Wednesday, 23 January 2013

News in Brief #8

Attack of the Killer Walrus (Or Not)

The two principal families of fully aquatic carnivoran are the seals and the sea lions. To the casual observer, they can most easily be distinguished by the shape of their hind limbs. Sea lions can walk on all fours (albeit with a comical waddle), while true seals have to drag themselves about on land, their hind flippers being of little use out of the water. It's this that tells us that fur seals, for example, are members of the sea lion family, and not 'true' seals. But seals and sea lions are not the only aquatic carnivorans: there are two others. Firstly, there's the sea otter, a member of the weasel family. And then there is the walrus.

Walruses are kind of weird, when you think about it. Although they're probably more closely related to sea lions than they are to true seals, they're actually neither, and the walrus family is one of those odd mammal "families" that has only one species in it. But, as is so often the case with such "monotypic " families, that's only true if you count the living species. Because there used to be more.

Part of the reason that the walrus is so obviously peculiar is that it's adapted to doing only one thing (eating shellfish) and, moreover, doing it in a fairly specific habitat. But not all prehistoric walruses were the same. Some, for example, used to live in subtropical waters, and by no means all of them fed on shellfish.

Take, for example, Pelagiarctos. This is a fossil walrus from California, that lived about 15 or 16 million years ago. First discovered in 1988, it was described on the basis of part of its lower jaw. That jaw showed that the animal was probably pretty big, and crucially, that it had particularly large and sharp teeth. Hardly the sort of thing you'd want to crush up shellfish with, but more the kind you'd want for slicing up flesh. The conclusion: this was a killer walrus, something that viciously attacked and ate things like seabirds and small seals.

A new fossil, however tells us rather more. It's still only a lower jaw (although that's rather the bit you want, if you need to prove it belongs to the same animal as the previous fossil), but there is more of it. It's enough, for example, to give us a better idea of how it relates to other walruses. This puts it about mid-way between the most primitive walruses known, and the living sort. As far as one can tell, when all you have to go on is a bit of the jaw, it looked quite a lot like modern sea lions - there's no reason to suppose, for instance, that it had large tusks. It was also, assuming its jaw wasn't unusually sized relative to the rest of it, about the same size as an adult male sea lion - which is, let's face it, not bad.

On the other hand, now that we have a better idea of what the jaw really looked like, there's less evidence that this was a deadly predator. True, it certainly wasn't eating clams - the teeth are entirely the wrong shape for that, and the apparent strength of the jaw still suggests that this was an animal with a viciously strong bite. The authors concede that they can't prove the animal didn't snack on large, warm-blooded prey, at least some of the time. But, they conclude, there's no good reason to suppose that it did, either. What it was probably best at was eating fish.

And, off the coast of California, there's plenty of those about.

Rumble in the Jungle

Generally speaking, larger animals make deeper calls, for much the same reason that a double bass has a lower pitch than a violin. It therefore comes as no great surprise to learn that one of the deepest calls made by a land mammal is that of the elephant. In addition to the more obvious trumpeting sounds, elephants make remarkably low rumbling sounds to communicate with one another. Because the sounds are so deep, they propagate for long distances, making it possible for them to hear one another even when they're a long way apart. Elephants being the intelligent creatures they are, they can pack a lot of information into these sounds, indicating things like their age, sex, and, especially in the case of males, their state of, ahem, hormonal arousal.

So there have been quite a few studies on what the elephants use these sounds for. But rather less, it would seem, on how they make them in the first place. A new study, however, has just looked into this question. The researchers used acoustic cameras , arrays of directional microphones linked to a video camera that can pinpoint exactly where a given sound is coming from. In this case, for example, the camera will tell you whether the rumble comes from the mouth (as it would in most mammals), or whether the elephants are actually making noises through their trunks.

You can see the results here. So - that would be both, then. But not, you'll note, at the same time. So there are two distinct kinds of rumble being used by the elephants, and it turns out that they're not for the same purpose. Females used their trunks to call to one another when separated, and their mouths to make rumbles when re-united with their friends, evidently as part of a social bonding process. The oral rumbles were louder, and higher in pitch than the ones made through the trunk - making them the elephant equivalent of an excited squeal.

The researchers only had access to two males, but it's interesting to note that they only ever used their trunks, and were mainly interested in calling from a distance. Since the trunk rumbles are deeper, they may have been using them to sound larger and more masculine, impressing the ladies and boasting to each other.

Either way, it's a new level of complexity to elephant communication that we were previously unaware of.


Marsupial Sheep... Up Trees!

I kid you not.

But I should probably clarify exactly what I mean. The diprotodontids are an extinct family of marsupials related to today's wombats and koalas. In the grand scheme of things, they lived relatively recently, with the last ones dying out only around 9,000 BC. A common name for them is "marsupial sheep". Not that they looked very much like sheep, it has to be said (they looked more like giant wombats), but as bulky, ground-dwelling herbivores, they did fill much the same ecological role. Although most were, indeed, about sheep-sized, the largest and most famous, Diprotodon, was a two-and-a-half tonne behemoth, about the size of a rhino, and the largest marsupial known.

