Sunday 1 March 2020

Sharing Your Burrow

One of the key features of animal behaviour is sociality - to what extent the animal associates with others of its kind. Many mammals are solitary, meeting up to breed, but otherwise spending their adult lives alone, except when mothers are raising their young. That so many aren't is probably partly due to that period of long parental care. Mammals are defined by their ability to produce milk, which necessarily implies some degree of mother/child bonding, and it may well not take too many behavioural modifications to get from there to just not leaving home at adulthood.

Social behaviour has both benefits and drawbacks. On the plus side, pack hunting makes it easier to take down larger and otherwise unavailable prey, if you're a predator. If you're not, there's safety in numbers, and the more of you there are, the easier it is to spread out the duties of looking out for threats. On the downside, large numbers do make you rather more obvious, and if you're all after the exact same kind of food, there'd better be a lot of it about or some of you will go hungry.

Understanding whether or not a given species of animal is social can have benefits when it comes to things like conservation as well. It can tell us how much resources the animals will need in order to function normally, and it can tell us a lot about how animals disperse once they do reach adulthood, and what they're likely to look for when they do.

Fortunately, for most animals, determining how social they are is pretty easy: you watch them. But it's slightly trickier with animals that are inherently difficult to observe. There could be a number of reasons why this might be so, not least related to where they happen to live (in the depths of a dense jungle, far out at sea, etc.) but one of the items on that list is the animal spending most of its life underground.

Sociality has an additional advantage for subterranean animals in that "many hands make light work" - the more of you there are, the less effort is required to dig the burrow system. Despite this, and presumably because of things like the scarcity of resources, a number of subterranean species are solitary, including, for example, the silvery mole-rat (Heliophobius argenteocinereus). On the other hand, fairly close relatives of that exact animal, such as Ansell's mole-rat (Fukomys anselli) and its famous naked cousin (Heterocephalus glaber) are very much social animals. There's a similar split in a group called the ctenomyids, which are gopher-like animals native to South America - most are solitary, but at least one species lives in multi-female groups sharing access to a single male.

Another possible oddity is the cururo (Spalacopus cyanus), a burrowing rodent native only to a stretch of central Chile. Within this area, it's quite a common animal, even if it's not well-known to the rest of the world. It lives in various grassland habitats, from the western Andean foothills down to the coast, and digs extensive burrow systems. It's a member of the degu family, which also includes the degus proper, which are social, and the viscacha rats, which are not.

Cururos are generally agreed to be social animals, and this isn't particularly surprising. Firstly, they are more closely related to the degus than to the viscacha rats, and their lifestyle is also closer to the former than the latter. Indeed, while viscacha rats do dig burrows, they live in far smaller and simpler ones than the other species do - which makes sense, given that they don't have anyone else to share the construction work with.

Cururos are, however, different from degus, and their other close relatives, the Andean rock rats, in a number of respects. These differences are primarily due to an even more subterranean lifestyle, which, in some respects, parallels that of the better-known mole-rats of Asia and Africa. They have unusually small ears and eyes, and large projecting incisor teeth with which they literally gnaw their way through the soil. Where the other species feed on a variety of plant foods, such as grasses and bark, coruros feed on almost nothing but roots and bulbs, food they can find without leaving the shelter of their burrows.

While the common degu is a fairly well-studied animal, the cururo is rather less so (although to be fair, studies on viscacha rats are even rarer). So, while we know it is a social, burrowing, animal, we don't have a detailed assessment of exactly how social they are. And, given that they spend so much time underground, where nobody can see them, how would we find out anyway?

Fortunately, however, cururos don't spend their entire lives underground. They regularly come to the surface, although quite what for isn't really clear. Degus, which spend rather more time above ground, are diurnal, and it turns out that this is a habit that evolution has yet to break among the cururos, too - they are almost exclusively active during the daytime, and sleep at night, even when they are underground and can't see the sun.  (This, incidentally, wasn't obvious until relatively recently, because, when kept in cages indoors they do seem to be mostly nocturnal, even though we now know that this isn't what they do in the wild).

Possibly the nights where they live are too cold to be comfortable for them, and that's why they prefer to be active during the day. But, whatever the reason, the fact that they come out at all makes it possible to capture them when they leave their burrows and fit them with radio-collars, so that we can then monitor them to see exactly where they go during the course of the day, and where they spend their nights.

When we do this, one of the first things we can demonstrate is that the typical cururo spends its time out of the burrow wandering about over an area of only around 500 square metres (0.1 acres), with some variation depending on the local habitat. Which really isn't very far when you consider that their burrow systems can stretch for up to 600 metres (2,000 feet). Which, in turn, raises the question of just how communal those tunnel systems really are. Are largely antisocial individuals just inhabiting different parts of the same complex?

To cut a long story short: no. We can tell that, not because of how the animals spent the day but how, or more accurately, where, they spent their nights.

While some cururos sleep alone, it turns out that many sleep in groups of two or three, so closely spaced on the radio telemetry that the only reasonable explanation is that they are sharing the same small sleeping chamber. None of these groups are all-male, but pairs of females (possibly sisters?) do sometimes share. Where three individuals share a single nest, they seem to always be mixed-sex groups, usually (although not always) with two females to one male.

This is clear evidence for sociality, especially since given pairs or trios consistently sleep together on different nights, evidently having preferred partners with which they choose to spend much of their time.

But not, it seems quite all. Turning back to the data for those occasions when they venture above ground, some pairs of animals seem to regularly meet up during the day, even when they don't sleep together at night. Indeed, this may even explain what they're doing above ground at all, giving them more of an opportunity to meet others of their kind.

It's probably noteworthy that these are almost always pairs of one male and one female. Which would tend to imply that they have... shall we say, "mutual short-term reproductive interests".

Hey, if you're going to be sociable...

[Photo by Marco Subiabre, released under CC-BY-SA 2.5.]

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