Sunday, 14 October 2012

Why are Elephants Hairy?

The Asian elephant is the hairiest of the three living species
Elephants are, as is well-known, the largest land-dwelling animals alive today. Being so large presents a number of problems, and these are only increased by the fact that elephants, being mammals, are warm-blooded. Specifically, they have to maintain a body temperature of 36°c, only one degree below that of humans.

Perhaps the most obvious problem is that they need to eat a lot in order to survive. The need to keep their metabolism going means that they need more calories for their size, than, say a crocodile, which - being "cold-blooded" - can rely on the sun to warm it up. However, that internal body heat presents another problem as well, and that's keeping cool.

The larger an object is, the smaller its surface area proportional to its volume. This is basic mathematics; if I double the dimensions of an object, while keeping it the same shape, the surface area increases four-fold (22), but the volume increases eight-fold (23). Since heat is lost through the surface, the bigger an animal is the more heat it will tend to retain. This is actually a greater problem for small animals than large ones, since they tend to lose heat rapidly, and therefore need a very high metabolism just to keep up. Mice, shrews, and the like need to eat a huge amount, relative to their size, in order to keep functioning. It obviously doesn't help if the weather is cold, which was the original reason proposed for Bergmann's Rule - the idea that animals are larger in cold climates than they are in warm ones.

It may not have escaped your attention that, at least today, elephants are not known for inhabiting the Arctic wastes. So, for them, this presents the opposite problem to that faced by shrews and weasels; namely, how to keep cool. They have a number of adaptations for this, including some that are behavioural, such as spraying themselves with water. Remarkably, though, given their environment, elephants can't sweat, although they do seem able to lose moisture through their skin by other means.

Another adaptation is their ears. The large, flap-like, ears of elephants are ideal for shedding heat, since, being relatively thin, they have the high surface-to-volume ratio that most of the other parts of the animal lack. This is probably the main reason why their ears are as large as they are, and they are full of blood vessels, something that also helps shed hear. Its notable that they can also flap their ears, wafting air over their bodies to help cool them down. This sort of thing, along with water-spraying and so on, will be particularly useful when the outside temperature is warmer than the elephant's body, preventing any normal heat loss regardless of their size.

Elephants are also, for mammals, not especially hairy. A thick coat of fur keeps an animal warm by trapping air close to the skin, and most mammals have one for this reason - it's very useful for a warm-blooded animal. There are a number of reasons why an mammal might not be very hairy, but large size in a tropical climate is certainly one of them. Think of rhinos and hippos, for example, which are more or less tied for the honour of "second largest land-dwelling mammal".

However, close examination shows that elephants are not as hairless as they might look. Certainly their hair is very sparse, but there's still quite a lot of it, and we might question why this is true at all.

One possible explanation is that it's simply a hold-over from evolution, like the human appendix, or, for that matter, the hair over most of our bodies. Until very recently, in evolutionary terms, species of elephant lived in much cooler climates than they do now, and its likely that they would have needed a coat of hair to keep warm, even with their greater body mass. Woolly mammoths are an extreme example here, although not necessarily a very relevant one. As I mentioned last week, although mammoths were members of the elephant family, they aren't the ancestors of modern elephants, and, indeed, diverged from the other elephants long before the Ice Ages began.

However, elephants were by no means the only elephants to live outside the tropics. The "straight-tusked elephant" (Elephas antiquus), for example, lived in Europe during the interglacials, and evidently preferred a moderately cool temperate climate to the heat of the savannah. We have no idea how hairy it was, but it's certainly plausible that it had a thin coat of fur. Since this was a very close relative of modern Indian elephants - certainly closer than we are to chimps, for example - it may be that living elephants simply haven't lost all their hair yet. After all, we haven't.

In any event, mammals don't just use their fur to keep warm. Whiskers are modified hairs on the face that serve a sensory function in a wide range of mammals, for instance. Elephants obviously don't have whiskers, but bristly hairs on the trunk may help make it more tactile.

In a new paper, however, Conor Mhyrvold and co-workers propose a different explanation, and, so far as I know, an entirely new one. They propose that elephant hair actually helps keep the animals cool.

Huh? Well, they start with the assumption that elephant hair is simply too sparse to retain heat the way it does in most other mammals. This is quite plausible, since (apart from our heads) it certainly doesn't do so in humans. Indeed, it's apparently been estimated that hair cover of less than 30 hairs / cm2 is insufficient to trap heat. Since elephants don't even manage one hair per square centimetre (less than humans, even ignoring our scalps), it's a fairly safe bet that it isn't doing anything much to insulate them.

If they aren't trapping any warmth, though, the argument runs that the hairs are acting like miniature radiator fins, shedding body heat into the air. Effectively, they're increasing the elephant's surface area, and allowing them to lose heat more readily - as the spines do on cactuses, for example. In itself, once you've assumed that it isn't trapping warm air, that's likely a difficult conclusion to avoid. What's significant, though, is that the researchers use equations to demonstrate that the effect isn't a trivial one.

They estimate that a hairy elephant should, under typical conditions, lose 10% more heat than a completely bald one. The effect drops off rapidly in windy conditions, although the wind itself would probably cool the elephant down then. It also may not apply in still air, although apparently the elephant flapping its ears can do enough to alleviate that - and, of course, we know that they do do that when they're hot.

This is, of course, all theoretical, albeit derived from what look to be some solid physical equations. It's not as if they've actually gone out and shaved an elephant to see what happens. Nor have they yet tried to see what the same equations have to say about, for instance, rhino skin. But it's an intriguing idea that sounds quite plausible. They even go on to suggest that this may be why mammals, or their immediate ancestors, evolved hair in the first place; an idea that's frankly much harder to test.

Still, the idea that, for at least some animals, hair keeps them cool rather than keeping them warm is an intriguing one. We'll see what future studies have to say.

[Photo by "Fir0002/Flagstaffotos", from Wikimedia Commons]

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