Hair, Fur, Bristles, and Spines

Trinomys

Hair is one of the key defining features of mammals. Apart from cetaceans, such as dolphins, no mammals are entirely hairless, although, for example, hippos and some babirusa species come very close. The original purpose of hair was almost certainly to maintain body heat, something that matters to warm-blooded animals in a way that it doesn't to, say, reptiles. Since then, however, hair has adapted to fulfil many other functions as well.

To begin with, there is colouration, a function that hair needs to take over once it obscures the skin. The most obvious way that hair colour can help an animal is camouflage. While this is instinctively true, there have been statistical studies that demonstrate certain hair colours are, indeed, more common in animals living in certain environments. For instance, multi-species analyses of lagomorphs and cloven-footed mammals have shown that animals with grey fur are more likely to live in rocky environments. The same studies, as well as similar ones on carnivores, show that pale fur is associated with open environments, especially deserts, while dark fur is most common in those animals living in jungles, dense forests, or heavily vegetated swamps. And, surely to the surprise of almost nobody, white fur is associated with the Arctic, or with the winter coats of those living where it snows.

It's worth noting here, however, that the above patterns of colouration are not necessarily just about concealment. It has been argued that dark fur aids water evaporation and/or UV protection, and so would be expected in tropical animals, while pale fur reflects heat and might be expected in deserts. It's perfectly possible that it's both, and evidence for different kinds of animal may point in different directions as to which is most important. I've previously discussed how the colouration of wild boar seems to favour the heat hypothesis, but I could equally point to a study that showed certain rodents have darker fur when they live on lava fields because owls are less likely to eat them.

That assumes that animals are generally the same colour all over, which is obviously not always true. Even where they are mostly uniform, most mammals have paler fur on the undersides than the rest of the body. This countershading is probably related to the fact that the underparts of animal are normally going to be in shadow, which could aid camouflage, but, equally, also reduces the UV that the animal needs to protect against. 

Perhaps the clearer example of fur colouration as camouflage comes from certain spotted animals such as woodland deer, where the pale dots mimic the dappled sunlight coming through the leaves. It's notable, for instance, that in both deer and pigs, the young are spotted even if the adults aren't, and spent the first few weeks of their life concealed in woodland vegetation. Concealment for a different purpose may lie behind the spotted coats of predators such as leopards and jaguars.

It's often thought that stripes can help to break up an animal's outline, making it harder for others to detect, whether the animal be a tiger or a zebra, but the evidence for this is more limited than you might think.

In many cases, hair colour is unlikely to have anything to do with either concealment or protection from the environment. Again looking at cloven-footed animals, there is a tendency for animals with dark or light patches, especially on the face, to be active during the day, live in open environments, and to live in herds. Taken together, this suggests that such markings are used to communicate with others of the same species. The white tail or rump of many herbivorous species is a clear example here, being used as a warning to others when the animal is fleeing because it's detected a predator.

In birds, brightly coloured feathers are often used in sexual signalling, but this is less true of hair in mammals. Primates are among the few mammals where colouration is often different in males than in females, and even here, it's more likely to be exposed patches of skin (often on the genitals) than anything to do with their fur. An exception occurs among lions, where not only is the mane a clear indication of masculinity, it is also darker in the most physically fit and virile males.

In some species, we see the exact opposite of camouflage, where the animal tries to make itself as visible as possible. While this may also help in intraspecies communication and identification, this is generally a warning to others. Termed "aposematism", such warning colours are best known in mammals from the skunks.

But there is far more to hair than simply its colour, or even its ability to keep an animal warm. Hair can also be a sensory organ, feeling out the environment through touch. This is most obvious in whiskers, which I have looked at before. These are specialised sensory hairs usually restricted to particular parts of the body - often, but not exclusively, the snout. However, there are some animals, such as naked mole rats and manatees, where tactile hairs are found across the body, arguably stretching the definition of what counts as "whiskers". While humans, of course, lack this specific type of hair, eyelashes are tactile, triggering the blink reflex as part of their function in protecting the eye from specks of dust and dirt.

Leaving whiskers aside, the fur of most mammals consists of three types of hair. The most common are the down hairs or "underfur". These are densely packed, and are short and soft, forming a woolly undercoat whose primary function is to trap air next to the skin and keep the animal warm. 

Next are the awn hairs, which are longer and stiffer at the base, but end in soft flattened tips resembling those of down hairs. Since they are longer, and still reasonably numerous, these often form the bulk of the visible fur and provide for much of the colouration. This has what's described as "definitive growth", which is to say that it reaches a certain length and then stops growing, usually being shed at this point.

Interspersed between these other two types are the guard hairs, which are longer and stiffer, with pointed tips. These have little to do with trapping heat, but they can shed water and serve a protective function. Most are otherwise similar to awn hairs, but there are two specific subtypes that have different functions.

In the technical parlance of mammalogy, "bristles" are longer guard hairs that grow continuously and form very visible patterns on the body. These are what comprise the manes of horses and male lions.

Then, there are spines: highly modified guard hairs that are exceptionally thick and pointed and that, like awn hairs but unlike bristles, have definitive growth. Spines have evolved multiple times in different mammal groups and there is considerable variation among their form. The ones that we typically think of are rigid cylindrical structures, often termed "quills". 

These are found in hedgehogs, porcupines, and echidnas, among others. Not only are hedgehogs and porcupines not closely related, but porcupines aren't even a single group of animals, representing two entirely different families of rodent that evolved separately on different continents. In the case of all of these animals, the function of spines is obvious; they are protective. The spines of a hedgehog, for instance, while they probably are a deterrent to predators, also act as shock absorbers, while those of porcupines are purely defensive, breaking off inside any animal that tries to eat them - with potentially fatal consequences.

Not all hairs classified as spines fit this description, however. Others, while still stiffer than normal guard hairs, are flattened or grooved in cross-section, and are often somewhat flexible, closer to the everyday meaning of "bristle". These, termed "aristiform spines" are found in certain small rodents, having evolved at least four times in different families, all of which go by some variant of the common name "spiny mice". It's hard to believe that they would be much of a deterrent to predators or offer much in the way of mechanical protection... so what are they for?

Here, we are on much shakier ground. The two main theories are that they help to keep raindrops off the insulating fur beneath and/or that they radiate away heat, preventing the animal from overheating in tropical climates. A recent study reasoned that, if either of these explanations was true, the spines should vary with the climate in which the animal lives, becoming larger as the weather becomes either wetter or hotter, as the case may be.

That study looked specifically at a couple of species native to the Atlantic Forest along the east coast of Brazil. Both are members of the "spiny rat" family, the Echimyidae which, despite the name and the animals' general appearance, is more closely related to guinea pigs and porcupines than it is to true rats. The species in question, the white-spined Atlantic spiny rat (Trinomys albispinus) and hairy Atlantic spiny rat (Trinomys setosus) are very closely related but the former lives further north than the latter.

To cut a long story short, the result of the study was a definite "maybe". There does seem to be some correlation between climate and spine length and shape in these species, but it's not very significant and doesn't follow any consistent pattern between the two. Here, at least, it seems, there is still more to be discovered.

[Photo by Guilherme Garbino, from Wikimedia Commons.]