Sunday 8 May 2011

The Not-Naked Mole Rats

Silvery mole rat
I recently discussed moles, and their adaptations to their unusual, underground, lifestyle. But, of course, they are not the only group of mammals to adapt to such a life. A great many rodents are burrowing animals, spending a significant proportion of their time below ground, often in a bid to escape predators and to have a safe place to store food for later use. However, the majority travel above ground to forage, and are not truly subterranean in the way that moles are. Not so the mole rats.

The most famous mole-rat is surely the naked mole-rat, a bizarre, hairless species known for reproducing in the manner of bees or ants - with a single reproductive queen supported by numerous infertile workers. But this is not the only mole-rat, and the other, furrier, forms are also highly adapted to underground life. Indeed, the naked species is regarded as exceptionally odd, even by the standards of a group that's fairly strange to begin with, and seems to have branched away from its kin at an early stage in their evolution.

Common Mole     Damaraland      Dune & Cape   
 Rat, etc.     Mole Rat, etc.    Mole Rats 
     ^               ^               ^     
     |               |               |       Silvery
     |               |               |       Mole Rat
     -----------------               |           |
             |                       |           |
             |                       |           |       Naked
             -------------------------           |      Mole Rat
                          |                      |         |
                          |                      |         |
                          ------------------------         |
                                      |                    |
                                      |                    |
                                    (original African mole rats)

Until recently, there were believed to be around sixteen species in the African mole rat family, but recent genetic studies have shown that there are considerably more than this, with the "common mole rat" actually turning out to be several different species, some of which aren't even particularly closely related. Such are the problems of studying animals that are almost always down below the ground where you can't see them.

To complicate matters further, a second group of subterranean rodents are also referred to as "mole rats", and have at least as many species. To keep things simple, I'll just discuss the species related to the naked mole rat (technically called the "bathyergids") for the moment. I'm going to call them "African mole rats", because that seems to be the most common name, even though its a bit misleading, since some of the others also live in that continent. Animals just want to make it difficult for us sometimes, don't they?

Anyway, African mole rats are only ever found south of the Sahara, although they may have ranged further in prehistoric times. Most are found in southern Africa, with just a few species further north, in places like Nigeria and Ghana. Again, the naked mole rat is unusual, living across in Somalia, far to the north and east of any of its relatives, and it may be this isolation that allowed it to evolve into such an odd animal.

African mole rats have many adaptations reminiscent of those of the moles, including a relatively cylindrical body that fits well into narrow burrows, velvety fur that won't clog with dirt, and almost invisible ears. But there are some significant differences. Where moles burrow with their spade-like paws, mole rats instead use their teeth. As with all rodents, mole rats have a single pair of large incisors in each jaw, which grow continuously throughout their life, allowing them to constantly gnaw on tough materials without fear their teeth will ever wear down.

The incisor teeth of mole rats are particularly large, and appear even more so because the animal's lips are actually positioned behind them. Because the lips are behind the teeth, instead of where you'd expect them, mole rats can keep their mouth closed even as they use their teeth to gnaw away at the soil in front of them, digging out a tunnel as they go. They then scoop up the loose earth with their front paws, push it underneath their body, and kick it away with their hind feet. The need to keep the tunnels clear means that, eventually, they have to push the excavated earth up to the surface, where they leave mounds very similar to mole hills, that may be the only visible sign of their presence.

For the most part, they prefer relatively solid ground to burrow in, which stops the tunnels collapsing, and probably also makes them more difficult for predators to dig out. The two species of dune mole rat are slightly different, however, and live in soft, sandy soils. These are much easier to dig through, and, unlike the other species, they mainly use their front paws to do so, instead of their teeth, which are smaller than those of their relatives.

