Going Underground

European (or "common") mole

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

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

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

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

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

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

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

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

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

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

Eastern (or American "common") mole

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

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

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

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

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


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

Endangered Cats of the High Andes

A few weeks ago in the UK, we had the 10-yearly treat of filling in our national census forms. Once all the information from these forms is collected, we will have an official figure for the number of humans living in our country. Unfortunately, finding out the same figure for wild animals isn't quite so easy. Yet studying the population densities and numbers of wild animals is often of more than academic interest. A great many species are endangered, and without accurate ideas of how many of them are, we can't tell how endangered they might be, what strategies we might have to use to protect them, and whether or not those strategies are working.

Of course, animals can't fill in census forms, but there's more to it than that. If the animal lives in large, highly visible herds, it may not be so difficult - although even there, you have to be sure you're not counting the same animal twice, and that you're picking a suitably representative area to study (since, realistically, you're not going to go out there and do a head count of every such animal in existence). But many animals aren't so obliging. They may be small, they may actively hide from humans, they may be nocturnal or crepuscular, or they may just live somewhere its hard to get to, or in a jungle so full of plants its hard to spot anything. The rarer they are, the harder it becomes to find them, and yet those are exactly the ones we're most interested in.

Horns of the Thunder Beasts

Megacerops, one of the largest brontotheres

Today, the group of odd-toed ungulates - those large mammalian herbivores that support most of their weight on their middle toes - is represented by just sixteen species in three families: the horses, rhinos, and tapirs. But they were once much more extensive, with whole families of animals that didn't make it through to the present day. One of these was the brontotheres, a family whose name literally means "thunder beasts".

These were big, heavily built animals, some of them standing over six feet tall at the shoulder. In many respects, they would have looked quite like rhinos, having a rather similar body-shape, and crucially, large blunt horns on their snout. However, the horns weren't really so rhino-like as they appear, since the horns of rhinos are actually made of compacted hair, while those of brontotheres were made entirely of bone. The surface of the horns are rugged, suggesting that they were probably covered in skin during life, which perhaps makes the closest living parallel the projections on the heads of modern giraffes.

In the grand scheme of things, the brontotheres didn't last very long, dying out around 34 million years ago, but they must have been very impressive animals when they were alive. Despite their appearance, they seem to have been more related to horses than to rhinos. This is more apparent when you look at the very earliest brontotheres, which were relatively small and hornless, and do look rather more like the early multi-toed ancestors of the horses.

Muscular Jocks and Randy Nerds - the Sex Lives of Wild Guinea Pigs

Brazilian guinea pig

The domestic guinea pig (Cavia porcellus) is a fairly familiar animal, but they've changed enough in the 5000 years of their existence that we don't actually know quite what they were domesticated from. There are at least five species of wild guinea pig alive today, and while one of them was presumably the origin of the domesticated form, we don't know which it is. In addition, the guinea pig family includes eight other species with a fairly similar appearance, and three that are manifestly different, including the largest living rodent, the mighty capybara.

The guinea pig family therefore represents a fairly diverse group of South American rodents, adapted to a wide range of different habitats, and found across much of the continent. Considering that, by the standards of rodent families, there aren't very many species in the group, and that, with a few exceptions, they don't look all that different, its perhaps surprising just how variable their behaviour is. That's largely because they have adapted to so many different environments; there are forest dwelling species, a couple of semi-aquatic forms, and others living in mountains, deserts, and grasslands. The different ways that these species behave was the focus of a couple of recent reviews in the Journal of Mammalogy.

Guinea Pigs    Mountain    Yellow-toothed     Rock      Capybara
                Cavies         Cavies        Cavies
     ^             ^              ^             ^          |
     |             |              |             |          |
     |             |              |             |          |
     ---------------              |             ------------
            |                     |                  |
            |                     |                  |
            -----------------------                  |
                       |                             |
                       |                             |
                       -------------------------------
                                      |
                                      | 

The Brazilian guinea pig (Cavia aperea) is one of the ones that might be the wild ancestor of the domesticated animal. It prefers savannah with plenty of long grass and undergrowth. They are moderately sociable animals, where males live in a stable home range with one to three females. (One suspects those with a single partner just haven't been able to find any others, rather than having a preference for monogamy). Obviously, this means that there are some males left over, and these, which are generally younger and smaller individuals, wander about quite widely, although never too far away from the mated groups - presumably waiting for the male to die, or at least weaken.

