Sunday, 18 December 2011

Grazers and Browsers - and how to tell them apart without watching

Wildebeest are grazers - note the squarish muzzle
One advantage of studying the fossils of prehistoric mammals, as opposed to dinosaurs, is that mammals are still around today, while non-avian dinosaurs aren't. That gives us the ability to compare fossil species with living ones, and be fairly confident that our comparisons make sense. That's not to say, of course, that we can't infer quite a lot from the shape and structure of dinosaur bones, and work out details of their lifestyle and habits. But there's nothing much like non-avian dinosaurs around today, so there will inevitably be some guesswork involved when we do - educated guesswork, to be sure, but guesswork none the less.

Although the same can be said of fossil mammals - especially the stranger ones - in many cases, we can be more confident that our educated guesses are likely to be accurate. For example, sabre-tooth cats were, well... cats. So we can look at, for example, the proportions of their limbs, compare them with living cats, and deduce whether they were more like, say, jaguars, than they were like  leopards. Because leopards, jaguars, and sabre-tooths are, in many respects, quite similar, it's pretty likely that inferences drawn from the first two will apply to the third, unless there's some good reason to suppose otherwise. We know what cats are like, and sabre-tooths were cats, so that tells us a lot.

And what about herbivores? Herbivory includes a range of different diets, such as animals that feed mainly on seeds, or fruit. But large mammalian herbivores tend to have two possible feeding strategies: grazing and browsing. The best way to tell the two apart would be by examining their dung, and, failing that, the structure of their digestive systems could well be helpful. Neither, of course, are possible, if all you have is a fossil skeleton, but, fortunately, there are other clues we can examine.

Sunday, 11 December 2011

Weasels in the Snow: Common Weasels and Stoats

A stoat in summer
In terms of the number of species, the weasel family is the most successful of the carnivoran families. That is, at least in part, due to their small size, allowing them to fill niches unavailable to larger animals such as bears, lions, or wolves. The members of the family that take this to the extreme are, of course, those for which it is named: the weasels themselves.

The term "weasel" isn't a truly scientific one. It's used to refer to all those musteline animals that are neither polecats, mink, nor stoats, and that isn't a natural group of animals. A true evolutionary unit should consist of a common ancestor and all of its descendants, but the term "weasel", while it would plausibly include the common ancestor, arbitrarily excludes some of that animal's descendants. In reality, therefore, the animals commonly referred to as "weasels" include some that are closer to, say, polecats, than they are to other "weasels", and, as a whole, they represent at least three, and probably four, different evolutionary lines.

The common weasel (Mustela nivalis) is the epitome of the idea that, for weasels, small is good. Known in America as the least weasel, at as little as 12 cm (5 inches) long ignoring the tail, it is the smallest member of it's family, and thus, the smallest of all carnivorans. Although it prefers forests or farmland, it is happy to live almost anywhere that there is cover, including mountains and semi-desert, and this adaptability has allowed it to inhabit a wider stretch of the world than any other carnivoran species except the wolf.

Sunday, 4 December 2011

Hanging Out with Other Species

Spinner dolphin
Animals interact with members of other species in a range of different ways. Most obvious, perhaps, are predator-prey relationships, but not all interactions necessarily have to have the potential for violence. Often, we find members of different species living side by side because they simply happen to like the same habitat, or one species may steal the burrows of another rather than making the effort to dig their own. But there are also some more organised relationships, where two or more relatively large mammalian species actively congregate together for some sort of mutual benefit.

We commonly see this in herd animal, especially where one species is relatively rare within a given region. So long as they don't irritate the other species too much, it may be to the benefit of the rarer one to join the herd of the more common species, gaining the advantage from large herd sizes that it cannot achieve on its own. Aside from grazing herd animals, other social animals that often congregate with other, related, species, include examples among both primates and cetaceans. (Examples from other groups of mammal are rarer, but have been reported).

Broadly speaking, there are three different reasons why animals might want to actively hang out with members of another species. There is no particular why two or more of these reasons cannot be true at once, and disentangling them can take a fair amount of observation. Let's take a look at one recent study as an example.

Saturday, 26 November 2011

The Secret Life of the Monito del Monte

There are something like a hundred species of marsupial in South America; hardly an insignificant amount. As I've mentioned before, the marsupials were, in fact, in South America before they ever reached Australia. While some headed south over the still green and verdant lands of Antarctica to reach the Australian continent beyond, others stayed behind, becoming the ancestors of the opossums and shrew-opossums that still live in the Americas today - including, of course, one species in the southern US.

But there is an oddity that confuses this simple picture, and that is a curious animal called the monito del monte (Dromiciops gliroides). In evolutionary terms, this gives every indication of being an Australian marsupial, being more closely related to animals like kangaroos than it is to American opossums. Which is a bit odd for an animal that lives in Chile and Argentina. How can this be so? The best guess is that the ancestors of Australian marsupials originated in South America before they crossed over to their new home, and that, for some reason, the monito del monte was the only one to survive in their original homeland. It is, however, also possible that the Australian marsupials really did originate in Australia - or, for that matter, in Antarctica - and that the monito del monte headed back to South America at a later date before the Straits of Magellan opened up.

