Tuco-tucos on the Edge

Flamarion's tuco-tuco (C. flamarioni)

There are a great number of endangered animal species in the world, and, as of the time of writing, 644 of them are mammals. These include, of course, such dramatic and visible animals as rhinos and pandas. But they also include many smaller, less glamorous animals. For example, over 200 species of rodent are endangered worldwide, and while that's not actually very many out of the total number of rodent species that exist, it's still quite high in absolute terms.

I realise, of course, that it's unrealistic to expect the public to get as concerned about obscure rodent species as they are about, say, endangered cats. Because, you know, snow leopards. But, in the semi-random style of this blog, inspired largely by whatever I happen to have seen in the literature recently, I want to talk about the social tuco-tuco (Ctenomys sociabilis).

For those who've not heard of them, tuco-tucos are a family of burrowing rodents native to South America. They look rather like voles, but are somewhat larger, at around 20 cm (8 inches) long, if you include the tail. They're good at digging, but they don't live underground in the same sense that moles or mole-rats do, since they leave their burrows during the day to feed on things like grass seeds. In fact, they've been described as the South American equivalent of gophers, although the two groups aren't really that closely related as rodents go. One of their distinctive features is their loose, wrinkly, skin, which apparently allows them to turn round easily in their narrow burrows.

We don't really know a lot about tuco-tuco classification. They're usually regarded as a family in their own right, although they're sometimes considered a sub-family (or even smaller grouping) within the larger degu family. They all look much the same, and, as a result, the precise number of species isn't really clear - there's probably around sixty or so, maybe more. Many of these species are relatively recent discoveries, because they are distinguished largely by having different numbers of chromosomes - something that will make it difficult for them to interbreed to produce fertile offspring.

Such is the case with the social tuco-tuco, which was only described in 1985. In its case, the fact that it wasn't noticed earlier is only partly due to the fact that it looks about the same as other tuco-tucos, and that not many people can be bothered to closely examine small South American rodents anyway. It's also because they just aren't very widespread. Indeed, social tuco-tucos live only in Nahuel Huapi National Park in Argentina. Even within the Park, they're restricted to a very small area, perhaps 20 x 20 miles at the most.

They can't spread any further than that because they're surrounded by wide rivers and lakes on three sides, and by densely forested hills on the other, a habitat for which they aren't suited. Within their range, they've been described as living only in well-watered meadows with dense grass to feed on, so the actual area they inhabit is likely even smaller than it appears. This has two effects.

The first, and most apparent, one, is that the animals are critically endangered, at high risk of going entirely extinct in the relatively near future. With their meadows being chomped upon by grazing sheep, their population is declining, from what was hardly a high figure in the first place - and, of course, they have nowhere else to go. The second effect is more speculative.

As its name implies, the social tuco-tuco does not live on its own, as most of its close relatives do. Rather than each burrow belonging to a single animal, they are shared by small groups of females. These females are related, typically being sisters, daughters, and various nieces. They are accompanied by a single, unrelated, male, and may live in their burrows for a few generations before moving on. This, it has been suggested, is precisely because of their narrow range of habitat options. Forced to make the most of a very limited area, they have to live close together, or there won't be room for them at all.

Well, as I say, that's the suggestion. But this isn't an animal that's had a lot of studies done on it. Just how restricted is it to these wet meadow habitats? And how relevant are they to its social life? In the interests of at least making some attempt to conserve an animal on the edge of extinction, it's important that we understand what it is that it needs. And a new study took a closer look at those habitats, to see just how accurate the previous ones had been. (One of the authors, incidentally, was also an author of the most recent, 2003, study - unsurprisingly, there's not a huge number of people looking into this).

For such a small area, the diversity within the distribution range of social tuco-tucos is surprisingly high. It lies on the eastern slope of the Andes, in just the region that the cypress and beech forests of the highlands give way to the grassy steppes of lowland Patagonia. Being on the margin, its something of a patchwork, with stands of trees, patches of open grass, and relatively barren areas of shrubland all close together. It's cold, with plenty of snow in winter, and not much rain in summer, but, over the course of little more than ten miles, the annual precipitation drops by two thirds, as the slope descends into the Andean rain-shadow.

Because there's nothing else in the area that digs burrows like those of social tuco-tucos, and the burrows apparently collapse within a couple of years if left untended, it's a relatively simple matter to see where the animals live just by counting the entrances. (Well, as simple as anything gets on the edge of Patagonia). That the animals come out in daylight, rather than being nocturnal, also helps.

The study showed that, yes, social tuco-tucos do live in "mallines" - the wet meadows with rich grasses and herbs where they had always been supposed to live. But, at the same time, it showed that over half of the burrows were in drier areas, or in small patches of woodland. Which means that they aren't quite as restricted as we had thought.

But is that good news for them? It certainly isn't bad, since it shows they're not absolutely reliant on one thing. But it isn't necessarily as good as it might appear, either. For one thing, we still know that they don't move out of the area, for instance, into the denser forests to the west, so they can't be all that adaptable. For another, they still weren't found in the truly open steppe. If that's true (as it seems to be), then the patches of land they do inhabit, while larger than we thought, are still separated by stretches of land which they can't. Of course, it's possible that they cross it from time to time, as new groups of sisters seek out new burrows, but it still means that the animals are clustered together in relatively small areas. Which may, as previously thought, explain why they're social in the first place.

It may be that this obscure and isolated rodent isn't in quite as perilous a situation as had appeared. But its population still seems to be dropping, and it's still only found in a pitifully small region. Perhaps we can use this information to help protect it, for instance, by directing sheep away from areas where it lives without necessarily also directing them away from prime pasture. But even if we can, this is still as much a species on the brink as are tigers or western gorillas.

In twenty years time, there will be probably be less species on the globe than there are now.

[Picture by Cláudio Dias Timm, licensed under creative commons-by-sa-2.0]