As you might expect, this leads to quite a bit of violence between the males, and if you keep them together in captivity, they will try and kill each other. The females, on the other hand, tend to ignore each other, although males have been seen trying to break up fights between their partners. All of this means that males are quite effective at ensuring a female's pups are their own. As a result, Brazilian guinea pigs have evolved so that males are noticeably larger than females - their primary means of passing on their genes is to beat up their rivals, and for that, you need to be large and muscular.

The greater guinea pig (Cavia magna) lives in regularly flooded wetlands, and isn't a strong contender for the ancestor of the domestic forms, not least because it has webbed feet and enjoys swimming. Unlike the Brazilian guinea pigs, they don't seem to stick to a home territory, possibly because they can't predict when the ground is going to be flooded. Indeed, they are relatively antisocial for guinea pigs, largely ignoring each other and wandering about on their own. In their fast-changing environment, males have to mate with whatever females they can find, but they also need to fight off rivals, and so the males are larger and more muscular.

The common yellow-toothed cavy (Galea musteloides) has a third tactic. Although there is apparently some debate on how they live in the wild, they generally seem to be quite sociable, with several males and females living together, and communally raising their young. There is a dominance hierarchy, with an identifiable alpha male, and we might therefore expect that, as with the Brazilian guinea pigs, the males would be bigger, to fight off their many rivals. But they're actually slightly smaller and weedier than the females, and that's probably because of the way the females behave.

When they are ready to breed, the females get very excited, and dash about all over the place, something that the males notice quite quickly. Eventually one of the males will catch her, and mate, but that isn't enough for the female, who will immediately start running around again until another male manages to catch her. She often doesn't stop until everyone has had a go (and usually more than once each), which makes male dominance a bit pointless. There isn't much purpose fighting off rival males if it's the female that calls the shots, especially if she's going to mate with everyone else no matter what you do.

So, if they can't fight off rivals, how do the males ensure that they sire the most offspring? Well, they might not be able to out-compete rivals in physical combat, but they can try to do it sexually. Relative to its body size, the common yellow-toothed cavy has amongst the largest testicles of any mammal. In fact, they are about three times the size of those of Brazilian guinea pigs. As with most mammals, they shrink dramatically outside the breeding season (you'll probably have noticed that humans don't have a breeding season...), but that's par for the course. The individual sperm cells are also smaller, and less likely to be damaged, which means that not only can they produce a large quantity of the stuff, but it contains more healthy sperm for a given volume. If you can't beat your rivals with teeth and claws, you can try and do it by producing more, and better, sperm.

Capybara - a 140 pound "guinea pig"

What of the largest rodent of them all, the capybara (Hydrochoerus hydrochaeris)? As you might expect for something weighing 140 pounds, capybaras don't look that much like guinea pigs, and they were once thought to belong to a different family altogether. Nonetheless, genetic analysis has shown that they are, in fact, gigantic, semi-aquatic relatives of the rock and climbing cavies.

They live in herds, which can, at times, reach quite large numbers. The females in a herd tend to breed at around the same time, so that their young are born together, and can be raised in a nursery group, with all the females cooperating. Their testicles are not particularly large, and, when examined under the microscope, seem to have a high number of testosterone producing cells, rather than sperm-producing ones. That suggests that, despite the large number of potential rivals, breeding males can be confident nobody else will get at their females, and that they would rather divert their energy into being larger, muscular, and generally more aggressive.

Which is what we see; the males are larger than the females, and establish a clear dominance hierarchy, with an exceptionally large alpha male controlling access to the females. He isn't perfect at this, of course, and younger males do get a look in from time to time, but he does have a few advantages on his side. Females are only receptive to mating for a few hours at a time, which gives the male the chance to stand guard, and courtship and sex last around ten minutes - quite long enough for the alpha male to step in and put a stop to anything he doesn't like the look of.

