Perhaps the most obvious of these are the ground squirrels. Most ground-dwelling squirrels belong to a single evolutionary group, which includes something like a quarter of all known squirrel species. Many of these - the animals most commonly known simply as "ground squirrels" - look much like their tree-dwelling kin, but the group also includes such animals as prairie dogs and marmots as well as the semi-arboreal chipmunks.
The variation in social habits of these animals is perhaps even greater than their physical differences, with some species that look very similar living vastly different lives. A number of different studies have tried to look at why this is, and how the specific environment in which a species lives, or whatever other factors there might be, affects how they live.
Part of the problem here is that sociality is a complex subject, with some considerable blurring between different categories. As a result, there isn't always agreement as to what those categories should be, or even if the same ones are really applicable between different groups, such as, say, primates and antelopes. Even in the case of ground squirrels specifically, there has historically been some variation in the schemes used, although, broadly speaking there is agreement that how much female squirrels interact with their sisters and adult daughters is the key factor - suggesting that sociality evolves when daughters don't bother to leave home and this turns out not to create any problems for the mother.
Taking that as our measure, it turns out that it is possible to make some generalisations as to under what circumstances varying levels of social living become suitable for the animals. That is, do they want to live entirely alone, close by but still separate, in female groups with the males out on their own, in male-dominated harems, or perhaps in even larger, more complex or egalitarian, societies? (Yes, all of these do happen). It's interesting to note that the more sociable a ground squirrel species is, the larger its brain tends to be, but that's likely a chicken-and-egg situation with the two factors going hand-in-hand.
In fact, one driving force for sociality is probably how much danger the squirrels are in from predators; there are clear defensive benefits to being in a large group that can look out for one another. This also extends to a more generally unforgiving environment, with ground squirrels that live somewhere with long winters and a shorter feeding season also being more likely to gather together - which may be partly because daughters are more reluctant to leave home in the first place if the weather is terrible, but also because they can benefit by huddling together to keep warm.
Given this, we would predict that, absent a particularly large number of local predators, ground squirrels that live in environments where the weather is reasonably warm year-round probably won't be very sociable. Which seems broadly true when we look at the main evolutionary group of ground squirrels, which are found across Europe, Asia, and North America. But it turns out that there's a second group of ground squirrels that live primarily in Africa. What about them?
This group, technically known as the Xerini, form a distinct evolutionary line that likely diverged from the marmots, chipmunks and so on over 20 million years ago. There are only six living species and three of them, as we'd predict from the climate, either live alone or in very small groups. One of the others, however, the Cape ground squirrel (Xerus inauris) of South Africa, Namibia, and Botswana just doesn't fit the pattern, with females living in kin groups, and males living in separate groups composed of individuals that aren't related to one another. So far as we know, no other ground squirrel has that particular social structure.
Of the other two species, we know next to nothing about one of them, which lives far apart from the others, in Central Asia. But there are conflicting anecdotal reports that the remaining one might be social, at least some of the time. But is it, and, if so, is it anything like its relative, the Cape ground squirrel? The first detailed study to try and answer that question was published earlier this year.
The species in question is the Barbary ground squirrel (Atlantoxerus getulus). It lives primarily in the Atlas Mountains of Morocco, with some populations reaching over the borders into Algeria and the disputed territory of Western Sahara. In 1965, however, a breeding pair was introduced to the island of Fuerteventura, where the dry rocky interior evidently suited their descendants, so that they are now quite numerous. It was this population that the study looked at, which could, of course, mean that their behaviour is different on the mainland where they would be less constrained by available land area.
At any rate, it turns out that these squirrels do, indeed, behave similarly to their relatives over at the other end of the continent. Most of the year, they live in communal groups, but once a female becomes pregnant she retreats to an isolated burrow, where she eventually gives birth and raises her children. Since there's a relatively short breeding season (something that isn't true of the Cape ground squirrel, which breeds year-round), her sisters will have become pregnant at around the same time, and around half the time, once weaning is over, the whole family group gets back together to raise all of their litters communally.
Once they are old enough to do so, the males leave home and form their own, sexually segregated, communities with other, unrelated, males. From then on, they mostly sleep alone in their burrow outside of the breeding season. In particularly dry years, however, some young squirrels delay the onset of sexual maturity, and mature males are apparently willing to sleep together with such subadults regardless of their sex - perhaps benefiting from the larger group size when their sleeping companion isn't also a rival.
This breaks with the pattern seen in the more temperate-dwelling ground squirrels. There, males either live alone or together with females; they don't form single-sex communities of their own. Nor can the usual pattern that long, harsh winters help drive sociability be true in the case of either Cape or Barbary ground squirrels. A plausible explanation is that, here, it's instead due to unpredictable rainfall patterns in an environment that's dry to start with.
With so many different kinds of squirrel across the world, it's inevitable that some will have to adapt to face different pressures and that they won't all come up with the same solution to similar problems.
[Photo by "Donkey Shot" from Wikimedia Commons.]