Thursday, 24 November 2011

News in Brief #2

Columbian ground squirrel
The Wooing Ways of Ground Squirrels

Male animals will go a long way to ensure that they become the father for a female's young. If the female isn't likely to be monogamous, they may chase off rivals, beat them up, display their masculinity, or just have giant gonads. But male Columbian ground squirrels (Urocitellus columbianus), it seems, like to try a bit of smooching.

Columbian ground squirrels (that's British Columbia, by the way, not Colombia in South America) live in complex, underground burrows. They mate in the early spring, after waking up from hibernation, and the females are sexually fertile for just one day during each year, making competition between males particularly important. Yes, the males do fight one another to establish territories, but, according to a new discovery, that isn't their only trick.

In short, they don't go too far on a first date. They sneak into the female's burrows at night, when she isn't sexually fertile, and spend the night with her, without doing anything. The next morning - and the more experienced males do seem to be pretty good at predicting in advance the one day when this is going to happen - the females wakes up feeling randy... and, well, who's already there?

Sunday, 20 November 2011

When Whales Walked the Land

Protocetus, a close relative of the new species
The Eocene, the second epoch of the Age of Mammals, was a time of many strange creatures. The mammals were well established by this point, but few of the modern groups of mammal we are familiar with had yet evolved, and those that had did not necessarily look the same as they do today. Take the whales, for example:

Today, there are two basic types of whale. The odontocetes, or toothed whales, are the largest group, and include the porpoises and dolphins, as well as several larger species, including the mighty sperm whale. The other group are the mysticetes, or baleen whales, which have no teeth, and are instead filter feeders. This latter group includes the right whales and, perhaps most famously, the blue whale, which, so far as we know, is the largest animal ever to have lived.

Both of these groups first appeared at the end of the Eocene, but in those days, they shared the seas with a third, older, type of whale, that would die out during the following, Oligocene, epoch. These were the archaeocetes, and they include the original whales from which all the others later evolved.

Sunday, 13 November 2011

Weasels on the Riverbank: Mink

European mink
While most are still terrestrial, the long sinuous bodies of weasels are easily adapted to an aquatic lifestyle. Within the lineage that led to the modern weasels and their relatives, semi-aquatic habits have evolved at least three times. The first of those lines led to the otters, but the most recent led instead to the European mink (Mustela lutreola), a much smaller animal that is clearly not an otter, and is, in fact, most closely related to the ferrets and polecats.

The European mink was once found throughout central and eastern Europe, from Germany in the west to European Russia in the east. It is almost as aquatic as an otter, and is never found far from fresh water, preferring dense vegetation along the banks of fast-flowing streams and small rivers. Its feet are partially webbed, and it is a good swimmer in comparison to most other members of its family, although not as skilful as the otters. Mink den in natural hollows, such as those beneath tree roots, and will also take over the burrows of their favourite food, water voles.

Even so, while they are undeniably well adapted to a semi-aquatic lifestyle, they are no match for otters. For example, mink do not see well underwater, and only dive after fish, crayfish, and so on, after they have spotted them from above the surface. Given that otters and mink both inhabit the same parts of Europe, why aren't the latter simply out-competed? It turns out that it's their very lack of adaptation that helps them survive.

Sunday, 6 November 2011

Why Marsupials Can't Fly

A sugar glider is as close as it gets...
One of the significant features in the early evolution of mammals was the development of a different posture from their ancestors. Living reptiles, such as lizards and crocodiles, have a sprawling gait, with the limbs splayed out to the side, but, in almost all mammals, the limbs are held erect, directly underneath the body. This has a number of advantages. For example, the body no longer has to bend from side to side as the legs move, which allows the lungs to operate more efficiently - you aren't constantly having to squeeze one of them at a time as your chest flexes. Having the shoulder blades placed more vertically also allows them to have more of a direct involvement in limb movement, effectively giving an extra segment to the limb.

This feature is not unique to mammals, because it also evolved in dinosaurs, and in some prehistoric crocodiles - which, rather alarmingly, could run rapidly across dry land. Today, of course, it is also found in birds. Nonetheless, it is a key feature of mammals, and the starting point for a whole series of adaptations in mammalian limb structure. From this beginning, mammalian limbs have taken on a wide range of different forms, adapted to all kinds of different lifestyles.

One example would be the development of hooves. The stance of hoofed animals, standing on the very tips of their toes, is thought to give the limb greater flexibility and allow them to run faster. Hooves have therefore evolved in herbivores, to, at least in part, allow them to escape from predators. Given the speed at which, for example, a cheetah moves, hooves clearly aren't essential for rapid running, but they do seem to help, and one might wonder why predators - as keen to catch prey as the prey is not to be caught - never evolved them. The answer, most likely, is that claws are just too useful to carnivores, which use them as weapons to bring down their prey; something that wouldn't work with a hoof.
With the great variation of mammals that exist, its quite obvious that the proportions and shapes of limbs vary considerably between them. Still, there are some limits to this variation if you want to be able to walk or run effectively, and a couple of years ago, Manuela Schmidt and Martin Fischer of Jena University conducted a study comparing the limbs of a wide range of mammalian species to look at those limitations. One of their conclusions was that the hind limbs of mammals vary far less than the fore limbs.

Sunday, 30 October 2011

On the Origin of Tigers

South China tiger, perhaps the most primitive living subspecies
The tiger (Panthera tigris) is surely one of the most iconic of all mammals; instantly recognisable, and widely used in images illustrating the beauty of the world's wild animals. But what is the history of this most familiar of beasts? How did the tiger come to be?

