Sunday 5 June 2011

Gnawing Through Soil: The Blind Mole Rats

Palestinian mole rat
The chisel like teeth of rodents are adapted to gnawing their way through relatively solid food, and are so evolutionarily effective that there are more species of rodent than of any other mammalian order. As we've seen, those gnawing teeth can also be used to burrow through soil, and, as I mentioned then, the African mole rats are not the only rodents to have evolved in this way - or even the only group to be called "mole rats". While the African mole rats are related (albeit not very closely) to guinea pigs, these blind mole rats belong to an entirely different branch of the great rodent radiation.

The blind mole rat family (more properly called the Spalacidae) is usually said to represent the very first branch within the "muroid" rodents - the superfamily to which the familiar rats and mice belong. This is true enough, according to the most common modern classification scheme, but its worth remembering that quite where you draw the lines between different groups can be fairly arbitrary. It would be just as valid to define the muroids more widely, and say that the jerboas were the first branch, or more narrowly, and say that the blind mole rats are weird enough to have their own superfamily.

Indeed, there have been a number of debates about the origin of the group, and whether it's even real - rather than consisting of a bunch of unrelated animals that just happen to look similar because of their shared lifestyles. It now seems likely that they are a genuine family, and that they first split from their relatives around 28 million years ago, before the sudden and dramatic radiation of the many, many, forms of mice, rats, voles, hamsters, gerbils and so on.

Root Rats    Bamboo Rats     Blind Mole     Zokors
    ^             ^             Rats           ^      (All other
    |             |              ^             |        muroids)
    |             |              |             |           ^
    ---------------              |             |           |
           |                     |             |           |
           |                     |             ?           |
           -------------------------------------           |
                             |                             |
                        (Spalacidae)                       |
                             |                             |

Note: so far as I can tell, the exact position of the zokors within the group seems uncertain, and may never have been properly analysed.

As the above chart shows, the family contains more animals than just the blind mole rats themselves. All of the rodents in the group spend most of their lives underground, as burrowing animals feeding largely on plant roots and tubers. Compared with the African mole rats, they have been little studied, and rather less is known about them. Of them all, the blind mole rats are noticeably the best adapted to a fully underground life, while the bamboo rats seem to be the least.

There are around forty recognised species in the family, but, as so often with animals that live underground, its very likely that there are more that haven't been specifically identified yet. They are widespread, with the root rats (often also called 'mole rats', just to confuse issues) living in east Africa, the bamboo rats in southeast Asia, the zokors in China, Mongolia, and southern Siberia, and the blind mole rats in and around the Balkans and the Middle East.

They all tend to have relatively cylindrical bodies for moving through burrows, short limbs, large teeth, and small eyes and ears - although the zokors are unusual in digging with their feet, rather than their teeth. It is, however, the blind mole rats themselves that show the most dramatic adaptations. Like the African mole rats, their lips are behind their teeth, which therefore remain visible even when their mouth is closed - allowing them to gnaw their way through the soil without getting a mouthful of dirt. Apart from the large teeth, they do look quite like moles, although they can be much larger, with the biggest species being over a foot long.

Perhaps the best studied member of the group is the Palestinian mole rat (Spalax ehrenbergi), although this almost certainly consists of multiple separate species that have yet to be officially named because they all look more or less the same.

The blind mole rats are so named because their eyes are not visible; while African mole rats never open their eyelids, the eyes of the blind mole rats are entirely covered in hairy skin. The eyes beneath the skin are small and degenerate, and may be the most rudimentary of any known mammal. They have no true lens, and often no pupil, with an irregular pigmented mass filling the front of the eye. Nonetheless, the retina at the back of the eye does have a relatively normal structure and contains what appear to be abnormal rod cells, normally used by mammals to see during low light conditions. Furthermore, the eyes are fully connected to the optic nerve, and are clearly sending the brain signals about something.

In fact, there does seem some evidence that blind mole rats are able to detect light (they run away from it), although given their anatomy, its clear they can't do any more than that. Intriguingly, however, they appear to have co-opted structures normally involved with vision to the other functions. The odd retina, for example, has some similarities to the pineal gland of birds.

The pineal gland is a small organ, located well inside the skull of both birds and mammals, and is believed to represent a "third eye" once present in our evolutionary ancestors. (This third eye, incidentally was always very small compared to the other two, and is covered by skin even in the few reptiles that still have one - this is why you don't see pictures of three-eyed dinosaurs). The pineal gland's function, at least in part, is controlling our body clocks in response to external light, and is responsible for the jet lag we get when we get out of tune with the timing of sunrise and sunset. Thus, blind mole rats may use their eyes, if not to see in the conventional sense, at least to synchronise their body clocks with the day-night cycle, and perhaps to determine the time of year from the changing day length.

Interestingly, the visual cortex in the brain appears relatively normal, rather than being shrunken. However, it is not connected to the optic nerve, as it would be in other mammals, but instead, via other parts of the brain, to the auditory pathways. Although it's not entirely clear how, there is evidence that blind mole rats can communicate with one another by making vibrations in the ground, and its unsurprising that hearing would, in any case, be important to an animal that can't see. That the auditory pathways have co-opted parts of the brain used for vision in other mammals suggests that, in a sense, blind mole rats can 'see' using the vibrations of the earth around them, allowing them to navigate through their strange environment in complete blackness.

Another oddity of the blind mole rats is their remarkable tolerance for low oxygen levels. Especially in the rainy season, when the soil becomes clogged with water, their tunnels have relatively little free oxygen, down to less than a third what it would be in the open air, and the rodents seem to have evolved to cope with this. This is partly due to modified muscles, containing an unusual mix of fibre types, and reservoirs for oxygenated blood, but it is also due to changes at the biochemical level. This is one of those areas where figuring out how non-human mammals work can have direct relevance to our own species - if we can work out how blind mole rats pull off this trick, it might be helpful in combating human diseases where oxygen can't get to the tissues, or perhaps finding a way to stop cancer cells doing the same thing. (Tumours tend to choke off their own blood supply, and yet this doesn't always stop them growing).

Unlike the African mole rats, the blind mole rats do not live communally, with only one living in each tunnel system. These tunnels can reach hundreds of feet in length, as the animals burrow in search for food. They breed in the spring, and it seems they can't wait to get away from home, leaving only a few weeks after being born. Digging their tunnels can create numerous mole hills, and because they eat very little except underground roots, they can be a pest when they reach agricultural lands.

For the most part, the blind mole rats are not endangered, although, since we don't know much about them, that may be due to a lack of proper information in any cases. Indeed, since it seems probable that there are rather more species than we've noticed so far, it's entirely likely that some of them are threatened by human activity without us having realised. There are a few we know are in trouble, for example the giant blind mole rat (Spalax giganteus) is officially rated as vulnerable, and seems already to have been extirpated from Chechnya, with the civil war there probably not helping matters any.

[Picture from Wikimedia Commons. Cladogram adapted from Steppan et al, 2004]

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