Just Cold Enough

Northern meadow jumping mouse

Hibernation is one of the most effective means that mammals can employ to withstand harsh winter temperatures. Many mammals, especially larger ones, use other means, such as growing a winter coat, caching food, or simply moving somewhere warmer, but, if the goal is to reduce energy requirements when you can't get out to feed, hibernation is the most effective means of doing so.

During hibernation, bodily metabolism slows right down, so that an animal may need as little as 1% of its usual calorie supply to stay healthy. This has negative consequences, so that the animal does need to wake at intervals to stave them off, and how often this happens varies from species to species. In this respect, true hibernation can be distinguished from shorter, often daily, bouts of torpor by the fact that each "sleep" can last for weeks or even months. 

Scratching the Surface

The environment of the Earth is shaped, not just by sunlight, wind, and the geography of mountains and seas, but by the living organisms living on it. This process has been described as "ecosystem engineering", and it inevitably affects far more than the organism that's doing the modification - sometimes negatively, but often in ways that are generally beneficial.

In its broadest sense, ecosystem engineering can include changes to a habitat caused by the mere existence of an organism. The existence of multiple trees in close proximity creates a forest, which is a very different sort of environment to, say, a grassland, and this obviously has huge effects on what the resulting habitat is like for other organisms. One could also think here of coral reefs or the nutrient-rich hotspots created by a whale carcass sinking into the depths.

A Good Winter's Sleep

Mammals have an advantage over reptiles in that they don't need the weather to be warm in order to stay active. This makes it easier for them to live in parts of the world that have cold winters, but even then, the scarcity of food at such times of the year means that they often need some additional survival strategy. Some, of course, simply migrate somewhere warmer during the winter - which typically means moving downhill from summer grounds on mountainsides, the long-distance migration of birds being less of an option. Others, such as polar bears, are just good at surviving cold weather anyway, and may not need to do anything significantly different in winter.

But, leaving those possibilities aside, three basic options for surviving the winter present themselves. They could do something behavioural, such as storing food during the summer and coming back to their hidden caches later in the year when food is short. Or they could change physically, such as by building up fat over the summer or having an extra-thick winter coat that falls out in the spring. (And these are not, of course, mutually exclusive).

A Hole of Your Own

Many animals dig burrows for shelter, whether from the weather or from predators. In some cases these are complex or extensive burrow systems, such as we find with rabbits or gophers, and some animals, such as moles, try not to leave their burrows at all, adapting to a subterranean life. Most are much simpler than this, a basic hole in the ground in which the animal can rest securely at night - or during the day, as the case may be.

At the opposite extreme to the specialised diggers, however, are those animals that don't dig burrows at all, but still find it useful to seek shelter in this manner. These are creatures that will either use natural cavities or take over an abandoned burrow originally dug by something else. If they can't find one, it's usually not a disaster, although it may make life a little uncomfortable. But that's not necessarily true when it comes to time to breed.

Small British Mammals: Rabbiting About Rabbits

Over the course of the last two thousand years, a number of native British mammals have been driven to extinction. These were mostly the larger ones that were considered either dangerous (wolves) or tasty (wild boar). But we have also managed to introduce a few, mostly smaller, animals that were not originally native to the islands but are now widespread. The grey squirrel, being relatively recent, is probably the best known of these, although the brown (or "sewer") rat is another, only arriving in the 16th century.

Another example was first introduced in Roman times, and so has been here rather longer - but, still, it could be argued that it's not technically native. This is the rabbit (Oryctolagus cuniculus).

Spot the Difference

Common European shrew

The shrew family contains what are, by the usual method of measuring such things, the smallest of all living mammals. Yet they are actually quite a large group, with well over 300 species so far identified, and, given their small size, probably quite a lot that we don't yet know about - especially in the tropics. Indeed, in the grand list of mammal families, they come fourth in terms of sheer number of species, beaten only by the rats and mice, the cricetids (most, of which, frankly, are also mice, although the group also includes the voles and hamsters), and the vesper bats. They may not have the greatest range of physical variation, at least to human eyes, but that's still quite a lot of diversity, and they certainly aren't "all the same".

