Sunday, 29 May 2011

The Fate of the Bison

Before Europeans arrived, vast herds of bison inhabited the Great Plains of America, ranging from Manitoba to Mexico and Pennsylvania. The story of their decline due to massive overhunting is a dramatic one that seems to be well-known in America, an object lesson in mankind's ability to damage nature. Today, around 30,000 American bison (Bison bison) survive in the wild, and those raised in captivity are often hybrids with domestic cattle, rather than genetically pure survivors of the original species. American bison are officially considered "Near Threatened", and it is only the ongoing conservation efforts that prevent them from being a truly threatened species.

If you think that's bad, the story of what Europeans did to the bison of their own continent is even more depressing.

The European bison (Bison bonasus) looks very similar to its American cousin, but is, nonetheless, a distinct species. It's somewhat slimmer than the American species (which admittedly isn't saying much - they're both very muscular), and the horns are a different shape, but that's pretty much it. To avoid confusion between the two, the European bison is sometimes called by its German name of "wisent", but I'll stick with the more common English name.

While we're on the subject of names, though, it's worthwhile mentioning the difference between bison and buffalo. In America, the two words mean the same thing; they're both referring to the American bison. In Europe, however, the term "buffalo" is reserved for a number of African and Asian species, such as the water buffalo, which are clearly cattle, but look quite different from bison.

European   American     Domestic     Yak
 Bison      Bison        Cattle
   |          |            |          |
   |          |          Auroch       |        
   |          |            |          |     Gaur, etc.
   ------------            ------------         ^
        |                        |              |
        |                        |              |       Buffalo,
        --------------------------              |         etc.
                    |                           |          ^
                    |                           |          |
                    -----------------------------          |
                                   |                       |
                                   |                       |
                                   -------------------------
                                               |
                                         (First cattle)
                                               |


One other species of "true" cattle (that is, as opposed to other members of the cattle family, such as sheep, goats, and antelope) probably also exists. This is the saola (Pseudoryx nghetinhensis), an animal so rare that it was only discovered, hiding in the jungles of Vietnam, in 1993. It doesn't look very cow-like, and so probably branched off from its relatives even before the buffalo, but the first genetic analysis was only conducted last year, so I have left it off the diagram. The auroch, ancestor of modern domestic cattle is, of course, extinct as a wild animal.

Once, herds of European bison ranged across most of central Europe, much as their American counterparts did across the Atlantic. They did not sweep across wide grassy plains, because Europe doesn't have many of those, and instead preferred woodland environments, much as the American bison in Canada do today. There were three subspecies, one on the south-eastern edges of Europe in the Caucasus mountains, one further west in the Carpathian mountains, and the other more widespread across the lowlands.

While the American bison did not begin to die out until the nineteenth century, the European species began to suffer much earlier, partly due to hunting, and partly due to the loss of their woodland habitat to encroaching agriculture. As early as the Middle Ages, only scattered populations remained, and the situation only got worse as human population soared. By the nineteenth century, the Carpathian subspecies was extinct, and only two populations of European bison survived: one in the Caucasus, and the other in a carefully managed royal hunting reserve in the Białowieża Forest in Poland.

In 1914, World War One began to sweep across Europe, and by 1919, the last lowland bison in Poland had been eaten by the German army. The Caucasian subspecies struggled on only for a few more years before, it, too, went extinct in 1927. Except... the lowland bison, at least, were not quite extinct. There were still animals in zoos - only a tiny number, to be sure, but that might just be enough to save them.

Of these captive animals, five bulls and seven cows successfully bred, and all European bison alive today are descendants of these twelve animals. In 1952, a group of forty purebred lowland bison were released into the wild, back in the Białowieża Forest. The herd prospered, and today, there are nearly 2,000 lowland bison in existence. Over half of them still live in the same forest, with the remainder scattered across Poland, Belarus, and Lithuania.

These animals are, however, remarkably inbred. They are descended from just seven of the founding twelve animals, and the male lines of all but one of the bulls have since died out - meaning that all living bulls have an essentially identical Y-chromosome. In domestic cattle, its considered unwise for animals to be more than 12.5% inbred, or they will begin to lose fertility, die young, and generally perform poorly. In the lowland bison, inbreeding has been estimated to approach 50%, although there don't seem to be any major problems. There are a number of possible reasons for this, but it could just be that we haven't noticed yet, what with most of the animals being out in the wild. On the other hand, they have suffered from a recent outbreak of balanitis, although this doesn't appear to be connected with inbreeding.

