrels head in earlier, perhaps because they're more confident of surviving the winter and don't want to spend more time foraging (and being visible to predators) than they really have to.

The hibernacula themselves are typically around 56 cm (22 inches) deep and are more likely to be found in wooded areas, rather than the more open pasture in which the animals spend their waking hours. It's though that such areas have warmer soil temperatures in winter, but cooler in summer, which would help the animal hibernate comfortably. On the other hand, while milder winters will mean that the weather outside isn't quite so cold, the lack of a thick layer of snow above the burrow could also mean that it still becomes uncomfortably cold (depending on just how mild the winters get, of course).

Such problems can likely be solved by digging deeper burrows, or shifting the locations of doing so to lower valleys or the like. There is some evidence that the squirrels can alter the depth of their burrows to suit local conditions - they already tend to be shallower at higher elevations where heavy snow is more likely. But that would depend on the squirrels judging the mildness of the coming winter when it's still only July, which seems unlikely.

That's the sort of thing that can be tweaked over the generations as squirrels adapt to changes in climate, but only if the changes are slow enough. Either way, it does seem likely that if this endangered species is to survive the coming decades, it may have to change the details of its behaviour... and we don't really know how well it can do that.

[Photo by the US Fish & Wildlife Service, in the public domain.]