It's perhaps worth pointing out, though, what a shrew is, and what it isn't. Firstly, while they may look kind of like mice, shrews are not rodents. Rodents are defined by the presence of great gnawing incisor teeth, and by not having the second set of incisors that rabbits have. Shrews don't look even remotely like this, having lots of small sharp teeth ideally suited for biting into insects. In fact, shrews' closest relatives are actually the moles and hedgehogs - relatively small animals that also eat a lot of invertebrates.

Mazes and Mole-Rats

The African mole-rats are a family of bizarre rodents, and one that I've discussed before. Their most famous member is the exceptionally weird naked mole rat, but the other, furrier, species are still pretty odd. They spend almost their entire lives underground, feeding on plant roots, and, as a consequence, are virtually blind. Famously, naked mole rats are 'eusocial' mammals, living in the style of ants or bees, with a single breeding queen and a number of sterile workers. However, some other African mole-rats live solitary lives, and others, while not as extreme as the naked species, are still strongly social.

We know very little about some of the species, but others have, on account of their peculiarity among mammals, been well studied. One of these is the Zambian mole-rat (Fukomys anselli). Much of the work on this species has concerned their hearing abilities. They turn our to be particularly good at hearing low-pitched sounds; the sort of noise that rumbles through the soil, and echoes through their narrow tunnels. Not only that, but they are also able to detect magnetic field lines, presumably to orient themselves in the absence of any cues from the sun.

How Deep is a Prairie Dog's Burrow?

The Utah prairie dog has the biggest burrows

If you live in Britain, or, for that matter, many other parts of Europe, a "squirrel" is a bushy-tailed rodent that lives up trees. But this is by no means an adequate description for all squirrels. A great many squirrels are ground-dwelling animals, and would rarely, if ever, climb a tree. This is even more apparent when we look at the squirrel family as a whole: there are something like three hundred different species of squirrel worldwide, and only around half of them live in trees. The others generally dig burrows.

Squirrel burrows include a deep nest chamber, in which the animal can sleep, rear its young, or simply hide from predators. Connected to this there will usually be a number of tunnels, and multiple entrances. In the event that, say, a weasel, chases you into your burrow, it is really important to have an escape route, so a single entrance is unlikely to be enough.

Considering that squirrels are relatively small animals, these tunnel systems can be quite impressive. Since they're only using the burrows for shelter, they won't necessarily be as dramatic as those of animals like moles, that also need them to travel in search of food, but they can still be quite long and deep.

How long, and how deep? Well, that depends on a lot of factors. Different species will dig different kinds of burrow, and there's no reason to assume that, even given identical circumstances, all members of the same species will dig identical burrows. In any event, circumstances aren't identical. The nature of the environment, and particularly the soil, will affect how big a burrow it's practical to dig at all, even assuming you wanted to - and, generally speaking, animals won't dig a burrow any bigger than they have to, because it's exhausting work.

The Time of Horned Rodents

Ceratogaulus hatcheri

The rodents are the most successful order of mammals, with literally thousands of species worldwide. There are a number of reasons for this success, but one of them concerns the arrangement of the jaw muscles, which are modified to improve their ability to gnaw with their powerful, chisel-like teeth.

The muscle in question is the masseter, a muscle that, in humans, runs from the forward part of the cheekbone down to the back of the jaw, and which is important in chewing - it is often particularly large in herbivores, for example. It has two parts, one deep and one superficial, and in rodents, one or both of these parts extend much further forward than normal, reaching up onto the side of the snout. Generally speaking, in squirrel-like rodents, its the superficial part of the muscle that does this, in guinea pigs and their relatives, its the deep part, and in mouse-like rodents, it's both. Because the shape of the skull has to be modified to allow for these attachments, it is possible to tell which of the three possible arrangements even a fossil species had.

