Just Cold Enough

Northern meadow jumping mouse

Hibernation is one of the most effective means that mammals can employ to withstand harsh winter temperatures. Many mammals, especially larger ones, use other means, such as growing a winter coat, caching food, or simply moving somewhere warmer, but, if the goal is to reduce energy requirements when you can't get out to feed, hibernation is the most effective means of doing so.

During hibernation, bodily metabolism slows right down, so that an animal may need as little as 1% of its usual calorie supply to stay healthy. This has negative consequences, so that the animal does need to wake at intervals to stave them off, and how often this happens varies from species to species. In this respect, true hibernation can be distinguished from shorter, often daily, bouts of torpor by the fact that each "sleep" can last for weeks or even months. 

Why Polar Bears Still Make Dens

Hibernation is something that's generally associated with smaller animals, such as bats and rodents. It's likely to be a particularly useful trait for them, since they otherwise need to eat regularly to survive and this would be difficult in winter. Bears are a clear exception to this, snoozing away the winter despite being particularly large animals, and it seems that hibernation has been a key feature of their evolution for some time, distinguishing them from other carnivorans, such as wolves, that live in similar climes.

One of the first things we have to point out in this respect is that it isn't universally agreed by scientists that bears do, in fact, hibernate. This is obviously a debate about terminology, rather than the bear's actual behaviour, and it hinges on whether a bear's body temperature drops enough while it's denning for the activity to be truly considered as "hibernation". It seems fair to say that the usual tendency these days is to consider that what they're doing is close enough to count, but not everyone agrees. For today's purposes, however, I'm going to use what some might argue is a layman's understanding, rather than a strict biological term.

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 Best Place to Den

Winter is a difficult time for many animals at high latitudes. Food is scarce, and the cold climate itself is a problem. Although many mammals do manage to keep going through the winter, others migrate to warmer climes, while many of those that don't hibernate instead. While this is by no means easy, for animals that can manage it, it allows them to preserve their energy, surviving off body fat without the need to search for food.

There has, historically, been some dispute as to whether bears truly hibernate or not. This is because the sort of undoubted hibernation practised by, say, bats, involves an almost total shutting down of normal metabolic functions with the animal effectively becoming cold-blooded for the duration. Bears do not do this; while they are asleep through the winter, their body temperature drops from a normal level of 37°C (99°F) to a low of 33°C (91°F). Now, this is not insignificant, since a human with a body temperature that low would be suffering from clinical hypothermia and in fairly urgent need of medical treatment. But still, you're not going to be so cold that dew literally starts forming on you, as happens with bats.

Small British Mammals: Dormice

Hazel dormouse

The general body plan of looking like a mouse or rat is one that's proven very successful for rodents; a huge number of rodent species, not all of which are closely related, broadly fit this description. Both the mouse family and the "hamster family" (which includes deer mice, pack rats, and voles, among others) are considered to be part of a larger grouping of related rodent families called the Myomorpha. This roughly translates as "mouse-like rodents" and it's a group that includes, under the most common current system, seven different families.

Historically, the Myomorpha was defined based on the structure of the jaw muscles, so that it included some animals, such as jerboas, that don't necessarily look all that much like regular mice. But modern genetic analysis has shown that this anatomic feature isn't as unique as we'd thought, and seems to have evolved at least twice, perhaps in response to a similar dietary requirement in the two different groups. As it happens, the jerboas got to stay in the "mouse-like" group (if only just), but one family of animals that manifestly do look like mice turned out to be something else entirely, and about as unrelated to true mice as it's actually possible to be among living rodent species.

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.

The Echidna's Incredible Expanding Testicles

The echidna is a pretty strange mammal. Most notably, it is the only mammal other than the platypus that lays eggs. I've discussed in an earlier post why that doesn't disqualify it from being a mammal, but it is undeniably unusual. It's not even the only odd thing about echidnas, since they can also sense electricity through their snout, and the males have a four-headed penis.

I'm not making this up.