But what of the Caucasian bison? It turns out that one bull of the founding five, appropriately named "Kaukasus" by his keepers, belonged to this subspecies, not the lowland one. He died in 1925, just two years before the last living member of his subspecies out in the wild, but he had managed to sire a number of calves. As the only animal of his kind in captivity, the only cows he could mate with were lowland bison, and so all of his calves were hybrids, and the Caucasian subspecies is therefore officially extinct. But his calves did survive, and they went on to breed with other lowland bison.

Because of this, the hybrid descendants of this solitary Caucasian bison are also descended (through the female line) from all eleven of the other breeding bison alive in the 1920s, and are much more genetically diverse than their purebred kin. Indeed, the hybrids are about 94% lowland, with only 6% of their genes coming from Kaukasus himself. There are estimated to be slightly more of these hybrids alive today than there are purebred lowland bison, although about half of them live in captive breeding centres. Nonetheless, some have been released into the wild in Poland, Slovakia, Russia, and Ukraine. They have even recolonised the Carpathians, former home of the long lost third subspecies.

Inbreeding among these hybrids is estimated at around 26%, just marginally worse than the 25% you would get if a brother and sister had children, but not as bad as in the purebred lowland bison. Even so, they seem to be worse off, although that may just be because so many of them are in breeding centres where such things are more obvious to researchers. For instance, the hybrids seem to suffer from abnormalities of the skull, and an increased chance of dying within a month of birth.

Because the total population of European bison is increasing - albeit from an incredibly low starting point - as of 2008, it is no longer considered an Endangered species. It is however, classified as Vulnerable, which is only one grade higher, and makes its position worse than that of the American bison. As you might expect from a species down to just twelve animals less than a hundred years ago, it is extremely inbred, and its continued survival is by no means assured. The effort that has been put into saving the European bison over the last century has reaped great dividends, but the task is far from over.

[Picture from Wikimedia Commons. Cladogram adapted from MacEachern et al. 2009]

Sunday, 22 May 2011

Cold-blooded Mammals

Short-eared elephant shrew, another member of the family
Mammals are warm-blooded animals. That is, they maintain a consistent body temperature regardless of, and usually higher than, that of their surroundings. It is, after all, one of the defining attributes of being a mammal. Yet, while virtually all healthy mammals have no real difficulty in keeping their body temperature high, a great many of them don't always bother.

Probably the best known example of this is hibernation. Many animals living in climates with cold winters enter a long "sleep" when their food becomes scarce, and don't wake up until the following spring. But this isn't sleep of the sort that humans experience daily, but something much more dramatic. Their whole body shuts down to a minimal level of activity, and body temperature drops to only a fraction above that of their surroundings - essentially, they become cold-blooded. Of course, unlike reptiles and amphibians, they aren't doing anything much while they are in this state, so we shouldn't draw too many parallels, but they can't be truly said to be warm-blooded at these times, either.

Sunday, 15 May 2011

When Sloths Roamed the Grasslands

The Altiplano - where ground sloths may once have grazed
I've mentioned before that the living tree sloths are the last remnant of a much larger group of ground-living animals. These animals originated in South America during the long time that that continent was an island, connected only to Antarctica. Eventually, the land mass collided with North America, bringing with at an array of more modern mammals, such as foxes, deer, and cats, as well as a few that have died out elsewhere since, like tapirs and llama.

The ground sloths actually did reasonably well after the other mammals arrived, and a very wide array of fossil species are known, the last of them having died out as recently as 9000 BC. Indeed, they crossed Central America heading in the other direction, leaving fossils in the northern continent, and at least one species, Megalonyx, even crossed the Arctic Circle to live in the cold wastelands of northern Canada. Many inhabited forests, but others preferred grasslands, or even semi-desert, while Thalassocnus may have been semi-aquatic. There was also a huge range of different sizes, from some that were barely larger than modern tree sloths, up to the multi-ton Megatherium.

Most of what we know about the ground sloths comes from the last 23 million years or so. This takes us back to long before the joining of North and South America, although the first monkeys had reached the continent (presumably by island hopping) around 25 million years ago, and rodents earlier still. Yet the sloths were there essentially from the beginning, along with opossums and some rather strange herbivores. The oldest known fossil that might be from a sloth comes from, of all places, Seymour Island in Antarctica, although it is only one tooth, and we really don't get good fossils until much later.