There are six living subspecies of tiger, and at least a further two that went extinct as recently as the twentieth century. One way to trace the origin of tigers is to examine the genetics and physical features of the different subspecies and see how they compare. When we do this, we find that the Sumatran tiger appears to be a distinct lineage (it has even been suggested that it should be a considered a separate species, although nothing has really come of this), although the critically endangered South China tiger may have arisen even earlier. One of the first splits after this involved some tigers heading out to the eastern parts of Indonesia, where they established themselves as the Javan and Balinese subspecies - both now extinct. Of those on the mainland, one headed to India as the Bengal tiger, while the other went further north to become the Siberian tiger. The remaining two subspecies, the Malayan and Indochinese, are clearly related to one another, although there is some dispute as to whether they are closer to their relatives in Siberia or those in Bengal.

Sunday, 23 October 2011

The Mother Elk's Dilemma

A male elk
Giving birth and looking after newborn young are particularly dangerous times for the large hoofed herbivores. That isn't to say that it isn't difficult for other mammals, too, or, indeed, other animals in general, but the larger herbivores are especially vulnerable. Smaller animals can hide their young in burrows, or other secluded dens, until they come of age, while the larger carnivores don't generally have to fear being eaten. Large herbivores do not have such a luxury.

However, hiding from predators while the young are still small and vulnerable is not the only concern that mothers have to face. Giving birth is costly in terms of resources, and after that, the young must be fed milk until it is large enough to search for food on its own. The mother, therefore, needs to keep herself well fed, and needs a good supply of food to keep both herself and her infant healthy. While this is obviously true at any time of the year, during the birthing season it becomes particularly important for the mother to find an area that is both free of predators, and has a ready source of available food.

That can be a problem if the areas that have the best food are also the ones that have the most predators.

Sunday, 16 October 2011

Weasels on the Farm: Ferrets and Polecats

European polecats
The great majority of the animals that mankind has domesticated are herbivores; horses, cattle, sheep, camels, guinea pigs, chickens, and so on. Of course, there are always exotic pets, and birds of prey kept for falconry, but when it comes to carnivorous species domesticated for long enough that they are distinctly different from their wild ancestors, there are really only three: cats, dogs... and ferrets.

Of course, ferrets have been domesticated for far less time than either cats or dogs. Quite when this first happened is unclear, although we know that the Romans bred them for catching small animals such as rabbits down burrows, and they may not have been the first to do so. Today, ferrets are often considered sufficiently different from their wild kin to be classed as a different subspecies (just as cats and dogs are). Unlike the wild forms, they are often white or pale yellow in colour, and many are true albinos, with bright red eyes. However, the coat colour can be quite variable, including tan, reddish-brown, dark brown and true black, often with markings that can be clear enough to give a 'Siamese' appearance. Although the colour variation is less than among, for example, cats, the American Ferret Association nonetheless manages to recognise a full 38 possible colour patterns for show purposes.

Sunday, 9 October 2011

Bats Can Be Colourful, Too

Spotted bat, Euderma maculata
It's stating the obvious to say that mammals have a range of different colours and coat patterns. The purpose of all these different markings can be varied: they may help to identify members of particular species to their kin, they may be used for sexual attraction, they can act as camouflage, and so on. Nor is there any particular reason to suppose that a given pattern has to serve only a single function. There are a great many colourful mammals, but bats are generally not among them.

The bats are the second largest order of mammals, after the rodents, including a total of nineteen different families - most of them with really obscure names - and well over a thousand different species. Yet, despite this great diversity, most bats are pretty much the same colour all over - usually a variation on the theme of "it's brown". But not all of them; many bats are surprisingly colourful, and you might wonder what the point of that is if they only come out at night, and spend the rest of the day sleeping in pitch black caves. The question really is not so much why are most bats so bland, but why aren't they all that way?

Except, of course, that not all bats do live in caves, and that may well have something to do with it. For the second largest order of mammals, bats have not been as well studied as most other groups. Most of the studies that have been conducted have tended to focus on the undeniably cool fact that bats navigate using sonar. Yet they are a very interesting, and one might even say peculiar, group of mammals. It may be, for example, that just looking at the colours of bats can tell us something about the reasons for coat patterns in mammals in general. A recently published survey by Sharlene Santana of UCLA, and colleagues, examined published descriptions of over nine hundred species of bat, cross-checking the patterns of their fur with their lifestyle. Was there... well, a pattern to the patterns?

Saturday, 1 October 2011

Following the Herd

A number of mammal species live in large groups, including dolphins, monkeys, and wolves, among others. Among the most familiar, though, are the various hoofed herd animals. In order to maintain a herd - or any similarly sized group - it is important that the animals all move together, and that there has to be some kind of communal decision-making process that everyone agrees on. It's no good one animal wandering off on its own, if nobody will follow it, but, equally, if all animals have an equal right to decide where the herd should go, its just going to mill about, not going anywhere.

Undoubtedly, different species will have different methods for making such decisions, depending on their biology, the nature of the environment, and so on. If the group is really big (as might be the case with, say, bison or wildebeest), options are fairly limited, and there is unlikely to be one single leader - if only because not all the members of the herd will be easily able to see him. Among animals that live in smaller groups, leadership by single individuals, or by a small group of individuals, becomes more of a realistic possibility, although alternatives do exist. But which individuals do the leading?