The eyes of mole rats are very small, and it had been assumed for a long time that they were completely blind. However, over the last decade or so, it has become clear the structure of their eyes isn't as odd as you might expect if this were true. They have an iris and pupil, a lens that is small, but nonetheless fairly normal, and crucially, the retina seems to have all the light-sensitive cells you'd expect to find in an animal that can see. There are far less nerve cells connected to the retina than in other mammals, but they do exist, which suggested that mole rats probably are able to see, albeit very poorly. When scientists looked at the visual centres of the brain, they found a similar story; the brain structures that would be expected to respond to the presence of light are all there, even though the parts involved with actually seeing objects are highly degenerate.

Of course, examining the anatomy is all very well - can they actually see, and if so, why? The first part of the question was answered in 2005 for the Zambian mole rat (Fukomys anselli). The animals were kept in an artificial burrow, with a choice of sleeping chambers, some of which had light, and some of which were dark. They chose the dark chambers, showing both that they could tell the difference, and that they considered light unpleasant - as well they might, living underground all the time.

More recently, studies have been conducted on the giant mole rat (Fukomys mechowii) and the silvery mole rat (Heliophobius argenteocinereus). Again, both species avoided light, although they didn't seem to mind red light, and presumably couldn't see it. However, while they clearly don't like light, they don't always run away from it; by poking holes in the artificial burrow and shining light through it, the researchers were able to persuade the mole rats to rush towards the breach and plug it with earth. In the wild, there would presumably be many other clues that a tunnel had a hole - the sound of a predator digging its way in, or just a change in air currents or the like - but it does seem that light by itself is enough to be disturbing to the animals.

The dune, cape, and silvery mole rats are all solitary animals, leaving their home burrow when they reach adulthood. It seems that they simply burrow away and block up the tunnel behind them, although they do venture above ground to find mates, a potentially very dangerous time for them. All the other species of mole rat are at least partially social, living in communal burrows. It is well known that the naked mole rat takes this to a remarkable extreme, but it is not alone since the Damaraland mole rat (Fukomys damarensis) of Botswana and Namibia has a similar lifestyle, albeit much less studied. It is generally thought that the other species of mole rat are less extreme, living socially, but not having a single reproductive queen, although even they tend to be dominated by a small number of long-lived individuals that limit breeding by their relatives to some extent. And, of course, its always possible that the lack of detailed research means that there are a few extreme social breeders that we've missed.

Given that they aren't very closely related within the African mole rat family, its perhaps unsurprising that this social system works slightly differently between the naked and Damaraland mole rats. The former are the more extreme, with a distinct 'worker' caste of infertile individuals incapable of breeding. Damaraland mole rats, however, live in smaller colonies consisting of a queen, a small number of reproductive males, and a number of offspring. Unlike their naked kin, Damaraland mole rats will not mate with their own parents or siblings, so it may not be that their workers are truly sterile, just that they don't have anyone to mate with. Nonetheless, there is at least some evidence that the queen is able to control access to visiting males from outside the colony as well, so it may not be quite so simple.

The question of quite why they evolved to live like this is an interesting one, especially since some of their relatives didn't. There are a number of possible explanations, but it may be worth noting that the two most social species live in the driest parts of Africa south of the Sahara, while other mole rats live in generally more hospitable savannah or veldt habitats. Unlike moles, mole rats are vegetarian, eating roots, tubers, and even above-ground plants that they can physically pull down into their burrows by their roots. In fact, while they do, of course, use their burrows for shelter and breeding, most of the tunnels are only dug in the ever-present search for food.

In a dry environment, food is most often available for only a short time after the rains, and the mole rats have to dig quickly in order to find suitable food, and store it where possible. That the soil is also likely to be hardest to dig in these environments makes it especially difficult, and it may be that a large colony working cooperatively makes this all much easier to do.

For the most part, the mole rats are not endangered by human activity, and they have few natural predators. A few snakes, most notably the mole snake of southern Africa, are able to get into their burrows, but in general, their cosy, relatively cool, and difficult-to-find underground burrows keep them safe. Although no species are officially listed as endangered, one, the Kafue mole rat (Fukomys kafuensis) of Zambia, is considered vulnerable, due to its very restricted range, and because locals consider it something of an agricultural pest.

[Picture from Wikimedia Commons. Cladogram adapted from Van Daele et al. 2007]

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