Cubs in the Snow

Both brown and American black bears spend much of the winter asleep. Whether this counts as true 'hibernation' is debatable, although, in recent years, most scientists no longer insist on a strict physiological definition, having given in and accepted the word as it is usually understood in English. They do this because their food is in short supply during the winter, and spending the entire time snoozing is a good way of conserving energy.

Before hibernating, they construct a den, in which they will spend the next several months, depending on the exact weather conditions. Significantly, mothers of both species give birth in the den, often while they are still asleep. All of which works for a large, omnivorous animal living somewhere that inclement weather makes it hard to forage in winter. But that's not something that will be true of all bears.

Friendship and Fission-Fusion

Mammal species have a wide range od different social systems into which they organise themselves. Many are essentially solitary outside the breeding season, others form long-lasting pair bonds, and others live in larger associations which may themselves have varying different structures. Among the latter, one common pattern is that of the fission-fusion society.

Here, rather than having a long-lasting association, perhaps bonded by ties of family, the animals live in groups but the membership of that group is not constant. New animals are constantly wandering in, while others break off and leave for other groups. On a larger scale, it may be that the individual fission-fusion groups - the actual bands of animals you would see travelling together - are themselves gathered into larger social networks that may have a relatively consistent structure. That is, the new animals joining the group aren't random; they're individuals already known to the group, and rival social networks may exist nearby that do not mix their members.

How Sloths Turned Upside Down

One of the things that sloths are noted for is spending a lot of time hanging upside down. This isn't, of course, something they literally spend their entire lives doing; they come down out of the trees to defecate, and they do sometimes travel over the ground - or swim across rivers - to move from one tree to another. They have also been observed, especially in zoos, casually sitting about in what we'd consider a fairly normal posture. But even so, being upside down is how they spend most of their time.

This is obviously pretty odd, from the perspective of a mammal (or, indeed, most other animals). You might think that what must have happened is that, at some point in the distant past, some ancestral tree sloth first headed up into the branches, decided to do so by hanging from them rather than by the more obvious method favoured by (say) primates, and that all of its descendants simply happened to do the same thing. In other words, it's a one-off quirk of evolution.

Except that can't be right.

How Baby Bats Learn to Fly

Small mammals, on the whole, live fast and die young; a great many of them live for only a single year. And that's assuming they don't get eaten first, which, unsurprisingly, they often are. In order to maintain their numbers, then, it is essential that they breed early and often. In particular, they tend to have large litters, which they then need to get to adult independence as quickly as possible. The common house mouse, for example, can breed at least five times a year, and gives births to litters of typically around five to eight pups at a time, each of which is sexually mature within less than two months.

This is not, however, what we see with bats.

The Fast Lives of Early Sperm Whales

Livyatan melvillei, a Miocene species
(It was supposed to be 'Leviathan', but the name was
already taken)

Sperm whales are magnificent and highly distinctive animals. While they may not be quite as large as the great toothless whales, they are still pretty huge: a fully grown male is typically about 16 metres (52 feet) in length and weighs over 40 tons. So distinctive is it, in fact, that today it is usually considered to be the only living species in its family, the Physeteridae.

The caveats in that last sentence - "today", "usually", and "living" - are all significant. On the first two points, the sperm whale family used to be considered to contain no less than three living species, and some researchers still define it that way. The other two species are the dwarf and pygmy sperm whales (Kogia spp.), and they're typically (but not always) placed in their own family these days. As their names suggest they are much, much smaller than the "true" sperm whales, being more like the size of a large dolphin.

Toothless Old Fossils

Artwork depicting extinct animals almost invariably shows them physically fit, unless they're actively engaged in some life-or-death battle. They're usually either adults in the prime of life, or still juveniles (often newborn). And this is actually quite reasonable, since few animals get to live to an old age out in the wild. Most will, after all, be eaten before then, and if they have any serious illness or crippling injury, they're not going to last long. Senescence, the gradual decline of biological function associated with becoming elderly, is not something we see much in the wild, for all that it's common in humans and domestic animals.

Which isn't to say that we don't see it at all, especially in very large animals that have few, if any, natural predators once they reach adulthood. The same must also have been true in the distant past, and it's reasonable to assume that at least some fossils, at least of the larger animals, belong to elderly individuals. It isn't, however, necessarily going to be all that obvious, especially if the skeleton is incomplete in the first place. Nonetheless, a recent report does describe a jawbone that the authors believe belonged to an elderly animal, and, if they're right, this offers some unusual insights into the creature in question.

Time of the Tiny Otters

An earlier fossil
(The new one still has the jaw attached to the rest of the skull)

A few months ago, the discovery of the "largest dinosaur ever" was announced. Again. Indeed, the "largest dinosaur ever" always seems to be being discovered, and, while we must at some point, surely get to the real end of the sequence, I wouldn't put any money on this latest one being it. But speaking more generally, I'd expect that when most people think of dinosaurs, "big" is going to be a fairly common adjective.

Even when we look at prehistoric mammals, it's the big ones that get most of the attention. Mammoths. Prehistoric rhinos. Enormous tank-like armadillos. Giant wombats. Big animals are cool, and there were some pretty large ones in the distant past.

Yet, at any point since their first appearance, small animals will always have been more common than large ones. Lots of small and interesting things doubtless scurried about under the feet of the dinosaurs, and so it was with the Age of Mammals, too. So today I want to look at the recently described fossil of a small mammal, and how it too, can tell us something interesting.

News in Brief #3

The Mandrill's Face

There are a number of colourful primates, but one of the more instantly recognisable is surely the mandrill (Mandrillus sphinx) whose adult males have strikingly blue and red faces. Under a strict definition, mandrills aren't really baboons, but they're very closely related to them, and the difference is pretty academic. Given that only the adult males have these extreme face-markings it's not surprising to learn that they are there mainly to advertise their masculinity and fitness as a potential father to female mandrills.

A study from 2005 confirmed that, yes, indeed, female mandrills find males with bright red noses to be particularly sexy. Oddly, though, until now nobody appears to have looked at what the bright blue patches on either side of the nose are for. In the new study, Julien Renoult and co-workers confirmed that the blue colour is more intense in dominant males, just as the red is. Indeed, it seems that what's really important is the contrast between the two. This suggests that, in the distant past, mandrills developed the red nose as a measure of their fitness, and the redder the nose, the more females liked them. So the noses of dominant males became ever redder... but there's only so red a nose can get. When some males began to develop a contrasting colour elsewhere on the snouts, the redness of their noses became more obvious, and, over the course of evolution, the bright blue/red contrast we see now developed.