Many other species of guinea pig show variants on these themes, with, for example, the southern mountain cavy (Microcavia australis) showing sperm competition, and having correspondingly large testicles, and the rock cavy (Kerodon rupestris) fighting off rivals. But there is a third way.

The Muenster yellow-toothed cavy (Galea monasterensis) was only identified in 2004, and is only known from one small location in Bolivia. In experiments, when females are given a choice between two or more males, they pick one and stick with him, largely ignoring the others. After they have been together for some time, if the female is removed, the male becomes stressed, and he visibly becomes much happier if she is returned. In short, the species appears to be monogamous.

The males are no larger than the females, since any potential rivals are likely to have mates of their own, so there's no need to fight them off. But equally, they don't need large testicles or increased sperm production, because the female is likely to be faithful. Significantly, and quite unlike the aggressive common yellow-toothed cavies, fathers of this species help to raise their young, something that makes sense in a monogamous partnership where you can be sure the child is your own. This may be explain why the behaviour evolved; Muenster yellow-toothed cavies inhabit the Andes mountains, where food is scarce, so having two adults to look after the young may well be advantageous.

Cooperation, for this species, beats sexual competition.

[Pictures from Wikimedia Commons. Cladogram adapted from Dunnam & Salazar-Bravo, 2010]

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]

Why Are There Marsupials in America?

So, what exactly are marsupials?

If you answered "mammals where the female carries her young in a pouch," I'm afraid the big QI buzzer has just gone off. That's because, while its mostly true, there are, in fact, some marsupials that don't have pouches.

Along with the placental mammals, the marsupials represent one of the two main groups of mammals alive today - although there is of course, also a third, much smaller, group that includes the enigmatic platypus. The oldest known marsupial fossil, Sinodelphys, dates back to 125 million years ago, as does Eomaia, the oldest fossil on the line leading to placentals. Our best guess is that the two groups diverged not long before that, probably no more than 131 million years ago. But its worth pausing to consider how far back that is; its a little over half way through the age of dinosaurs, and long before such famous animals as Tyrannosaurus, Velociraptor, and Triceratops evolved. Indeed, its as far back from Tyrannosaurus as that animal is from us today - half the history of marsupial evolution occurred before dinosaurs went extinct.

How the horse began to run

Hyracotherium, a close relative of Arenahippus

One of the advantages of studying fossil mammals, compared with dinosaurs, is that there are many close parallels still alive today. There isn't anything remotely like a Tyrannosaurus stalking the plains of present-day Africa, but comparing sabre-tooth cats to animals such as tigers and leopards can tell us quite a lot, with rather less need for guesswork. Also, while complete mammal fossils are still quite rare, they are, nonetheless, more common than those of animals from the more distant past.

Still, when it comes to early mammals, complete skeletons are rare enough that finding one can provide a significant opportunity to learn more about them. Species are often described on the basis of their skulls alone, since skulls tend to be the most distinctive parts of the skeleton, and you can tell a lot just from that, but having the rest of the skeleton stll attached to the skull is obviously pretty useful.

A recent report in the Journal of Mammalian Evolution described a remarkably complete skeleton of the early horse Arenahippus, with the tail being almost the only part missing.

As a side note, exactly what this animal should be called is a matter of some controversy. When specimens of the species were first found, they were thought to belong to the genus Hyracotherium, which may (or may not) be an alternative name for Eohippus, the "dawn horse" that appears at the beginning of so many charts of the evolutionary history of horses you see in museums and the like. But its probably neither, so we'll stick with the name it was given in 2002, even though there are counter-arguments to that one, too.

At any rate, whatever its called, Arenahippus is one of the most primitive members of the horse family known. It lived in the early Eocene epoch, just ten million years after the extinction of the dinosaurs, when many of the modern groups of mammals were just getting started. We don't know that later horses evolved from it, because there were lots of species of early horse living alongside each other, and while one of them must have evolved into the later ones, there's no way to tell which it was - if its even we've found yet. Those step-wise evolutionary charts you see of horse evolution don't really show exactly what evolved from what, just general pictures of what horses at a particular point in time looked like.