A new fossil species recently described from Bolivia may add more to the story of early ground sloths. Authors Bruce Shockey and Federico Anaya estimate their fossil to be around 26 million years old, putting it firmly in the late Oligocene epoch. It consists of much of the skull, and possibly some parts of the tail and right foreleg. It belongs to the same family as Mylodon and its kin, and has been given the rather cumbersome name of Paroctodontotherium calleorum.

One of the first questions that's often asked about fossil animals is "how big was it?" As you might imagine, when you only have a skull to go on, that's not too easy, and even with a reasonably complete skeleton it isn't that straightforward. Nor it is unusual for a fossil species to be identified from no more than a skull - fossils are notoriously incomplete, and the skull is usually the most distinctive bit of the skeleton. In these cases, the best we can do is make some guesses based on the size of the bits we do have and the probable shape of the animal.

In this case, the authors used three different measurements to estimate the size that the whole sloth would have had when alive - and got three completely different answers. Based on the length of the row of teeth in the upper jaw, the animal would have weighed about 15 kg (33 lb), about the size of a Staffordshire bull terrier. Based on the total length of the skull, it would have been more like 47 kg (100 lb), similar to a Labrador retriever. Finally, using the width of the muzzle as the baseline, the usual calculations make it 354 kg (780 lb), at which point I run out of dogs, and have to refer you to a male grizzly bear.

Even allowing for a degree of uncertainty, and ignoring the difference in shape, its difficult to see how a Staffie could ever be mistaken for an adult grizzly, so what's going on? The animal obviously has an extremely broad muzzle, making any estimate based on that alone essentially useless, and leaving us with the two more reasonable figures. Its more likely that the 47 kg figure is closest to reality, but even that assumes that the sloth did not have a particularly long or short head relative to the rest of its body, so its really no more than an educated guess. The reason that the lower figure is probably wrong is that the equations used to work it out assume a relatively normal set of mammalian teeth, and sloths don't have normal teeth - in fact, they have far less than usual, meaning that the figure is probably an underestimate.

Aside from misleading us as to the size of the whole animal, that broad muzzle may, in fact, be very significant. Living tree sloths, especially the three toed ones, have fairly rounded heads, with short snouts, but the extinct ground sloths generally didn't. Indeed, only one other ground sloth - Lestodon, which lived much later, during the Ice Ages - had a broader snout than this new species. When we look at living herbivorous animals, we find that long, narrow snouts are associated with animals that carefully snip out high quality buds, fruits, and so on from the plants they are feeding on. Herbivores with wide snouts are generally grazing animals that shovel in large quantities of plant material without much in the way of precision. This makes sense if you're feeding on something like grass, because its not all that nutritious, and so you want to eat a lot in one go.

This suggests that the new sloth probably fed on grass or sedges, and this fits with the environment at the time, which was most likely a mix of pampas and altiplano, as well as with the known diet of the closely related species Mylodon. But there is a problem here. In addition to the shape of their muzzles, there is another difference between careful browsers and grass-eating grazers: the latter have very high teeth. They have to, because grass is relatively tough compared with most other green plant material, and because, if you're chomping stuff directly off the ground, you're probably going to end up with some soil and grit in your mouth from time to time. And these things wear your teeth down - if they don't start off high, they'll soon become useless, and you die.

This animal, like other sloths, and unlike most other fossil herbivores from the same neighbourhood, does not have particularly high teeth. Yet, here again, sloths are different from other big herbivores. Their teeth grow continuously throughout life, so, as long as that happens quickly enough, they will never wear down, simply replacing the worn enamel with new material from underneath.

Grasslands first spread across South America not long before this sloth and its relatives first walked. The climate had cooled, as Antarctica split away from the south, allowing the creation of a cold circumpolar current around the newly ice-bound continent, in addition to more worldwide effects. The fact that this new sloth does not have particularly primitive features, and is well adapted to a grassland habitat, suggests that its ancestors had been evolving for some time, probably longer than had previously been thought. Furthermore, we know of at least seven other species of ground sloth that lived at the time, and the fact that this one is (so far as we can tell) much smaller than any of the others suggests that they were already quite varied.

Indeed, it could even be that the change in the climate, and the spread of the grasslands was what first caused the sloths to diversify, and begin evolving into the even greater range of forms that they later took on.