But there is one rodent species alive today that doesn't have any of these features. (Actually, mole rats don't have the full muscle arrangement, either, but we can tell from the shape of their skulls that their ancestors once did). That animal is the mountain beaver (Aplodontia rufa), and it's only found along the Pacific Coast of North America. Despite the name, it is not particularly closely related to real beavers, and it doesn't look much like them, either, looking more like some kind of giant vole. Indeed, its a burrowing animal that doesn't like water, and usually only ventures above ground at night.

Secrets of the Mound-building Mouse

In temperate climates, such as we have in Europe, and in the northern parts of North America, winter can be a difficult time for animals. The weather is cold, there is often snow on the ground, and food is in short supply. Some animals hibernate through the winter, while others struggle on through the harsh weather. Smaller mammals, such as rodents, often store dry food in caches that they can return to when there is nothing fresh available. Squirrels, for example, hide nuts to bide them through hard times. But some rodents go to greater lengths.

Out of the over six hundred members of the mouse family, the steppe mouse (Mus spicilegus) is one of the three closest species to the familiar house mouse (Mus musculus). It is found from the easternmost border of Austria, through Hungary, Romania, and Serbia, and out into the steppe lands of Ukraine and western Russia. They are found primarily in and around agricultural land and orchards, and rather less in the wild grasslands where they presumably originated. Indeed, while the species seems to have diverged from the house mouse around three million years ago, long before modern humans were around, today it seems to rely upon us to create the unnatural habitats in which it thrives best. In this respect, it resembles the house mouse, which only rarely lives outdoors.

The Strangest Marsupial?

So, I finally reach the end of the mini-series on mole-like mammals. I've looked at the moles themselves, two different kinds of mole rat, and the golden moles of Africa, and that leaves just one group. But its perhaps both the strangest and the least studied of them all.

As you can see (sort of) from the picture, these animals look remarkably like the golden moles. There's probably a good reason for that; like most golden moles, they burrow their way through dry, sandy soils, and they have a very similar lifestyle. Yet, apart from both being mammals, they are entirely unrelated. These animals dig through the sands, not of Africa, but of Australia. For these are the marsupial moles (Notoryctes spp.)

The marsupial mole family contains just two species, imaginatively named the northern and southern marsupial moles. They're pretty much impossible to tell apart just by looking at them, and for a long time it wasn't clear that they were two separate species at all. They live only in the deserts of central and north-western Australia, where they like especially sandy soils, and feed primarily on ants and termites.

Swimming in Sand: the Golden Moles

As we've seen with the moles and the two types of mole rat, spending one's entire life underground requires a number of unusual adaptations. However, because it has the great advantage of making you difficult for predators to find, it is a lifestyle that has evolved more than once among the mammals - and there are also subterranean reptiles, and even amphibians. The moles and mole rats are the best studied subterranean mammals, but they aren't the only ones. There are a lot of animals we could consider to fit this description, depending on exactly how broadly we want to define "subterranean", but I'll conclude this mini-series by looking at just two, which, like the moles and mole-rats, include particularly extreme adaptations.

The first are the golden moles. They inhabit southern Africa, often in much the same areas as the African mole rats, but the two types of animal don't seem to compete much. That's probably because, while the mole rats are herbivores, the golden moles are carnivores, feeding off various invertebrates, such as insects, earthworms, and spiders. That obviously makes them seem more like the true moles, but, in fact, the two groups are really not related at all.

Indeed, they are about as far from the true moles as its possible to be, while still being a placental mammal. Exactly what their closest relatives are has been debated, but they're probably the tenrecs, shrew-like animals now found only on Madagascar, and some rather odd African animals with a remarkable resemblance to otters.

