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.

Steppe    Macedonian    Algerian     House
Mouse       Mouse        Mouse       Mouse
   |          |           |           |
   |          |           |           |         Asian
   ------------           |           |         Mice
         |                |           |           ^
         |                |           |           |
         ------------------           |           |
                 |                    |           |
                 |                    |           |
                 ----------------------           |
                           |                      |
                           |                      |

As a close relative, it also looks very similar to the house mouse, and the two are very hard to tell apart. So similar in fact, that this created a problem with the scientific name for the animal. The steppe mouse was first formally described by Alexander von Nordmann in 1840, under the name of Mus hortulanus. One of the key things you have to do when describing a new species is find a specimen of your animal, and use that as the holotype - the individual against which all other members of the species will be measured. When somebody else finds a new specimen, in theory what they do is check to make sure it belongs to the same species as the holotype specimen, and, if it is, they know what they've found. Nordmann already knew what a steppe mouse was, so he went out and found one to use as his holotype, wrote it up, and gave it its scientific name.

Only, unfortunately for him, it later turned out that he'd picked up a house mouse by mistake. His scientific name was associated with a specimen of the wrong animal, and had to be stricken from the record. A newer description, from 1882, was used instead, and that author's choice of name, Mus spicilegus, became the official one.

The steppe mouse is also called the "mound-building mouse", and that's because of an unusual habit that is quite different from those of its closest relatives. In the autumn, several mice gather together to collect seeds and other plant materials, and deposit them in a large mound that they then cover with soil. The mounds can be large - a foot or so high, and four or five feet across. Deep in the soil underneath the mound, the mice dig burrows centred around one or two nests, and there they spend the winter.

The burrows are shared between a group of mice. They aren't necessarily siblings, but more of an extended family, with related mothers, but unrelated fathers - effectively cousins related through their aunts. How many mice share each mound is not entirely clear, and may well vary from place to place. Estimates have proved difficult, because whenever researchers try to dig up the mounds to count the mice underneath, the mice tend to run away (who'd have thought?) A combination of having a few exit tunnels some distance away from the mound itself, and the habit of dashing into the tunnels of other nearby mounds while researchers are still chasing them does not help matters. But somewhere between six and twelve seems about typical.

Its natural to assume that the mounds are food caches to allow the mice to survive through the winter, so that they can emerge refreshed in the spring, and start breeding again. But just because they are giant larders doesn't necessarily mean they can't have any other function. Indeed, evolution often works like this, when a structure or habit evolved for one purpose turns out to be useful for something else as well. In this case, for example, it has been suggested that the mounds may also help protect the mice, whether from bad weather or from potential predators, or both.

Péter Szenczi and colleagues of Eötvös University in Budapest wanted to know how effective the mounds were as a shelter from the weather (since, after all, the mice are already digging burrows underground), and how useful they are as a winter food source. They looked at a large number of mounds in two different areas in Hungary, measuring them, analysing what they were made of, and checking to see how good they were at keeping the mice alive through the winter.

Their first finding is, perhaps, something of a surprise. The mounds are made of seeds and other parts from a number of different plants, depending on what was available nearby, with the main components (other than soil) being either barnyard grass or goosefoot, and the remainder being various other, mostly grassy, plants. But when they looked at the animals' dung pellets, right through the winter, that wasn't what they'd been eating. Instead, they preferred amaranth, ragweed, and cocksfoot.

The "obvious" explanation for the mounds, one you'll frequently see online or in books whenever the steppe mouse is discussed, apparently isn't true. The mice are not eating the stored seeds. That their burrows don't extend into the mounds, but remain quite separate in the soil underneath, one or two feet underground, also supports this idea. That's not to say that, if the winter got really bad, they wouldn't eat the seeds in the mounds at all - it would presumably be better than starving - but it doesn't seem to be something they normally do.

So, if the mounds aren't food caches, as most people have generally assumed, what are they for? Why bother to store a huge pile of food and then not eat any of it? Temperature probes and soil analysis provided what seems to be the answer: the soil below the mounds is warmer than that around them, even allowing for the height of the mounds themselves. Moreover, it's also drier, which helps to keep it more comfortable. By digging their burrows under a pile of insulating vegetation that soaks up rain and snow, the mice can huddle in cosy nests, protected from the worst of the winter weather.

It seems to work, too, because when the researchers counted the numbers of mice they found under mounds at the beginning and end of the winter, they came to about the same value. In other words, pretty much all the mice that went into shelter in the autumn were still alive in the spring. Whether the mounds also help protect against predators that might want to dig the mice up is less clear, and we can't really tell how much difference they make without disrupting them to see what happens, but that they go to the trouble of collecting around 140,000 cm3 (five cubic feet) of soil and plant matter would suggest its probably significant.

So, yes, rodents often do cache food to bide them through the winter. But that isn't necessarily their only reason for collecting plants. Because sometimes, the "obvious" answer isn't the right one.

[Image from the Carpathian Basin Digital Collection. Cladogram adapted from DeBry and Sashadri, 2001]

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?