Thursday, 29 September 2011

News in Brief #1

Sometimes weeks go by and I have difficulty finding anything very new to post, at least that isn't too similar to something else I've already done recently. Just as often, though, I have to pick between a number of possible stories, and some end up being pushed to the back of the queue, and never leave it. So, every couple of months or so, I'm going to gather up stories that didn't quite make it, and post a short summary here. So, without any more ado:

Are Foreign Mating Calls Still Sexy?
Sika deer
Mating calls are hardly an unusual feature in mammals, and deer are no exception, especially where the males like to gather a larger number of females around them to mate with. You'd think that part of the point of a mating call is to attract females of your own species, so that you end up with a suitable partner. A group of British, Austrian, and French scientists recently tested this out with female red deer (this is the European version of the animal Americans call an 'elk'). They put up loudspeakers emitting recordings of male red deer, and of male sika deer, a closely related Japanese species with similar mating habits, but that looks quite different.

Sure enough, most of the females wandered over to where the calls of the male red deer seemed to be coming from. But ten percent of the females actually seemed to prefer the calls of the sika males, apparently finding them more enticing than the ones from their own males. The researchers say that this may lead to "permeability of pre-zygotic reproductive barriers"... by which they mean a willingness to have sex with the wrong species. And, indeed, after sika deer were introduced into parks in Europe, some hybrids between the two have been reported. Some red deer does, it seems, just find the exotic attractive.

The Insightful Elephant
We've known for a while that we aren't the only species to use tools, even if we ignore instinctive use of objects - such as birds smashing open snails on rocks. Chimpanzees, for example, have the intelligence to work out how to use simple tools to acquire food, and, outside the world of mammals, even some species of crow have been shown to do the same. Elephants seem a reasonable candidate for another animal that might do the same. They are intelligent animals, and they have a trunk that can pick up and manipulate objects.

Sunday, 25 September 2011

A New Species of Dolphin

Common bottlenose dolphin
On September 14th, the discovery a new species of dolphin, the burrunan dolphin (Tursiops australis) was officially announced. There has already been a fair bit of coverage of this in the media (see, for example, the BBC story), but I want to focus here on how this all came about. How exactly do you go about naming a new species?

In theory, it's a fairly straightforward, if somewhat laborious, process. You find your new species, write up a description of what it looks like, and how to tell it apart from similar species, designate a holotype (more on this later), think up a name, and get it published. Leaving aside the difficulty of the first part of that - "first, find your new species" - that's often all there is to it. But, with dolphins, the story has been rather more complicated than that.

First, let's get our bearings. The sort of dolphin we're talking about here is a bottlenose dolphin, a particularly well known type, and commonly seen in sea mammal parks, where they have been trained to perform a number of tricks. They live in every ocean, avoiding only the very coldest of polar seas and are therefore extremely widespread.

Sunday, 18 September 2011

Griphotherion - the Puzzling Beast

The skull of Mesotherium, a large, beaver-like, typothere
- note the shape of the teeth
For millions of years after its separation from Antarctica and before its collision with North America, South America was an island continent, isolated from much of the rest of the world. Many of the groups of mammals we are familiar with had not evolved at the time of the separation of the continent, and its long period of isolation allowed many strange animals to evolve to take their place, quite different to those elsewhere in the world.

Once Central America began to form, and the animals we are more familiar with began to flood south, these odd native animals began to die out. It took a long time, and, in fact, four groups do still survive today - opossums, armadillos, anteaters, and sloths are all remnants of this once more diverse group, and two of those even crossed the land bridge in the other direction, and can now be found in the North.

But the others were less fortunate. Large carnivorous mammals never got a foothold in the continent when it was still an island, and the arrival of sabretooths and the ancestors of jaguars (to name two obvious examples) doubtless contributed to the decline of the native fauna. Herbivores, however, were a different matter. I've mentioned this before, in the context of some rather odd-looking long-nosed herbivores, but there are many other examples.

Sunday, 11 September 2011

The World of Weasels

The common, European, weasel, Mustela nivalis
The best known of the carnivorous mammals are surely the large, dramatic, species, such as lions, tigers, wolves, and grizzly bears. Yet, if we determine evolutionary success by the number of species in a group, the most successful carnivoran family is not that of the cats, dogs, bears, or hyenas, but the weasel family. In a way, this should not surprise us too much, since there is always going to more food going around for a small animal than for something the size of a tiger or polar bear.

The weasel family is also, arguably, the most diverse of the carnivoran families. Nobody would doubt that a tiger is a kind of cat, and its hardly surprising to learn that foxes are members of the dog family, but the majority of members of the weasel family are not animals that, in everyday speech, we would call weasels. True, ferrets, for example, do look rather like out-sized weasels, but its probably less obvious that the family includes such animals as badgers, otters, and wolverines.

But what, exactly is a family of animals? The latest edition of Mammal Species of the World lists 144 families of mammal, 121 of which are placentals, but such a list can never be truly definitive. The modern rules for defining any natural group of animals, whether it be a family, subfamily, order, or anything else, is that all the species in that group must be more closely related to each other than to anything outside the group. This means that a family, like any other meaningful group of species, includes a single common ancestor and all of its living descendants.