Golden Moles        Tenrecs      Otter-shrews
     ^                 ^              ^
     |                 |              |
     |                 |              |
     |                 ----------------           Elephant
     |                        |                    Shrews
     |                        |                      ^
     --------------------------                      |
                 |                                   |
                 |                                   |
                 -------------------------------------
                                  |
                                  |

There are at least twenty one species, most of which don't have particularly memorable names, but one of them was only discovered in 2000, so there's no particular reason to assume we've found all of them yet. The first genetic survey to figure out how the different species relate to one another was only conducted last year, and differs in a number of points from what we previously thought based on their appearances. This sort of thing isn't very surprising, especially when you consider that they all look pretty much the same anyway, so that we were relying on some fairly minor points of difference.

So what do they look like? As the name suggests, most species are golden in colour, and, of course, they all have the generally compact body form that truly underground animals all tend to. But, even ignoring the colour, they do look rather different from true moles. That's because they don't burrow in the same way. Moles have spade-like forefeet and very powerful arms, to shovel soil out of the way. Golden moles, however, have much narrower forefeet, that look more like picks than spades.

The forefoot generally has just two functional toes, each ending with a long claw, although those that burrow in sand have three or four. In addition to the claws, the animal uses its head like a wedge to force its way through the earth, something that probably helps in looser soil. But it also means that, even more so than moles or African mole rats, the animal's eyes are likely to get grit and dirt in them. So, like the blind mole rats, golden moles have tiny eyes - just 1% the size of the head - that are entirely covered in hairy skin, and likely to be useless for anything beyond merely detecting the presence of light. So far as I can tell, there has been no research on whether they can even do that much, although it would be a useful ability for an animal that is going to be much safer in the dark, so I'd guess it's quite likely.

Perhaps the strangest feature of the golden moles, however, are their ears. The malleus, or hammer bone, of the middle ear is, in many golden moles, truly enormous. Relative to their body size, some species have the largest mallei of any mammal - in extreme cases being somewhere between sixty and a hundred times heavier than you would expect. In most cases, they are heavier simply because they are bigger, with a huge spherical or elongated head, but in the desert golden mole (Eremitalpa granti), at least, the bone is also remarkably dense. All of this apparently helps them to detect vibrations in the ground, and even work out the direction they are coming from, so that they can run for cover in the very deepest parts of their tunnels.

Golden moles tend to dig relatively shallow tunnels, just beneath the surface. Indeed, in the case of the desert-dwelling species they are effectively swimming though loose sand, using their pick-like claws in a kind of butterfly stroke. The tunnels have to be shallow, because that's where all the tasty insects are, but they also dig much deeper passages for protection, and its then that they use their relatively normal-looking hindfeet to kick up molehills on the surface. In addition to those they use to hide from predators, such deep passages can also end in breeding chambers, or latrines, making it hard for other animals to scent their spoor or find their young.

They seem to breed whenever they feel like it, rather than having a defined breeding season, and to give birth to small litters of no more than about three pups. Like most subterranean animals, other than the African mole rats, they are solitary animals, leaving their mother's nest as soon as they can. At least some of them are adapted to survive at a wide range of temperatures, including the ability to lower their body temperature and hibernate, if it gets really cold - as it sometimes does, for example, in the Drakensberg Mountains.

Although the group as a whole is well-adapted to a range of different habitats, from sandy desert to sphagnum-covered swampland, many individual species have highly specialised requirements, and that puts them at risk from encroaching human activity. Five of the species are considered endangered, but two further species are known from only a single specimen each, which doesn't exactly bode well. Indeed, one of those, the Somali golden mole (Calcochloris tytonis) isn't even a complete specimen - scientists just found a bit of one in the pellet of a barn owl in 1964, and have never seen another since.

Leaving that aside, the most endangered species that we definitely do have information on is De Winton's golden mole (Cryptochloris wintoni), which inhabits coastal sand dunes in the Namaqualand strandveld of South Africa. Or at least, it did, because, despite having tried, nobody has seen one since the 1950s, and its habitat has been largely destroyed by diamond mines. Our lack of knowledge of golden mole genetics may mean there are more species than we have found so far, but our own activity may also mean that there are rather less.