Diprotodon may be famous, but it's one of the smallest species that was also among the most common. This was Nimbadon, which lived in Australia about 12 to 15 million years ago. When I say small, I only mean relative to others of its family; it was about 80 cm long from nose to rump, and probably weighed around 70 kg (150 lbs). So hardly a lightweight. And, when you have what appears to be the skeleton of a 70 kg wombat, your obvious conclusion is going to be that it lives on the ground.

But apparently not. A detailed analysis of the skeletons of these animals turned up some surprising conclusions. The forelimbs seem to have been unusually mobile, with the sort of flexible joints that you don't really need just to lumber along. Moreover, the claws are curved and long, just the sort of thing for digging into wood - these are herbivores, remember, so they won't be for taking down prey. The arms also appear to have had large muscles, big paws, and semi-opposable thumbs. Overall, the researchers conclude, the arms of Nimbadon are virtually identical to those of koalas.

The conclusion that Nimbadon climbed trees, and presumably in the same tree-hugging way that koalas do, is hard to escape. The idea of giant wombats climbing trees is a pretty odd one at first glance, although we should remember that there are some fairly large primates, and, for that matter, that bears can climb trees if they have to. In this case, the teeth suggest that Nimbadon ate fairly soft food, such as fresh leaves and maybe fruit, so it probably climbed to do a bit more than, for example, escape from marsupial lions.

Because of the way that the fossils are found, and the relatively large number of them, it had previously suggested that Nimbadon was a herd animal, lumbering its way across the forest floor in great packs. Now it seems that those herds of "marsupial sheep" were climbing through the branches up above.

New Species Discovered

Another month, another new species of bat. This one, Coleura kibomalandy, is a type of sheath-tailed bat native to Madagascar. It's one of those instances where we already knew that the bat existed, but had previously thought it to be something else - in this case, a species native to continental Africa. It was discovered through genetic analysis, turning out to be more distinct from the continental form than is a bat from the Seychelles that we already knew was a separate species. A new species of rice rat has also been announced recently, but, in its case, rather too late. Native to Barbados, and identified from skeletal remains, it apparently went extinct in the mid to late nineteenth century, possibly having been eaten by mongooses introduced to the island.

Our remaining new species for today are much older, all being fossils. First up is a hyaenodont that lived in Libya around 40 million years ago. Despite the name, hyaenodonts are not close relatives of hyenas, although some of them do have similarly shaped teeth. In fact, they aren't even carnivorans, but rather belong to an earlier order of mammalian predators, one that the carnivorans eventually supplanted and replaced. There were many different kinds of hyaenodont, and this turns out to be an especially odd one.

That's not to say its entirely unanticipated, though. It belongs to the genus Apterodon, first named back in 1880. It's just that we already knew Apterodon was odd, from the five species of it we'd previously identified. Now, however, we have a fairly good skeleton; there's not just part of a jaw here, but several bits of the skull, and, crucially, quite a lot of the limbs. Skulls can be easy to identify, being distinctive in many different ways, but other parts of the skeleton can be hard to pin down to a particular species when they're alone. Which means that, when they're attached to a skull, as they were here, you suddenly have a lot more information than you would otherwise.

In this case, we can tell quite a bit from the shape of the forelimbs and shoulder bones. These appear to be adapted for digging, swimming, or both (otters, after all, dig burrows). The authors suggest that the animal was semi-aquatic, something really quite unusual among hyaenodonts, most of which, like the famous Hyaenodon itself, are known for being not unlike, well, hyenas. This one is a good deal smaller than Hyaenodon, of course - roughly cat-sized - and it must have had a very different lifestyle. Judging from the shape of its teeth, for instance, it ate fairly hard-shelled food. Which, given its habitat, may mean things like crabs or lobsters.

Finally: pandas. Giant pandas are, of course, bears, and its hard, looking back from our current perspective, to see why anyone ever thought otherwise. So far as I can tell, somebody decided that, since they both eat bamboo and live in the same area, they just had to be closely related to red pandas, which look kind of like raccoons. They aren't.

In fairness, though, pandas are fairly strange bears. Bears are fairly omnivorous at the best of times, but a vegetarian carnivoran - you have to admit, that's odd.

So, while, yes, pandas are bears, they have been distinct from all other bears for quite a long time - 18 million years or more. We've actually done pretty well at tracing the panda's fossil history back through time, but a new specimen represents the oldest fossil panda yet. This one, Kretzoiarctos, dates back to 11 or 12 million years ago, and was discovered in, of all places, Spain. (I should add, incidentally, that it's just the name that's new - the fossil itself was first described in 2011). The fossil clearly needs more analysis to tell us what the earliest pandas were like, and perhaps shed some light on why they became so different from the other bears, but it does show that pandas were once much more widespread than they are now, and that they may not have originated in Asia at all.

[Picture by Peter Schouten, released under Creative Commons Attribution 2.5 Generic license]

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