[Picture from Wikimedia Commons]

Sunday, 8 May 2011

The Not-Naked Mole Rats

Silvery mole rat
I recently discussed moles, and their adaptations to their unusual, underground, lifestyle. But, of course, they are not the only group of mammals to adapt to such a life. A great many rodents are burrowing animals, spending a significant proportion of their time below ground, often in a bid to escape predators and to have a safe place to store food for later use. However, the majority travel above ground to forage, and are not truly subterranean in the way that moles are. Not so the mole rats.

The most famous mole-rat is surely the naked mole-rat, a bizarre, hairless species known for reproducing in the manner of bees or ants - with a single reproductive queen supported by numerous infertile workers. But this is not the only mole-rat, and the other, furrier, forms are also highly adapted to underground life. Indeed, the naked species is regarded as exceptionally odd, even by the standards of a group that's fairly strange to begin with, and seems to have branched away from its kin at an early stage in their evolution.

Common Mole     Damaraland      Dune & Cape   
 Rat, etc.     Mole Rat, etc.    Mole Rats 
     ^               ^               ^     
     |               |               |       Silvery
     |               |               |       Mole Rat
     -----------------               |           |
             |                       |           |
             |                       |           |       Naked
             -------------------------           |      Mole Rat
                          |                      |         |
                          |                      |         |
                          ------------------------         |
                                      |                    |
                                      |                    |
                                      ----------------------
                                                 |
                                    (original African mole rats)
                                                 |

Until recently, there were believed to be around sixteen species in the African mole rat family, but recent genetic studies have shown that there are considerably more than this, with the "common mole rat" actually turning out to be several different species, some of which aren't even particularly closely related. Such are the problems of studying animals that are almost always down below the ground where you can't see them.

To complicate matters further, a second group of subterranean rodents are also referred to as "mole rats", and have at least as many species. To keep things simple, I'll just discuss the species related to the naked mole rat (technically called the "bathyergids") for the moment. I'm going to call them "African mole rats", because that seems to be the most common name, even though its a bit misleading, since some of the others also live in that continent. Animals just want to make it difficult for us sometimes, don't they?

Anyway, African mole rats are only ever found south of the Sahara, although they may have ranged further in prehistoric times. Most are found in southern Africa, with just a few species further north, in places like Nigeria and Ghana. Again, the naked mole rat is unusual, living across in Somalia, far to the north and east of any of its relatives, and it may be this isolation that allowed it to evolve into such an odd animal.

African mole rats have many adaptations reminiscent of those of the moles, including a relatively cylindrical body that fits well into narrow burrows, velvety fur that won't clog with dirt, and almost invisible ears. But there are some significant differences. Where moles burrow with their spade-like paws, mole rats instead use their teeth. As with all rodents, mole rats have a single pair of large incisors in each jaw, which grow continuously throughout their life, allowing them to constantly gnaw on tough materials without fear their teeth will ever wear down.

The incisor teeth of mole rats are particularly large, and appear even more so because the animal's lips are actually positioned behind them. Because the lips are behind the teeth, instead of where you'd expect them, mole rats can keep their mouth closed even as they use their teeth to gnaw away at the soil in front of them, digging out a tunnel as they go. They then scoop up the loose earth with their front paws, push it underneath their body, and kick it away with their hind feet. The need to keep the tunnels clear means that, eventually, they have to push the excavated earth up to the surface, where they leave mounds very similar to mole hills, that may be the only visible sign of their presence.

For the most part, they prefer relatively solid ground to burrow in, which stops the tunnels collapsing, and probably also makes them more difficult for predators to dig out. The two species of dune mole rat are slightly different, however, and live in soft, sandy soils. These are much easier to dig through, and, unlike the other species, they mainly use their front paws to do so, instead of their teeth, which are smaller than those of their relatives.

The eyes of mole rats are very small, and it had been assumed for a long time that they were completely blind. However, over the last decade or so, it has become clear the structure of their eyes isn't as odd as you might expect if this were true. They have an iris and pupil, a lens that is small, but nonetheless fairly normal, and crucially, the retina seems to have all the light-sensitive cells you'd expect to find in an animal that can see. There are far less nerve cells connected to the retina than in other mammals, but they do exist, which suggested that mole rats probably are able to see, albeit very poorly. When scientists looked at the visual centres of the brain, they found a similar story; the brain structures that would be expected to respond to the presence of light are all there, even though the parts involved with actually seeing objects are highly degenerate.