Sunday, 4 September 2011

The Scariness of Tigers

Animals are naturally wary around signs of predators that might want to eat them. Even if they don't necessarily run away at first sight, they are at least likely to spend more time keeping watch and less time, for example, eating. Many animals may have a range of possible predators, and so have to be on the lookout for a range of different signs that one might be in the area. On the other hand, it makes no sense to be frightened of anything unexpected, so its unsurprising that they watch out for sounds, smells, and so on that are particularly associated with animals that regularly attack them.

Sometimes this behaviour may be learned, and at other times it can be ingrained deep in the genes, depending not least on what animal we're talking about. But what happens when the predator is not around any more? That completely removes the possibility of learning that the predator is dangerous, but it may also, over a number of generations, disappear from the genetic record as well - there isn't much point wasting energy looking out for signs that you'll never see.

Sunday, 28 August 2011

Our Jurassic Mother

Eomaia scansoria - no longer the oldest known eutherian
Today, almost all mammals fall into one of two major groups: the placentals and the marsupials. These two groups represent a fundamental split in the lineage of the mammals, a split that occurred long, long before the dinosaurs died out, when mammals were, for the most part, small shrew-like animals living in the shadow of the great reptiles. But how long ago, exactly?

Since 2002, the oldest known fossil of a possibly placental mammal was that of Eomaia. It is remarkably well preserved, even including impressions of the animal's fur, and dates from 125 million years ago. The oldest known marsupial fossil, Sinodelphys, dates from around the same time and place, and there had been reason to suppose that the two lines separated not long before that.

But just how long ago is that? I have recently been using charts to show the age of various fossils within the Age of Mammals. That entire era has lasted (so far) 65 million years, so to get back to Eomaia we need to head back almost as far again as the whole period covered by the chart. It was a time before such famous dinosaurs as Tyrannosaurus and Triceratops had evolved, and the best known animals of the time are perhaps Iguanodon, Kronosaurus, and Deinonychus (on which the 'raptors of the Jurassic Park films seem to have been based, although they decided to use the cooler sounding name of a rather smaller dinosaur instead).

Sunday, 21 August 2011

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.

Sunday, 14 August 2011

Sex Without Pregnancy: how female macaques get what they want

Tibetan macaque (Macaca thibetana)
The females of most mammal species have some means of signalling to males that they are sexually fertile, and able to become pregnant. There is often some sort of breeding season, timed so that birth is likely to happen at a point in the year when food is abundant - spring, the rainy season, or whatever. But the cycles of female hormones mean that the female is only actually capable of becoming pregnant for periods of a few days, or even hours, during that season. Essentially, she comes into heat, and the males get excited.

That's the usual pattern. But there are some mammals that don't seem to do this. While they may not bother to mate outside of a mating season, they don't seem to be indicating to males when, within that season, they are likely to become pregnant. But is that really the case?

Sunday, 7 August 2011

Why Cows Have Four Stomachs

(If all you want to know is what animals do, and what animals don't, have a four-chambered stomach, the shorter answer is here).

The stomach is an organ found in almost all vertebrates. Although there is considerable variation among the different groups, and there are some fish that don't have one at all, in general it has two functions. Firstly, it helps to store food for later digestion so that you don't constantly have to be eating, and secondly, it begins the digestion of food both by physically grinding it up and by mixing it with acid and digestive enzymes. Sometimes these functions are separated to some extent - for example, birds have a large crop for food storage, and a smaller true stomach below it that digests the food, and includes a muscular, grinding gizzard (a useful thing when you have no teeth). But the most complex stomachs of all are found in mammals.

A cow does, indeed, have four stomachs. Or, at least, it has a stomach divided into four separate chambers, which amounts to the same thing. Nor, of course, are cows alone in this. It's a feature found throughout the cattle family, which is a fairly large group consisting of over a hundred species - most of them antelopes, although it also includes the sheep and goats. The cattle were not the first family to evolve the feature, and we know that because its also found in all their close relatives, including such animals as deer and giraffes. In fact, the only truly cloven-hoofed animals that don't have four stomachs are the pigs and peccaries - which is why they aren't kosher.

In fact, we can group all the mammals that have this four-chambered stomach together. This group are called the "pecorans". Why not just "ruminants"? We'll get onto that later.

Saturday, 30 July 2011

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.

Sunday, 24 July 2011

Should I Stay or Should I Go?

Although they are hardly alone in this, mammals are noted for their care of their offspring. Yet many of them are solitary animals, and the young leave home as soon as they are able to survive on their own, so that the only groups seen are when a mother is caring for her children. But, of course, many mammals live in herds, packs, or other groups, with at least some individuals staying with their parents once they grow up.

But, even in herds, some animals do leave home, to establish or join new groups elsewhere. How do they make this decision as to whether to leave or stay? For many, its a fairly simple rule: the males leave, and the females don't. That means that a herd or other group is dominated by a core of females descended from a single matriarch, while the males have generally come in from outside. Initially, in most cases, the males wander about looking for a new group, perhaps together with one or two other young males in a similar situation, until they eventually find someone that will take them in. This ensures that they don't end up mating with their own female relatives, and keeps the gene pool as wide as possible.

