A History of the Bamboo Rats

In terms of number of species, the mouse-like rodents are the most successful group of mammals alive today. The group is typically considered to consist of six families, with the vast majority of species crammed into just two - the mouse family itself, and the cricetids, which includes the voles and hamsters. These, however, are relatively late arrivals, and there are two families that split from the main line of mouse-like rodents long before the ancestors of the mice and voles diverged from one another.

One of these consists of a grand total of three species (maybe), living in the forests of southern Asia. The other goes by the technical name of the Spalacidae, and it consists of animals that have the rather unusual trait of spending almost their entire lives underground. There are at least 35 species of these animals, found in various places across Europe, Asia, and Africa. This is a fairly wide distribution, which raises the interesting question of how they managed to spread that far when they seem to have fairly narrow habitat preferences. Until recently, the evolutionary history of these oddities has been something of a mystery but now, it seems, we are starting to get enough information to piece together an overview.

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.

Does Colder Mean Bigger?

Amne Machin, Qinghai

In 1847, German biologist Christian Bergmann formulated what has since become known as Bergmann's Rule. Over the following 165 years, the Rule has been somewhat modified and re-interpreted from Bergmann's original statement, but a common modern version is that warm-blooded animals tend to be larger the colder the environment they live in. Bergmann was originally thinking of closely related species; for example, polar bears tend to be larger than most brown bears. However, the rule is now often applied to individuals and populations within species; for instance, Siberian tigers are larger than those that live in the tropics.

The rule has, as I've noted before, been the focus of some controversy. Is it, in other words, actually true? Certainly, there are many counter-examples - snow leopards are the smallest cats in their genus, for example. But are these rare exceptions to a general rule?

The Red Queen and the Court Jester

Greater blind mole rat

What is the primary driving force behind evolution? If an animal is well suited to its environment, and able to reproduce effectively, then why does it change over time? Why, in short, don't humans still live up trees, where monkeys are still perfectly happy today?

There are two broad types of explanation for what drives evolution: they're called the Red Queen and the Court Jester. It's important to note that there's no reason why they can't both be responsible. Indeed, it would be astonishing if they weren't. The debate, then, hinges on which is more important, and when.

The Red Queen hypothesis, first formalised in the 1970s, is named for the character in Through the Looking Glass, who had to run as fast as she could, just to stay still. The idea is that no animal (or other living organism) is alone, and, if you don't evolve, the creatures around you will, and you'll be doomed. One species may be very good at eating a particular type of plant, for instance, but if another species gets slightly better at doing the same thing, if the first species doesn't improve, it will be out-competed and go extinct. You always have to be better than your competitors, and they have to try to be better than you, leading to a constant drive for change - towards better adaptation to your environment and chosen food source, whatever that may be.

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]

Churning out babies - pregnancy in the naked mole rat

By any standards, the naked mole rat (Heterocephalus glaber) has to be among the strangest of the placental mammals - and, to be honest, also one of the ugliest. It spends virtually its entire life underground, is hairless, apparently can't do the decent mammalian thing and control it's own body temperature, and may well be entirely blind. Perhaps even stranger, though, is its method of reproduction.

Remember the brown antechinus? Well, this is almost the exact opposite.

The mole rat family is itself a fairly strange group of animals, and actually more related to guinea pigs than they are to rats. The naked mole rat is almost certainly the best known member of the group, but there are several not-naked mole rats as well (I plan to discuss these in another post, coming up soon). In evolutionary terms, the naked species lies outside the main group, having diverged long before they did, and it's often given its own subfamily to highlight the differences between it and its furry kin.

Naked Mole      Other       Degus,       Guinea Pigs,
   Rat        Mole Rats      etc.           etc.
    |             ^            ^             ^
    |             |            |             |
    |             |            |             |
    ---------------            ---------------
           |                          |              Cane Rats,
           |                          |                 etc.
           ----------------------------                  ^
                       |                                 |
                       |                                 |
                       -----------------------------------
                                        |
                                        |           


The naked mole rat lives in huge underground colonies, with anything up to 300 members, although around 80 is more common. What's particularly strange is that, out of all of the rats in the colony, only one of the females breeds. This, of course, is a pattern we often see in insects, with bees, ants, and termites all being good examples. In those insects, we have a single queen that lays all the eggs, tended to by hordes of sterile workers and a small number of reproductive males. Mammals don't work like that... but the naked mole rat comes remarkably close.

Just as with bees, all the breeding in a naked mole rat colony is performed by a single queen, and a slightly larger number of males. That the other males don't get a look in isn't so unusual; they have to wait until the dominant males die off, as happens with many other species of mammal. It's the fact that the other females don't reproduce, and are, effectively, sterile workers, that makes the naked mole rat so unusual. (Not absolutely unique, though, since at least one other mole rat species seems to do the same - it just hasn't been studied so thoroughly yet).