[Picture from Wikimedia Commons. Cladogram adapted from Seiffert 2007.]

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]

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]

Going Underground

European (or "common") mole

Mammals have adapted to a wide range of habitats and lifestyles, from Arctic wastelands to the deep sea. In addition to taking to the water and to the air, there are a number of mammals that spend almost their entire lives underground. I'm not talking here about living in caves, or digging a burrow to sleep in, but animals that spend their whole lives burrowing through the ground.

There are a number of disadvantages to this lifestyle. You have to constantly dig for one, and your fur and other body parts will get constantly clogged with earth. There is no light, and its difficult to find food or mates. On the other hand, there are also some clear advantages. For one, burrowing animals are safe from most (though not all) predators. Anything small enough to get into their burrows is probably small enough not to want to eat them, and, being underground, they're pretty difficult for anything above ground to find. In addition to that, unless the ground really gets saturated, they don't have to worry much about the rain, and they are protected from extremes of heat and cold.

Although there are a lot of mammals that dig burrows, there are four main groups that have taken the subterranean lifestyle to an extreme. Perhaps the most familiar example are the moles. The mole family consists of forty-one species, although a number of them aren't truly underground creatures, and two of them are semi-aquatic. The group seems to have diverged from its closest relatives, the shrews, not long after the dinosaurs went extinct, and possibly even earlier, making them quite an ancient group. The true underground lifestyle probably evolved at least twice, because the "common mole" of Europe (Talpa europaea) and the "common mole" of North America (Scalopus aquaticus) are not particularly close relatives within the family.

  European      "American"    Star-nosed
& Asian moles     moles       mole, etc.
      ^             ^             ^
      |             |             |
      |             |             |       Desmans
      |             ---------------          ^
      |                    |                 |
      |                    |                 |       "True"
      ----------------------                 |     shrew-moles
                |                            |          ^
                |                            |          |
                ------------------------------          |
                              |                         |
                              |                         |
                              ---------------------------
                                           |
                                     Earliest moles
                                           |

The chart above is somewhat simplified; the "American" moles in fact include one species from China, while the close relatives of the star-nosed mole include some species commonly called "shrew-moles". Brown represents fully subterranean species, blue the semi-aquatic species.

Moles are reasonably widespread, being familiar animals across Europe, much of Asia, and North America. They avoid the colder environments where the ground freezes solid for much of the winter, and so truly subterranean moles are not found in places like Norway or most parts of Canada. They also avoid the deserts, which explains their absence in the Middle East, North Africa, and the central US, despite the presence of two species in the eastern US and three along the west coast. They've never colonised Africa or (less surprisingly) Australia, although there are other mammals with a similar lifestyle on both continents, which I plan on discussing in later posts. Perhaps more surprising is their absence over much of India and in South America; it's not that they dislike tropical environments because there are moles in South East Asia, from Vietnam and Myanmar down to southern Malaysia. For its size, there are also rather a lot of mole species in Japan, along with several species from China, Korea, and the warmer parts of southern Siberia.

True moles are well adapted to their burrowing life. They have powerful front legs with spade-like claws for digging through the earth, and their velvety fur clogs with soil less than that of most other animals would. Unlike bats, which can usually see pretty well when they need to, the eyesight of moles really is quite poor. The European mole (that is, the one we call the "common mole", and which is the only one found through most of Europe north of the Mediterranean) has eyes that superficially appear quite normal, but the cornea is degenerate, and has been compared to that found in humans with the rare visual disorder keratoconus.