Sunday, 17 July 2011

Badger Badger Badger

The European badger (Meles meles) is a very familiar animal across Europe, the animal we immediately think of when we think of "badgers". Its distinctive black-and-white face markings make it instantly recognisable, but, in fact, it has quite a range of different appearances, with variations in size, fur colour, and skeletal anatomy that allow it to be divided into a number of subspecies. This variation has been the source of some confusion when it comes to separating this species from its closest relatives.

It used to be thought that the European badger could be found throughout almost the whole of Europe and Asia, avoiding only the tropics, deserts, and the depths of Siberia. As a result it was called, quite reasonably, the "Eurasian badger". Over the last decade, that view has changed. Firstly, it became clear that the badgers of Europe and Asia were different enough to be considered separate species, and then that the badgers of Japan were likely different from those elsewhere in Asia. The suggestion has a venerable history; it was first made in the 1840s, but subsequently ignored when scientists decided that Eurasian badgers were really just one species that happened to be quite variable in appearance.

Sunday, 10 July 2011

How Black Bears Trekked to Ontario

Can you show me the way to Ontario...?
The Pleistocene is the geological epoch immediately before our own. It ended just ten thousand years ago, a time so recent on a geological timescale that the modern epoch isn't even visible on the chart of the Age of Mammals shown below - its effectively just the line at the very top of the column. It's hardly surprising therefore that, for the most part, the animals alive in the Pleistocene are the same ones that are alive now, barring those that have gone extinct over the course of human history.

The Pleistocene, however, represents the Ice Ages, when vast glaciers covered much of the landmass of the northern hemisphere (rather less so in the south, though, because Australia and Africa, in particular, don't get as close to the south pole as Europe, Asia, and North America do to the northern one). It ends with two events that occurred roughly at the same time: the warming of the world at the end of the last Ice Age, and the dawn of human agriculture. In the case of North America, humans only arrived at the very end of the Pleistocene, meaning that it represents the last time that the continent was untouched by human hands.

Sunday, 3 July 2011

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.]

Sunday, 26 June 2011

Second Time Mums - Luck, Fitness, or Callous Calculation?

Oddities such as the brown antechinus aside, unless they die young, most female mammals can expect to give birth to several litters over their lifetime. This is a useful strategy, ensuring that you don't put all your eggs in one basket, so to speak. However, as human mothers may have noticed, looking after young can be a stressful and exhausting task, especially in species where the male can't be bothered to help out (which is most of them). As a result, in the wild, raising a litter of young is not always as successful as it might be, and, at times, the mother doesn't even survive the experience.

In general, it seems that mothers raising their first litter of young are the least likely to manage; those who have done it at least once before cope much better. That might not sound very surprising, but consider that there at least three different reasons why this might be the case. Perhaps the most obvious is that they just aren't very good at it the first time round. That may be partly due to a lack of experience, but raising young is fairly instinctive for most species, so it could also be that older mothers are larger, and better able to survive the experience of diverting some of their food resources to their offspring.

But another possibility is that the mothers might be deliberately not doing such a good job the first time round. This explanation, which I also discussed for the brown antechinus, is "terminal selection theory". The idea is that it's sometimes better for the mother to lose a litter and live to breed again than it is for her to die raising the first one. As she gets older, the less likely she will have another litter anyway, and the less she has to lose by putting all of her effort into raising the current one (assuming that is, that your primary objective is to have as many healthy young as possible, which in evolutionary terms, it usually is). So, if you don't completely wear yourself out on the first litter, maybe they'll survive anyway, but, even if they don't, at least you'll get to grow bigger and raise more young in the future.

The third possibility is that some mothers are better than others. If the 'bad' mothers are going to die trying to raise young, its far more likely to happen on their first attempt. If this theory is right, the only reason that older mothers do a better job of raising young is that they've proven they're good at it - all the bad mothers are already dead. They survive to raise more young, not because they were lucky, but because they have some advantage that continues to stand them in good stead as they age. Finally, of course, we should acknowledge that there's no reason why there can't be some truth in all three of these options, or that some might be more important for some species than others.


The northern elephant seal (Mirounga angustirostris) is one of the largest of all seals, with the adult males reaching over two tons in weight. They breed along the coast of California, spending the rest of the year offshore and travelling at least as far as southern Alaska. They belong to the true seal family, which is to say that they don't have visible ears or the ability to waddle about on land as fur seals and sea lions do; their hind legs are so well adapted to swimming that they can't be used for much else, and they have to drag their bodies with their front flippers when out of the water. Their closest relatives all live in Antarctic waters, from which the ancestors of the northern elephant seals probably headed north about three million years ago.

Leopard Seal   Crabeater     Northern        Southern
    etc.         Seal      Elephant Seal   Elephant Seal
     ^             |            |                |         Monk
     |             |       (move north)          |         Seals
     |             |            |                |           ^
     ---------------            ------------------           |
            |                           |                    |
            |                           |                    |
            -----------------------------                    |
                         |                                   |
              (migration to Antarctic)                       |
                         |                                   |
                         -------------------------------------
                                          |
                                          |
 
Interestingly, monk seals today are found in only two locations - the Mediterranean and Hawaii, which aren't exactly noted for being close to each other. Since seals as a whole seem to have originated in the North Atlantic, the Hawaiian ones must have travelled west by swimming in between North and South America, before they collided. Indeed, fossil monk seals have been discovered in the Caribbean, which would support this idea. For that matter, this is probably also the route that the Antarctic seals themselves took, much earlier, since we know of fossils of them from the Pacific, but not from the South Atlantic.