There are differences between naked mole rats and insects, of course. Most notably, the workers in insect colonies are permanently sterile, and completely incapable of breeding - the queen is born to her status. Among naked mole rats that isn't true, and any female can leave the colony to found her own, becoming fertile in the process. The same happens when an existing queen dies off, often after some vicious fighting between her potential replacements. Nonetheless, once fertile, the queen's appearance changes from that of her siblings. Her ovaries and uterus greatly enlarge as her body floods with the female hormone progesterone, and her back actually lengthens to accommodate her eventual pregnancy.

Pregnancy in naked mole rats is unusually long. It lasts 70 days, which may not sound like much, but is at least twice what you would expect compared with other rodents of similar size - they only weigh around 30 to 35 grams (just over one ounce). Small rodents generally breed rapidly, producing as many litters as possible in a short time, not least to compensate for the fact that a lot of them are going to get eaten. So why not this one? A recent study by Kathleen Roellig et al. may shed at least some light on what's going on.

The researchers established a captive colony of naked mole rats, and monitored the pregnancies of the queen, using ultrasound equipment much the same as that used for human pregnancies - albeit somewhat smaller. There are a couple of ways that mammals can extend their apparent period of pregnancy. Some, such as the silver-tipped myotis, seem to be able to store sperm in their reproductive tracts for a few months until the time is right to have children.

More common is delayed implantation, in which the egg is fertilised as normal, but, after dividing into a tiny ball of cells, then suddenly stops its development, and does nothing for a few months. Eventually, it attaches itself to the uterine wall, and begins to grow and develop again. This is especially useful to animals such as seals. Seals only come ashore to breed and give birth once a year, so their pregnancy has to last just under 12 months. Since it wouldn't take that long for a properly growing embryo to develop into a seal pup (seals, are, after all, not far off the size of a human) its obviously useful for that to be prolonged somehow. And, of course, it means they don't spend their entire adult life feeling the symptoms of pregnancy.

No such luck for the naked mole rat, though. In this study, once she'd settled down into the new colony, the queen rarely waited more than a couple of weeks after giving birth before getting pregnant again. The ultrasound showed that there was a delay in the womb, with implantation probably not happening until around the end of the second week, but that's not long enough to explain the whole of the longer pregnancy. From then on, the embryos kept growing.

We're talking quite a lot of embryos, too. On average, the queen gave birth to around eleven pups at a time, which is plenty, even for a rodent. But even that isn't really the full extent of her alteration into a baby-making machine. The first ultrasound scans were given at about 20 days, and, at that point, she had an average of just over thirteen embryos. On average, a couple of them died and disappeared before birth - assuming, of course, that none had already gone by 20 days. It's at least possible that this has something to do with being in captivity, but it might also be that naked mole rat queens can control the size of their litter. It could even be a bit of both - a tunnel system in a lab would naturally have a limited size, so having too many young would make things rather crowded.

On one occasion, the researchers took an infertile female away, and gave her her own colony. She developed into a queen, and became pregnant, but never gave birth. No miscarriages, either; the embryos just vanished (as they do if they die young enough). Of course, that could be due to illness of some kind, but it might be that she had the ability to cancel her own pregnancy if she didn't feel comfortable enough. A second attempt to found a new colony was more successful; although the two females placed in it fought viciously to the death, the survivor went on to become a queen and raise a litter of thirteen pups.

Like many other mammals, the young of many rodents are born relatively undeveloped - hairless, blind, and essentially helpless (quite unlike, for example, deer). Its hard to tell whether naked mole rats follow this pattern, since they're hairless and virtually blind even as adults, but they certainly aren't able to walk from birth, so greater development doesn't seem likely to be the whole story behind their longer pregnancy.

The researchers suggest that the real reason may be down the almost perpetual pregnancy required of an animal that's the sole source of babies for a colony. Because they mate again so soon after birth, if naked mole rats had a normal gestation period for their size, of around 30 days or so, their first batch of young would still be suckling when the second litter was born. Far better then, to have double-sized litters half as often, giving you enough time to raise them all, while still having the same number of children in the long run.

And how many children is that, anyway? The researchers also measured something called the "lifetime reproductive effort", which essentially compares the total mass of a female's offspring when they become independent with her own adult body mass. For most mammals, around 1.4 is quite normal, and its rarely higher than 2.2 in rodents. For the naked mole rat, the researchers calculated a value of - wait for it - 139.8.

That's what you get for being the only reproductive female in a colony of over a hundred adults.

[Picture from Wikimedia Commons. Cladogram adapted from Mikko's Phylogeny Archive and Deuve et al. 2008]