Among Old World monkeys - the type of monkeys to which we ourselves are most closely related - the females are often sexually promiscuous, mating with a number of different males. Although, in general, it tends to be males that sleep around, so that they can have as many children as possible, that's not relevant to females, who can have only one child (or litter) at a time. There are, however, some possible benefits for female promiscuity, which I have alluded to before, when discussing the common yellow-toothed cavy.

By mating with lots of males, you can increase the chance that the fittest one becomes the father of your children, whether that fitness is indicated by physical bulk or, as in the case of those cavies, sexual prowess. Even so, while they may want to give others a chance, just in case, female Old World monkeys normally have some way of indicating when they are most likely to become pregnant, allowing the largest male to drive off rivals around that time. For example, they may make specific calls, or have visible genital swellings during their peak of fertility.

The Assamese macaque (Macaca assamensis) is one of the exceptions - others include the Tibetan macaque, shown in the picture above. The macaques are a relatively large group of monkeys, found across southern and eastern Asia, and they include such well known species as the rhesus monkey. Perhaps more surprisingly, they also include the Barbary "ape" of north Africa and Gibraltar, although the Assamese macaque of south-east Asia is much more closely related to more typical species in southern India and China.

  Assamese         Rhesus       Lion-tailed
Macaque, etc.   Monkey, etc.   Macaque, etc.
     ^               ^               ^
     |               |               |        Barbary
     |               |               |        Macaque
     -----------------               |           |
             |                       |           |      Baboons,
             |                       |           |        etc.
             -------------------------           |         ^
                         |                       |         |
                         |                       |         |
                         -------------------------         |
                                     |                     |
                                 (Macaques)                |
                                     |                     |

Over the last fifteen to twenty years, there has been some debate over the relationships among the twenty-two species of macaque. However, there does seem to be broad agreement that the Barbary macaque is out on its own (as might be expected, given that it's the only one not found in Asia), and that all the others fall into at least three general groups. I have used here the most up-to-date classification I could find.

The breeding season for Assamese macaques lasts a full four months, from mid October to mid February, resulting in the birth of a single infant after a pregnancy of around 164 days. Although the females do show some degree of genital swelling during the breeding season, this is not noticeably different when they are actually fertile than at any other time during the four month period. Since they don't seem to behave any differently during these times, there does not seem to be any obvious way for the males to tell when the females are fertile.

Of course, just because its not obvious to us, doesn't mean that it isn't obvious to the monkeys concerned. Perhaps, for example, they smell different, or there's something subtle in the way that they act which we humans have missed. Ines Fürtbauer and colleagues, of Göttingen University, recently published a study that took a closer look at the sex lives of female Assamese macaques to find out what they're really doing, and why.

They did this by watching the sexual activity of wild Assamese macaques in Thailand throughout their breeding season, and analysing their dung to find out when the females were fertile - a completely non-invasive way of assessing their hormonal status. Perhaps the first thing that was obvious was that the monkeys were mating quite a lot - pretty much every day, even assuming they never managed to hide from the researchers. Nor did the females much care who they mated with; on average each one mated with at least 80% of the available males at one time or another.

What's more, on a whopping 94% of occasions, the females mated when they were not fertile. Indeed, they carried on mating for at least two months after they became pregnant. Now, like other macaques, these monkeys have a strong social hierarchy, with a few powerful alpha males and a number of younger, smaller, subordinate males. Sometimes these alpha males would monopolise particular females for weeks at a time, protecting them as male mammals often do when they want to ensure any resulting child is going to be their own. The thing is, they were just as likely to do this whether or not the female was fertile, and whether or not they had even the slightest chance of getting her pregnant.

Which would suggest that they really didn't know.

So, if the females aren't giving their partners any clues as to when they're fertile, why not? The answer probably lies in an ugly fact of life as a monkey: infanticide. Dominant males, in particular, are quite likely to kill the offspring of other males to ensure that their own are the ones that survive to adulthood. But you can't do that if you don't know which ones they are. If you've mated with all of the females, and can't tell which ones you got pregnant, any child might be your own.

Infanticide is obviously bad news for the infant, but its also bad news for the mother, who invests a lot of time and energy in raising her young. So it may be in her interest to keep the males in the dark, and sexual promiscuity is one way to do that. That's not to say that there aren't advantages to monogamy too, or to mating only with the fittest and strongest males, but, in this species, that doesn't seem to be the way that the balance has swung. This would be an advantage if, for example, alpha males don't last very long so that they won't be much good at protecting you in the future.

However, while they would mate with just about any male, the females did seem to have preferred partners. That is, 30-40% of their matings would be with just one individual, even if they weren't exactly faithful to them. That these partners were as likely to be subordinate as alpha males would suggest that it's probably the females doing the choosing, although there's no way to know for sure. Again, if the alpha males don't last long, today's subordinate will be tomorrow's alpha, so you aren't necessarily losing much.

Indeed, there may well be some advantage to having a particular "boyfriend", even if you don't stay faithful. Humans, after all, tend to have a preferred partner, and to mate with them even when they won't get pregnant. Aside from the obvious, in evolutionary terms, this creates a social bond that comes in useful when you need to raise your children, and it may be that the Assamese macaques are doing something similar. There is some evidence that males help the females to raise their young, and a strong social bond, engendered at least partly through regular sex, may make them more likely to do that.