In a recently published study, Derek Lee of Dartmouth College in New Hampshire, looked backed over twenty years of records of northern elephant seals breeding at the Farallon Islands near San Francisco. There are a number of reasons why first-time elephant seal mothers might find it harder to raise their young than their more experienced relatives. Seals have to come ashore to give birth, but the rest of the time they are at sea. Since mating also occurs on land, it makes sense for it to happen at the same time, so they courtship takes place as soon as the previous litter are weaned, if not before. The rookeries where they breed become raucous and violent places with enormous males charging about all over the place, so the females can't afford to leave their young alone, which, since the males certainly aren't going to help, means they have to starve themselves for all the time that the pups are growing up.

This makes the life of a female elephant seal a pretty tough one, at least once they reach sexual maturity at three or four years of age. They spend eight months of the year pregnant, another two months raising a pup, and then two months stuffing themselves with food to make up for all the weight they have lost during their enforced crash diet. First time mothers, being smaller than their older relatives, would be expected to have a harder time surviving that catastrophic weight loss, and maybe also defending their patch of land and protecting their pup against aggression.

Its unsurprising, then, that first time mothers do not do as well than more experienced ones. They are less likely to raise their young to the point that they can be weaned and go off to live on their own, and also less likely to survive the experience long enough to return the following year. Not that this stops them trying; the study found that they were just as likely to mate after giving birth as experienced mothers.

Notice that, if this study is correct, it does rule out the possibility that this population of elephant seals, at least, are purposely expending less effort in looking after their first pup to increase their odds of raising a second or a third later on. If that were the case, while first time mothers would be less effective at raising young, they would be at least as likely to survive to breed again as experienced ones, and they weren't. So if they are trying to maximise their own survival at the expense of their pups, it clearly isn't working.

Its tougher to decide between the other two possibilities. Do experienced mothers do better and live longer because they got lucky the first time round, or because they have always been more skilled or fit than those which died? By examining the statistics, the study concluded that it was fitness as a mother, not sheer luck, that made most of the difference. Apparently, there is a Darwinian cull of unfit mothers, ensuring that only the best get to raise multiple pups in their lifetime.

The study doesn't end there, though, because it also looked at the effects of El Niño over the twenty year period. The main fish that northern elephant seals eat are Pacific hake, and, when El Niño brings its periodic warming of the eastern Pacific waters, the hake do less well, and the elephant seals suffer. Of course, hake aren't the only thing they eat; they are quite happy to consume squid, and even some small sharks and rays, but it is clear that the elephant seals do not put on so much weight in years when El Niño strikes.

We might expect that first-time mothers would do worst, especially if their smaller size is a handicap for them, but the study seemed to show that they did no better or worse than experienced mothers during warm weather. Which isn't to say that climate change, with increasingly warm waters and reduced numbers of tasty hake, doesn't affect elephant seals, just that it affects all of them equally, and that there are some things that being a good mother just can't help you with.

So, if you're an elephant seal, and you want to live long enough to have a second child, and more after that, you don't have to rely on luck, and you don't have to callously withhold all your energy from your first-born. Thanks to Darwin, you just have to be a good mother.

[Picture from Wikimedia Commons. Cladogram adapted from Fyler et al. 2005]

Sunday, 19 June 2011

The Bone-Crushing Dogs of Oregon

Aelurodon, a borophagine dog
The mammals are an ancient group, far older than the birds, and, by certain definitions, as old as the reptiles. But the true rise of the group - the period often called the Age of Mammals - only begins with the extinction of the dinosaurs, 65 million years ago.

But 65 million years is an incredibly long time. In comparison, mastodons and sabre-tooth cats both died out round about 10,000 years ago, not even 0.1% of that great timespan. Over the course of those millions of years, many groups of mammals (and, of course, of other animals, too) came and went, and the faunas and landscapes of the world changed time and time again. If sabre-tooth cats prowled the countryside just a few thousand years ago, you can see that you don't have to go back very far to find a world that looks, in many respects, quite different to that of today, and the further back you go, the greater the differences become.

For example, compare Africa and North America today. Africa has lions, elephants, leopards, dozens of different kinds of antelope, rhinos, hyenas, giraffes, hippos... the list of large and cool-looking animals just goes on. North America, while not truly impoverished, has far less in comparison. There's pronghorns, a few types of deer, three bears, wolves and pumas and, in terms of big animals, not a lot else. But it used to be like Africa just a few tens of thousand of years ago, never mind millions. America had elephants - or at least mastodons - along with many more large and fierce cats, and plenty of odd-looking antelope like creatures, and more besides. If you could go on a safari trip to the North America of millions of years ago, there would certainly be plenty to see.

This 65 million year span is conventionally divided into seven epochs. The last of these, in which we currently live, only starts with the end of the last Ice Age around 10,000 BC, and is therefore so short that you can't even see it on the chart I've used here. Most of the others are so long that they need to be divided into much shorter chunks for them to be of any real use when we try to get a picture of what appeared when.