There are, in fact, four different species of echidna. The one most people are likely familiar with, though, is the short-beaked echidna (Tachyglossus aculeatus), on the grounds that it's the only one found in Australia. (The others, if you're wondering, live in New Guinea). In fact, they're found pretty much across the whole of Australia, albeit generally in open woodland or rainforest, rather than true desert. Even so, that's a fairly broad range of habitat types and variations in climate, and their behaviour does change somewhat across the continent.

Giving Birth While You're Asleep

A number of mammals living in temperate environments hibernate during the winter, or when food is otherwise scarce. The great majority of them are small animals, such as rodents, which normally have a high metabolic rate, and need to eat constantly to fuel their bodies. While hibernating, their body temperatures drop to virtually that of their surroundings - effectively making them temporarily cold-blooded. They do, however, have to wake up from time to time, if only to relieve themselves, and possibly have a light snack.

Bears, on the other hand, cannot really be described as "small mammals". Yet most of them do seem to hibernate. That hibernation is, however, rather different to what, for example, hamsters, are doing. In fact, it's sufficiently different that it's been argued that bears don't really hibernate at all. To be honest, it's a matter of definitions, of what exactly you consider the word 'hibernate' to mean. Today, most zoologists seem to consider 'hibernation' to be a perfectly good word for what bears are up to, although it's certainly possible to come up with a stricter definition.

The Secret Life of the Monito del Monte

There are something like a hundred species of marsupial in South America; hardly an insignificant amount. As I've mentioned before, the marsupials were, in fact, in South America before they ever reached Australia. While some headed south over the still green and verdant lands of Antarctica to reach the Australian continent beyond, others stayed behind, becoming the ancestors of the opossums and shrew-opossums that still live in the Americas today - including, of course, one species in the southern US.

But there is an oddity that confuses this simple picture, and that is a curious animal called the monito del monte (Dromiciops gliroides). In evolutionary terms, this gives every indication of being an Australian marsupial, being more closely related to animals like kangaroos than it is to American opossums. Which is a bit odd for an animal that lives in Chile and Argentina. How can this be so? The best guess is that the ancestors of Australian marsupials originated in South America before they crossed over to their new home, and that, for some reason, the monito del monte was the only one to survive in their original homeland. It is, however, also possible that the Australian marsupials really did originate in Australia - or, for that matter, in Antarctica - and that the monito del monte headed back to South America at a later date before the Straits of Magellan opened up.

Cold-blooded Mammals

Short-eared elephant shrew, another member of the family

Mammals are warm-blooded animals. That is, they maintain a consistent body temperature regardless of, and usually higher than, that of their surroundings. It is, after all, one of the defining attributes of being a mammal. Yet, while virtually all healthy mammals have no real difficulty in keeping their body temperature high, a great many of them don't always bother.

Probably the best known example of this is hibernation. Many animals living in climates with cold winters enter a long "sleep" when their food becomes scarce, and don't wake up until the following spring. But this isn't sleep of the sort that humans experience daily, but something much more dramatic. Their whole body shuts down to a minimal level of activity, and body temperature drops to only a fraction above that of their surroundings - essentially, they become cold-blooded. Of course, unlike reptiles and amphibians, they aren't doing anything much while they are in this state, so we shouldn't draw too many parallels, but they can't be truly said to be warm-blooded at these times, either.

Bat Swarms of Colorado

When we think of bats, the environment that first springs to mind is that of a cave. While there are a number of bats that roost in trees during the day, a great many are, indeed, cave-dwellers. Because caves aren't all that common in most places, bat colonies within them can be huge, which creates something of a problem when they all try to fly out in the evening. But, if you watched a bat cave throughout a year, in most cases, the activity level would change radically as the seasons progressed.

This is largely because most bats eat insects. In temperate climates, insects aren't around much during the winter; in many cases they've died off and left their eggs to hatch the following spring when the weather has improved. Any animal that relies heavily on eating insects to survive has to deal with this scarcity somehow. Insect eating birds, such as swallows and thrushes, cope with the problem by migrating in the winter, heading to warmer climes where insects are still common. There are some bats that do exactly this - for example, the hoary bat migrates annually between Canada and the southern US - but more commonly, they deal with the absence of food by hibernating.