In reality, like all evolutionary stories, that of horses is a branching tree, although its interesting to note that Arenahippus appears to branch off even before the more famous Eohippus did, putting it even closer to the origin of the horse family:

True Equines    Mesohippus     Eohippus
     ^              |             |
     |              |             |      Arenahippus
     |              |             |           |
     ----------------             |           |
            |                     |           |
         (3 toes)                 |           |
            |                     |           |
            -----------------------           |     Palaeotheres
                       |                      |          ^
                       |                      |          |
                       ------------------------          |
                                   |                     |
                            (First horses)               |
                                   |                     |
                                   -----------------------
                                              |
                                              |

In fact, even the tree above is greatly simplified - there are a great many other fossils branching off in between the steps shown above. Nonetheless, we can see that Arenahippus diverged at a point when horses still had four toes on their front feet (although, like Eohippus, they only had three on the hind feet). In terms of its size, and to some extent, its shape, it looked more like a dog than like a modern horse.

So what can this new skeleton tell us about the life of these earliest horses? Perhaps the most obvious place to look is the legs, since the one-toed foot of modern horses is one of their most distinctive features. The tops of the thigh and upper foreleg bones are clearly rounded, with flexible hip and shoulder joints. This is quite different from modern horses, where the shape of the joints means that the limbs can only move forward and back, with very little flexibility in any other direction. The authors suggest that this would have helped in an environment more cluttered with bushes and other obstacles, rather than the open grassland that favours the gallop of modern horses. Since other evidence suggests that the area of Wyoming where the fossil was found was woodland with dense undergrowth, this makes sense.

Furthermore, the shape of the bones of the hind limb show the presence of powerful muscles, especially the calf muscle. Taking into account the shape of the knee and ankle joints, this indicates that the hind limbs would have been bent as the animal pushed itself forward and began to run - something you see in dogs, but not in horses, whose hind limbs are fairly stiff.

However, its not just the shape of the limbs that show us how the animal would have moved, but also the backbone. Reconstructions of early horses tend to show a straight backbone, as can be seen in the photograph at the top. This is how the backbone of modern horses look, and the way that the individual vertebrae lock together makes the whole structure quite rigid, a pattern also seen in other fast-running hooved animals, such as antelopes. But there haven't been many good fossils with intact backbones before, and, looking at this one, it seems the pattern isn't quite so simple.

Back flexed, legs pushing towards the midline

The vertebrae at the far end of the back, just before it joins the pelvis, were, indeed, rigid, with processes that would have locked them tightly together. But just before this was a more flexible region where the bones would have prevented the back from twisting, or from bending upwards, but would not have prevented it from bending downwards. Thus, unlike modern horses, Arenahippus could have arched its back, and most likely did so just as it began to run.

All in all, Arenahippus seems to have been a more flexible animal than a modern horse, or even than its more horse-like later relatives, such as Mesohippus. That may be partly because the later animals were bigger, and a more stable body would have made them more energy efficient while running. Arenahippus's movable knees, strong calf muscles, and flexible hips would have enabled it to push off the ground with some force, while the arching back ensured its centre of mass stayed in line. Still, it does seem to have been more rigid than, say, a modern dog and was, perhaps, just beginning on a path that would lead its later relatives (if not, necessarily, its literal descendants) to their fast-running lifestyle.

That leaves aside the question of why later horses became larger at all, requiring the change to the more familiar shape and posture we see today. That may be due to the changing climate of the time, and the spread of grasslands. Arenahippus, like other very early horses, mainly ate herbs, and perhaps fruit, browsing on low-lying vegetation, while later horses grazed on grasses. Grass is harder to digest than herbs, so that a longer digestive tract is needed if you're going to eat it. One way to increase the length of the digestive system is to increase the size of the animal its inside, and its at least possible that this was a major reason for the change. The more open environment of grasslands may also have meant that the longer stride that the size and body shape of later horses promotes would have been more useful for them than for something living among dense undergrowth.

[Pictures from Wikimedia Commons, cladogram adapted from Mikko's Phylogeny Archive]