The Miocene is the fourth of these epochs, and its name means "less recent" because there were less familiar looking animals alive then than in the three epochs that follow it. It is, in a sense, the last epoch before the sorts of animals we see today began to really gain a foothold. One illustration of the way that this happened is the story of the dogs, and a recent review of local fossils by Eric Ekdale and Timoty Rowe of the University of Oregon provides a snapshot that feeds into the bigger picture.

Oregon today has coyotes, wolves, and two species of fox - the red and grey. The wolves have only recently returned to the state, and are still struggling to gain a foothold, but the other three species are reasonably secure. Like all living dogs, all four of these animals are true "canines", in the true technical meaning of the word (which includes foxes, which, in more common parlance, we would call "vulpines"). But these are not the only sort of dogs that have ever lived.

"True"     "True"
 Dogs      Foxes        Borophagines
  ^           ^              ^
  |           |              |
  |           |              |         Hesperocyonines
  -------------              |                ^
        |                    |                |
     Canines                 |                |
        |                    |                |          Bears,
        ----------------------                |           etc.
                  |                           |            ^
                  |                           |            |
                  -----------------------------            |
                                |                          |
                          (First dogs)                     |
                                |                          |
                                ----------------------------
                                             |
                                             |            
             
The group of "true" dogs (technically "canins" - note the missing 'e') includes the grey fox, among others that we traditionally think of as foxes. The red fox of Europe and North America really is a fox, though!

As the chart above shows, the canines - modern dogs and foxes - lived alongside the borophagines ever since both groups diverged from the more primitive hesperocyonines at around the end of the Eocene. During the Miocene, however, the borophagines were by far the dominant group, with there being (so far as we know) relatively few species of true dogs or foxes around at that time. The fossils in the review date from around 10 million years ago, just as the balance was starting to switch, and the borophagines began to go into decline. The hesperocyonines had already gone by this point, apparently out-competed by their relatives during the previous Oligocene epoch.

I've mentioned before that many fossils are very incomplete, and give scientists very little to go on. This was the case with many of the fossils in this review, which included five teeth, one foot bone, and one leg bone, all from different animals. The researchers were fairly confident that these all belonged to borophagines of some sort, but had no way of saying any more than that. For example, one tooth was a premolar. It was the wrong shape to belong to a cat, too well-developed to belong to the only other sort of large carnivore around at the time, and too large to belong to any sort of fox. That only leaves the borophagines, but beyond that, who can say?

Another pair of teeth, however, both seemed to come from the same animal, and had been described as belonging to a fox (Vulpes sp) in a previous study back in the 1960s. The authors of the review thought that that was a bit over-optimistic, not least because that sort of fox isn't thought to have appeared for another three million years. They do agree, however, that it isn't a borophagine, and therefore is a true "canine" dog. Although there's not really any way to know, it might have belonged to Leptocyon, which lived across much of North America at this time. Leptocyon was smaller than a red fox, and slightly larger than a kit fox, and probably looked quite like both of these animals.

Its significant that these remains were the only ones that belonged to something resembling a modern canine. With so many borophagine specimens, its may be that this animal - whatever it was - was rarer than they were, confirming the story that it was the borophagines that were the top dogs of their day.

The other three fossils all belonged to different species of borophagine. That means that, 10 million years ago, like today, Oregon had at least four species of dog, although there's every chance that there were more we don't know about. The most complete fossil belonged to a coyote-sized animal called Carpocyon, that had previously not been found this far north and west. Unfortunately, it was missing the skull. Palaeontologists like skulls; they're usually the most distinctive bit of an animal, and in this case, it meant that we can't tell exactly what sort of Carpocyon it was - there are at least two possibilities.

It's the other two animals that really show how the dogs of modern Oregon are no match for those of ten million years ago, and they're also the only ones we can identify the exact species of. One, Epicyon saevus, was perhaps about the size of a rottweiller, but the other, its close relative, Epicyon haydeni, was a really, really, big dog. It was larger than a black bear, and had powerful, bone-crushing jaws that could presumably have demolished the entire carcass of anything it ate, skeleton and all. Unlike Africa, North America never had hyenas, but it had the great pack-hunting Epicyon haydeni instead, which lived in the same way and was considerably larger.

It must have been one of the most fearsome predators of its day, and it's hard to imagine anything would have chased it away from a kill, except possibly a sabre-tooth. Apart from the large size, it probably looked more like a wolf than a bear or hyena, but was surely a muscular and impressive animal. These are the animals that once terrorised the wilds of Oregon.


[Picture from Wikimedia Commons. Cladogram adapted from Wesley-Hunt & Flynn, 2005]

Saturday, 11 June 2011

How many friends does a dolphin have?

Studying the behaviour of animals in the wild can be difficult, especially if they live in inaccessible or hostile environments, such as precipitous mountains or dense jungle. Animals that live out at at sea are particularly problematic. Seals and sea lions come ashore once a year to mate and give birth, and spend the rest of their lives some distance from land, where we have far less idea what they're doing. Fortunately, zoologists tend to be obsessed with sex, so at least they're on dry land when they're doing what we're most interested in.

Cetaceans, of course, don't even come ashore to do that (although they aren't quite alone in this respect, even among mammals). These animals are superbly adapted to an aquatic life, even more so than seals, and much, much more so than we decidedly non-aquatic humans. This means that studying cetaceans in their native habitat is a difficult, and rather specialised field - and, depending on what you want to know, studying them in the laboratory may not even be an option.

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]