Besides, while you might not want to risk killing off the young of other females you've mated with, that of your own partner has at least a fair chance of being your own. If you're going to put any effort into being a father at all, you might as well do it for that one.

When we see rutting stags or powerful bull seals, it's easy to think that, at least among non-human mammals, male sexuality is all that really matters. But, for a number of species, females sexuality is at least as important, and they can use it to get exactly what they want.

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

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.

This is one reason why its often more useful to trace the past movements of an animal population by examining its mitochondrial DNA (which is inherited from the mother) than by doing the same for the Y chromosome (which is inherited from the father). Males wander all over the place, and the picture would get blurred and confused quite quickly. But what of animals where both males and females may either stay close to their parents, or choose to leave - how do they make the decision?

The red fox (Vulpes vulpes) is an animal familiar to many people across the northern hemisphere. It's often the case that very similar animals from North America and Europe will be referred to by the same name, although they are actually separate species - this is true, for example, of otters, bison, and badgers. The red fox is an exception; it really is the same species on both continents. Fossil records show that they first appeared in Asia, and it seems that they crossed the Bering land bridge to reach America when sea levels were lower during the last Ice Age.

In fact, the term "fox" is a fairly vague one, biologically speaking. It's generally used for any small member of the dog family that, well, looks like a fox. Most of these animals are, indeed, genuinely related to the red fox, and form a natural group within the canines, but some of them - specifically the "foxes" of South America - are actually closer to wolves. The red fox, however, is definitely a fox. Indeed, as the type species of its genus and tribe, it is, in a sense, the very definition of what it means to be fox-like.

Red fox,    Kit fox,     
  etc.        etc.        Grey fox,
   ^           ^            etc.
   |           |             ^ 
   |           |             |      
   -------------             |       Bat-eared fox    
         |                   |             |
         |                   |             |
         ---------------------             |         "Wolf-like"
                  |                        |            Dogs
                  |                        |              |
                  --------------------------              |
                              |                           |
                      (first "true" foxes)                |
                              |                           |
                                     (first canines)

The red fox is an adaptable animal, that will live anywhere north of the tropics, other than the coldest of wastelands or the driest of deserts. In particular, it has successfully colonised urban environments, and many foxes live on the fringes of human society. Even in Britain, which lacks such things as raccoons, I find that foxes go through my rubbish bags, and I can often hear them at night, or see them on the streets in the early morning, even though I live in quite a large town.

For the last thirty years, there has been an ongoing study of urban foxes in the English city of Bristol. Helen Whiteside and colleagues of the University of Bristol, used that data to examine how foxes in the city are related to each other, and what they do when they reach adulthood. Red foxes are reasonably sociable animals, and form small packs with a dominant breeding male and female, and a number of younger individuals. The younger foxes do get a chance to breed, but they have less opportunity than the dominant pair, so the question arises of why they bother doing it.

Why, in other words, don't they all just leave - or at least all the males - as soon as they get the chance? What convinces at least some of them to stay at home? At least part of the answer seems to be that they help to look after the young of close relatives within the pack, giving the pack as a whole a better chance at survival. But that really only makes sense if you know that the cubs really are your close relatives, and has to be offset against the fact that you want to find a partner - ideally one that isn't a close relative - that can give you cubs of your own.

So it makes sense that whether a young fox will stay with its parents or strike out on its own will depend on the role its parents have within the pack. Surprisingly, though, it turns out that it doesn't really matter who the father is. That may be because of the urban environment, where there's more than enough food lying about in rubbish bags and the like for access to that not be an issue. And if younger males within a pack slink off every now and then to find a mate from a neighbouring group, the dominant male in their native group probably doesn't care much whether they're around or not; either way, they aren't competing with him.

But it turns out that who your mother is does make a difference: male cubs born to dominant mothers within the pack are much more likely to leave. That's probably because, if they stay, since the dominant female does most of the mating, if they stay around, they'll end up mating with her, which clearly isn't a good idea. But, if the young male's mother is subordinate, she mates less often, and that's less of a problem - the male might as well stay where he is rather than take the risk of finding somewhere new.

But, for young females, exactly the opposite is true. They stay if their mother is dominant. That means that most future cubs will be their close relatives, giving them a reason to help them out. They have little reason to compete with their own mother, and can, in fact, wait until she dies and become the dominant female themselves. If the mother of a young female is herself subordinate though, the young fox leaves. Whether she does that voluntarily, or she whether she gets chased away is unclear, and it may be a bit of both, but, either way, the dominant female has little to gain from having too many unrelated potential rivals hanging about. Her own daughters are one thing, but anyone else is a possible risk.

All of this sounds quite reasonable, and its perhaps surprising that nobody has really shown it to be true before. The fact that an animal's choice to leave home is influenced by its mother, but not its father is, perhaps not so obvious, and shows why its worth checking this sort of thing from time to time. That males and females pick opposite strategies may also show us how this sort of group living got started and that, as so often, it's your mother that's really important.

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

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.]