Nonetheless, while it prefers to feel its way about with the help of sensitive whiskers and specialised touch receptors on the snout (the ears are also very small, again, to stop them clogging with dirt) the European mole can see, if not very well. The Iberian mole (Talpa occidentalis) of Spain and Portugal, and the eastern mole (that is, the American "common mole", which is found throughout much of the eastern US) are a different case. Their eyelids never open, permanently protecting their eyes from the dry gritty soil of their homelands at the cost of their vision. Still, while the fact that their eyes are permanently shut presumably prevents them from making out any shapes, they aren't completely blind.

The structure of the eyes in Iberian moles has been studied in some detail. The cornea is even more degenerate than in European moles, the iris has no muscle fibres to allow it to widen or narrow in response, and in any event is stuck to the lens, the lens itself has a disorganised structure and is less transparent than in most other animals, and the retina is unusually small. Nonetheless, apart from its size, the retina does look relatively normal, which suggests that it must be at least partially functional. Indeed, Iberian moles do respond to light - by running away from it, which makes sense if you live underground, where there really shouldn't be any.

Nor does the strangeness of moles end with their poor eyesight. In a further bid to stop holes filling with dirt, the female's vagina is sealed shut for most of the year, and only opens when they are sexually receptive. Rather odder, though, is the fact that female moles are, in a certain sense, actually hermaphrodites. This is perhaps rather stretching the term, but it is the case that the ovaries contain a large amount of what certainly looks like testicular tissue - indeed, there's rather more of it than there is ovary. It doesn't produce any sperm, but it does produce male hormones. Of course, all female mammals produce some male hormones - they're made by the adrenal glands - but the female mole produces considerably more than usual.

Eastern (or American "common") mole

This only seems to have been studied in detail in four of the five species of mole found in Europe - the European and Iberian moles, and the Balkan (Talpa stankovici) and Roman moles (Talpa romani) of the Balkans and Italy, respectively. Those four species are quite closely related, having diverged only a few million years ago after their ancestors left Asia, and with their current distributions probably having been heavily shaped by the need to avoid the advancing permafrost during the Ice Ages. But it isn't unique to them, and has also been reported in the star-nosed mole (Condylura cristata), that remarkably distinctive beast of the eastern US and Canada, and, for that matter, the Japanese mole (Mogera wogura) - although, not, apparently, in the eastern mole.

Quite what the point of this is is a little unclear, although the glands do shut down during the breeding season and when the female is pregnant. Being awash with male hormones for the rest of the year though, does lead female moles to develop some masculine anatomy as they grow; they have an enlarged clitoris, and what appears to be a prostate gland. None of this hampers their ability to be good mothers, though, because moles have a higher reproductive effort (that is, the mass of offspring one mother can produce in her lifetime) than their closest relatives, the shrews. This seems to be due, at least in part, to the production of some very nutritious milk, and may offset the high death rate of young moles when they leave home and make the dangerous journey above ground to find somewhere to establish their own burrow.

They have to make their own burrow, because moles are pretty anti-social animals, and adults normally only meet up to mate. Having said that, they don't need particular large burrow systems, so there can be several moles in a relatively small area. Population densities of 5 to 25 moles per hectare have been reported for the European mole, and 5 to 12 for the eastern mole. Moles, of course, are mostly carnivorous, eating insects and worms, largely, it would appear, by hoping that they accidentally wander into the tunnels.

While the less subterranean members of the mole family are basically nocturnal, its a little more complicated for the true moles. Day and night aren't really an issue for them, and they sleep three times each day for most of the year - although males can be active for rather longer periods when they're looking for a mate.

Because of the protection that hiding below ground offers them, and the fact that they generally don't mind agricultural land, most mole species are not overly threatened by human activity - which isn't to say that farmers don't poison them, just that they don't do it enough to place entire species at risk of extinction. The clearest exception is the Echigo mole (Mogera etigo), which lives in only one small area on the west coast of Japan's main island, Honshu. It prefers soft soils, and changes in agriculture in the region make those less common than they used to be, and the animal is officially considered Endangered.


[Pictures from Wikimedia Commons. Cladogram adapted from Mikko's phylogeny archive and Motokawa 2004]