Of course, examining the anatomy is all very well - can they actually see, and if so, why? The first part of the question was answered in 2005 for the Zambian mole rat (Fukomys anselli). The animals were kept in an artificial burrow, with a choice of sleeping chambers, some of which had light, and some of which were dark. They chose the dark chambers, showing both that they could tell the difference, and that they considered light unpleasant - as well they might, living underground all the time.

More recently, studies have been conducted on the giant mole rat (Fukomys mechowii) and the silvery mole rat (Heliophobius argenteocinereus). Again, both species avoided light, although they didn't seem to mind red light, and presumably couldn't see it. However, while they clearly don't like light, they don't always run away from it; by poking holes in the artificial burrow and shining light through it, the researchers were able to persuade the mole rats to rush towards the breach and plug it with earth. In the wild, there would presumably be many other clues that a tunnel had a hole - the sound of a predator digging its way in, or just a change in air currents or the like - but it does seem that light by itself is enough to be disturbing to the animals.

The dune, cape, and silvery mole rats are all solitary animals, leaving their home burrow when they reach adulthood. It seems that they simply burrow away and block up the tunnel behind them, although they do venture above ground to find mates, a potentially very dangerous time for them. All the other species of mole rat are at least partially social, living in communal burrows. It is well known that the naked mole rat takes this to a remarkable extreme, but it is not alone since the Damaraland mole rat (Fukomys damarensis) of Botswana and Namibia has a similar lifestyle, albeit much less studied. It is generally thought that the other species of mole rat are less extreme, living socially, but not having a single reproductive queen, although even they tend to be dominated by a small number of long-lived individuals that limit breeding by their relatives to some extent. And, of course, its always possible that the lack of detailed research means that there are a few extreme social breeders that we've missed.

Given that they aren't very closely related within the African mole rat family, its perhaps unsurprising that this social system works slightly differently between the naked and Damaraland mole rats. The former are the more extreme, with a distinct 'worker' caste of infertile individuals incapable of breeding. Damaraland mole rats, however, live in smaller colonies consisting of a queen, a small number of reproductive males, and a number of offspring. Unlike their naked kin, Damaraland mole rats will not mate with their own parents or siblings, so it may not be that their workers are truly sterile, just that they don't have anyone to mate with. Nonetheless, there is at least some evidence that the queen is able to control access to visiting males from outside the colony as well, so it may not be quite so simple.

The question of quite why they evolved to live like this is an interesting one, especially since some of their relatives didn't. There are a number of possible explanations, but it may be worth noting that the two most social species live in the driest parts of Africa south of the Sahara, while other mole rats live in generally more hospitable savannah or veldt habitats. Unlike moles, mole rats are vegetarian, eating roots, tubers, and even above-ground plants that they can physically pull down into their burrows by their roots. In fact, while they do, of course, use their burrows for shelter and breeding, most of the tunnels are only dug in the ever-present search for food.

In a dry environment, food is most often available for only a short time after the rains, and the mole rats have to dig quickly in order to find suitable food, and store it where possible. That the soil is also likely to be hardest to dig in these environments makes it especially difficult, and it may be that a large colony working cooperatively makes this all much easier to do.

For the most part, the mole rats are not endangered by human activity, and they have few natural predators. A few snakes, most notably the mole snake of southern Africa, are able to get into their burrows, but in general, their cosy, relatively cool, and difficult-to-find underground burrows keep them safe. Although no species are officially listed as endangered, one, the Kafue mole rat (Fukomys kafuensis) of Zambia, is considered vulnerable, due to its very restricted range, and because locals consider it something of an agricultural pest.

[Picture from Wikimedia Commons. Cladogram adapted from Van Daele et al. 2007]

Sunday, 1 May 2011

Surviving on the Edge

The spread of humanity across the globe has been bad news for a number of animal species. Species have always gone extinct, as the climate or other circumstances change, but the pace of extinction, and the number of species that are currently teetering on the brink has greatly increased over the last few thousand years. In many cases, humans are the direct cause of the problem: we kill animals for food, for sport, for make-believe medicines, to protect our livestock or crops, and so on.

But just as often the effects are indirect, as natural habitats are broken up into smaller isolated pockets or destroyed altogether. A few species have adapted well to human environments, and are rarely, if ever, found far from our settlements - the house mouse and the pigeon are both examples, before we even begin to look at pets or livestock. But most have not fared so well, and the question of how our destruction of the habitat affects animals is obviously an important one in conservation.