Are Other Animals Ever Right-handed?

Humans normally display a preference for using one hand, rather than the other, for performing complex tasks, such as writing. It's most commonly the right hand, although exactly how common left-handedness is is debatable - largely due to insistence on the use of the right hand in some cultures, regardless of individual preference. While a small number of people are ambidextrous, this seems to be something that's learned, rather than something that you're born with.

Do other animals also display a similar preference? For most, that's a difficult, if not impossible, question to answer. A right-handed horse, for example, would prefer to use it's right front hoof to do... what exactly? When an animal has no manual dexterity to start with, we can't really say which side is the more dextrous. Studies looking at handedness in other animals have therefore focussed on those which are capable, to at least some degree, of grasping, and of performing tasks that are least somewhat analogous to how we use our hands.

Dire Wolves and Direr Dogs

Reconstruction of a dire wolf

[For more on dire wolves specifically, see: Dire Wolf Redux]

We are all familiar with the idea that, for much of the Age of Mammals, there have been large, scary cats with enormous teeth. They died out relatively recently, and as a result, we know a fair bit about them, and they remain one of the most popular prehistoric mammals with the general public - although mammoths do pretty well, too.

But, while the sabre-tooth cats stalked the world, what were the dogs like?

Perhaps the best known prehistoric dog is the dire wolf (Canis dirus). Do a google image search for "dire wolf" and you'll find far more images than you will searching for "sabre tooth", let alone something more specific like "Smilodon". But a look through those images gives you some idea why, and it doesn't have much to do with real-world fossils. The term "dire wolf" has been co-opted by fantasy fiction and games, and many of the pictures show the animals as they appear in Dungeons & Dragons, World of Warcraft, or Game of Thrones. They are generally portrayed as immense wolves, at least the size of a small horse, and sometimes with sabre teeth of their own, or bizarre spikes on their bodies.

Weasels in Warm Rivers: Otters of the Tropics and the South

Hairy-nosed otter

The body plan of otters is evidently a successful one in evolutionary terms. While the two species of "common otter" are the only ones inhabiting the rivers of the northern temperate zone, there are at least ten species in other parts of the world, and most of them look remarkably similar. They all have long, sinuous, bodies with short legs and webbed feet. Their tails, quite unlike the slender or stumpy appendages of other members of the weasel family, are powerful and muscular, and somewhat flattened to better help propel them through the water when swimming. Their fur is dense and, in most cases, short and sleek.

Furthermore, they are all more or less the same colour. Otters, of all species, are brown over most of their body, and they usually have paler underparts with a particularly noticeable patch of pale fur on the chest, which may extend up onto the chin. Compared with the variety of weasels, or even martens, the visible differences between most species of otter can be subtle, and it's hard tell them some of them apart.

The closest living relative of the familiar Eurasian otter is the hairy-nosed otter (Lutra sumatrana) of South East Asia and western Indonesia; the two species probably diverged during the early Ice Ages. It is also a highly endangered species, and, combined with its remote jungle location, that means we know relatively little about it. This story of threats to otters is, sadly, one I'll come back to a lot in this post. Of all the members of the weasel family, the otters are the ones most at risk, probably because their riverine habitat makes them particularly susceptible to pollution, and may also make them easier to hunt for their valuable fur.

Battle of the Sabretooths

Machairodus giganteus

The cat family is divided into two living subfamilies. One group are the pantherines, or large cats, which include lions, tigers, jaguars, and the various kinds of leopard. The other are the felines (which means that, in the technical, scientific sense, lions are not felines - the correct collective term that includes all members of the cat family is "felid"). The felines are quite variable in size, from cougars and cheetahs at the high end, through a range of mid-sized cats such as lynxes and ocelots, down to wildcats and their familiar domestic relatives.

But there once used to be a third subfamily of cats, and they represent, with the possible exception of mammoths, what are probably the most widely known of all extinct mammals. In fact, I'd guess that they are the only extinct mammals that come at all close to dinosaurs in terms of popularity. I refer, of course, to the sabretooth cats.

Chances are, especially if you're American, the sabre-tooth cat you've heard most about is Smilodon. This is actually three different species of cat, albeit very closely related. The largest was about the size of a lion, but with a short tail, like a bobcat. They used to live in both North and South America, and died out only around 10,000 years ago, perhaps not coincidentally shortly after humans became numerous on those continents. (Not, one assumes, that humans would have hunted Smilodon themselves - but they may well have eaten much of the cats' preferred prey).

News in Brief #5

Mountain long-eared bat

Can Bats Fly Like a Bird?

This is a blog about mammals, so it's only right to point out, every now and then, that birds have no monopoly on animal flight. The only mammals capable of true flight are, of course, the bats - flying squirrels and their ilk can only glide, not truly fly. (Nor are mammals and birds alone, of course; there are many flying insects, and once upon a time there were also pterosaurs). Still, birds have been flying for much longer than bats have, so might they be better at it?

A recent study by Florian Muijres and colleagues reveals, perhaps surprisingly, that the answer is 'yes'. They put birds and bats in a wind tunnel, and measured their ability to generate lift, and the lift-to-drag ratio, which indicates how much energy they have to put into it. By both measures, birds outperformed bats of similar size. Why so? After all, while birds have been flying for three times as long as bats have, the oldest known bat fossil is still very old, and you'd think they'd be pretty good at it by now.

Being a Little Bit Pregnant

Sun bear

In my ongoing series on the members of the weasel family, I have, on a number of occasions, referred to the concept of delayed implantation. A number of members of the weasel family exhibit this phenomenon, and they are far from alone among mammals.

To begin with, the egg cell starts to divide immediately after fertilisation. In most mammals, it then rapidly forms into a hollow ball of cells that move down through the reproductive tract into the womb. During this time, it really doesn't change much in size; the ball of cells, known as a blastocyst, is not much larger than the unfertilised egg, and the individual cells within it are much smaller.

Once it reaches the womb, the blastocyst attaches itself to the uterine lining and begins to send out tiny tendrils of cells that penetrate the wall in search of blood vessels. Once it finds them, the embryo can obtain nutrients from the mother, and starts to grow and develop. The side of the structure in contact with the uterine lining develops into the placenta, while the remainder forms the embryo proper, and the amniotic membranes surrounding it. This even occurs in marsupials, although, in their case, the placenta is primitive and short-lived, and, for that matter, there's something similar in some reptiles, such as sea snakes.

The Importance of Grooming

Primates are amongst the most social of mammals. They tend to gather together in bands or aggregations, often based on female kinship, and with a relatively small number of dominant males, born outside the group. The details vary considerably between species, as we might expect, but one feature that is relatively common is the way that such social bonds are formed and cemented. A particularly common way that primates maintain these bonds within a group is through grooming.

In grooming, the animals pick through a partners hair, removing lice, ticks, and other parasites, and generally keeping each other clean. This has obvious health benefits, and, in fact, these may be wider than is clear at first glance, since there is evidence that grooming causes the release of endorphins, hormones that, in humans, reduce pain and generate a feeling of well-being. Probably as a result of this, in at least some species, monkeys with better social relationships apparently live longer and have healthier offspring than those who feel more isolated.

Such benefits generally apply to the recipient. What does the animal performing the grooming from the practice? The most apparent and immediate benefit is that the other animal may groom you back, so that everyone benefits. Indeed, it's entirely possible that we humans don't cement social bonds this way simply because we haven't got the fur to make it worthwhile. But, given the complex, and non-egalitarian, social lives of most primates, there might be other benefits as well, which could explain why it's not simply the case that every monkey in a troop grooms every other one equally.