How does social living get started, in evolutionary terms? Perhaps the simplest way is that children simply fail to leave their mother, creating a fairly permanent family group. According to one theory, such groups are likely to become particularly stable if there are not enough resources around (for whatever reason) to allow the children to wander off and have young of their own. In these situations, the theory proposes that the older children hang around in order to help their close kin, such as younger siblings, and thus have at least some chance of passing their genes on to the next generation.
If this theory is correct, we would expect animals to be more likely to live in social groups where resources are limited, and for long-lasting groups to be based around extended families, rather than individuals who initially came together more or less randomly. It doesn't take too much thinking to realise that this is often not the case, since it is quite common for a very high proportion of the females in a group to be breeding in any given year. Or the groups may be long-lasting as a whole, but change their membership regularly - this latter is particularly common in primates, for example. For many creatures, it seems, the advantages of not being eaten (say) are quite enough on their own to persuade them to gang up together.
There are also potential downsides to living in extended families, rather than in groups of unrelated individuals. You might become more susceptible to disease, because of a lack of genetic diversity, for instance. And if the younger individuals do breed, they are at risk of doing so with their own kin, which leads to inbreeding.
This does not, however, mean that the theory is wrong, merely that it is one possibility among several. We do know of several social mammals that live in extended families, and that are more likely to do so when resources are limited enough that they can't all breed, but not so limited that a larger group will simply run out of food. But it's by no means true of all social animals, and there are at least some that vary their strategy as the level of resources fluctuates.
One animal of which this might be true is the common degu (Octodon degus), a rat-sized rodent native to the scrublands of central Chile. Although it is often simply called a "degu", without qualification, as the full name suggests, other species of degu do exist, although they are not as... well, common. In recent years, they have become a moderately common exotic pet, although fears that they might escape and damage the local ecology mean that it is illegal to own them in several jurisdictions. In respect of which, it's perhaps worth noting that they are more closely related to guinea pigs than to actual rats.
At any rate, common degus are highly sociable animals, digging communal burrows in which to spend the night. (Interestingly, all the not-common degus are nocturnal, but the "common" species is not, which likely increases its attractiveness as a pet). It was already known that degus in resource-rich environments do not live in obvious extended families, instead forming groups of unrelated individuals that change over time. But even in that earlier study, the researchers suggested that the same might not be true of those living in more marginal environments, where it would be hard for all of them to raise their own young. A more recent study took up the challenge to see whether this apparent preference for friends over family was a common feature of degus' social life, or something more opportunistic.
To do so, they examined two communities of degu, one living in a low-lying shrubland habitat, and the other much higher up the side of the Andes where food is available, but more patchily distributed. They counted the number of times that degus shared their burrow overnight with other specific individuals, and whether they were more likely to share with family members at the high altitude site than at the more abundant one.
They found... nothing. Common degus, it seems, are no more likely to gather round their families when resources are limited than at any other time. This, of course, as noted above, doesn't mean that the entire theory is sunk, since there are a number of other species where it does seem to be the case. Indeed, they may be relatively unusual in this respect. But, of course, that just raises the question of why it should be so, and whether there's anything exceptional about the lifestyle of these particular rodents.
That, of course, is trickier to answer, when all you've got is data showing that there isn't a pattern. But, in fact, there are a few unusual features of degu lifestyle that may be relevant. For one, what seems to happen is that children leave home when they reach adulthood, and then find another group to join that's about the same size as the one they just left. Male mammals do this all the time, thus preventing inbreeding, but there's no reason for both sexes to do it - so long as one moves, you're fine.
Yet, in degus, it seems that both sexes leave home, implying that there's some specifically about staying with their parents that they don't like. Perhaps the risk of competing for resources with their own close relatives outweighs any advantage they might get by helping to rear their siblings or their sibling's children. Or perhaps it's something to do with disease susceptibility. Or... well, make your own guess; at the moment, it's likely as good as anyone else's.
Somewhat easier to answer is the question as to why they bother to live in groups at all, if they usually leave home at the first opportunity. It's likely that defence against predators - more eyes to keep a lookout, and so on - is a factor here, as it is for many herbivores. But perhaps more importantly, cooperating in groups makes it easier for the degus to dig their burrows, a phenomenon that is also thought to help explain the vastly more social behaviour of naked mole-rats and their kin.
At any rate, for whatever reason, it seems that degus would rather hang out with their friends than stay at home with their families...
[Photo by ManyJanos, from Wikimedia Commons.]
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