Sunday, 16 January 2022

Why Animals Have Whiskers

One of the key defining features of mammals is that they have hair, or are at least descended from other animals that once had hair. The primary purpose of hair is to keep the animal warm, something useful for any warm-blooded animal, especially if it's small (elephants and rhinos, for example, while they do have hair, aren't really what you'd call "furry"). But, over millions of years of evolution, hairs have also evolved to carry out other functions, such as the protective spines of hedgehogs and porcupines.

Another example of specialised hair is that of whiskers. Technically known as vibrissae, whiskers are remarkably common in mammals. When we think of whiskers, it's likely that most people's thoughts jump immediately to the long, mobile, whiskers of cats and mice. But whiskers can also be shorter and less mobile than this, as we see in such animals as horses. Indeed, even if we look at a cat, whiskers are not restricted to the long ones on the snout; they also have whiskers on the eyebrows, on the cheeks and chin, and on their forelegs just above the paws. 

In fact, the great majority of mammal species have whiskers of some kind, making them a worthwhile subject of study. So, with the aid of a recently published scientific review of the topic, let's see what we can say about them. For a start, what are they?

Whiskers are long hairs, with the same internal structure as any other hair. That is, they are primarily composed of keratin, with an outer cuticle of scale-like cells surrounding an inner cortex of more typical-looking cells that migrate outwards to form the cuticle and which create the keratin. As in other thick hairs (although not finer ones), the cortex itself surrounds a central core of relatively large cells called the medulla which may be there largely to provide structural integrity.

Whiskers typically have a smoother surface than other hairs when viewed under the microscope and they are more likely to have a circular, rather than oval, cross-section. They usually taper and become more flexible towards the tip, and they almost always have a curve along their length, although this isn't always very obvious in the shorter ones. 

The hair follicle of a whisker is enlarged, not least because it contains multiple nerve endings that allow for detailed sensation as to how the whisker is moving when it touches something. This is usually in the order of a few dozen nerve endings per whisker, but bearded seals (Erignathus barbatus), for example, have well over a thousand for each of their 244 whiskers. In the more mobile whiskers on the snout of many animals, the nerve endings are embedded within a blood-filled sinus that likely gives the base of the whisker freedom to move. The individual nerve fibres from the whiskers enter the brain through the trigeminal nerve, which also carries sensory fibres from touch and temperature-sensitive nerve endings from across the face.

Early scientific research on whiskers largely focussed on counting how many there were and where on the animal's body they might be found. From around the 1960s onwards, the focus moved more towards the neural anatomy associated with the structures and then towards how this information might be processed by the brain. Here, most attention has been paid to the mystacial (i.e. "moustache-like") whiskers of the upper lip, since these tend to be the largest, most mobile, and most sensitive ones, although it's likely that similar principles apply to those elsewhere on the body. Likewise, most detailed studies have been performed on rats and mice, since these are readily available in the laboratory environment, although that isn't to say that other species have been ignored.

Mystacial whiskers tend to be arranged in a grid pattern, something that's especially true in nocturnal, arboreal, and marine mammals, where it's likely that they partly compensate for difficulty in using vision to sense the animal's surroundings. Animals that rely heavily on daytime vision and that don't climb about in trees, such as horses and ground-dwelling primates, tend to have fewer such whiskers, and they often lack the organised grid pattern.

Where the grid is present, though, it's often possible to identify a direct mapping from the whiskers to a similar grid-like pattern in the physical microstructure of the brain. This has been shown, not just in rats and mice, but also wallabies, sea lions, and even nocturnal primates such as bush babies. Something similar is seen with the visual cortex, where it is merely the first layer in processing visual information from the eyes; this is probably true with whisker sensation, too, since many other centres of the brain are also involved, likely integrating the information into a more complex sensory map.

These mystacial whiskers are mobile because muscle fibres loop around them, with one set for each row on the grid, so that all the whiskers in that row move simultaneously when the muscle is twitched. These fibres are attached to the orbicularis oris - the muscle that puckers the lips - but are capable of independent movement. Significantly, this arrangement is found in both mice and wallabies, which are hardly close relatives. In fact, it's even found in humans, one of the few mammalian species not to have whiskers at all, leaving the fibres, so far as we can tell, purely vestigial.

Given all of this, it should come as no surprise that the primary function of whiskers is sensory. They are very sensitive to touch, enhanced by that flexible tapering tip, and their primary purpose is to enable the animal to feel its way about in conditions of low lighting. In this respect, it's worth noting that animals such as mice are born with whiskers - which is significant when you consider that they are also born blind, and without a fully functioning sense of hearing. In other words, the ability to sense the world with whiskers is something they need right from birth, with sight and hearing almost an afterthought. Even kangaroos are born with whiskers, which may, perhaps, help them find their way to the pouch, well before most of their other senses are functioning.

Newborn rats don't have the muscular control to twitch their whiskers, but this develops by the end of their first week of life, and over the next couple of weeks or so develops into the full adult pattern of whisker movement. (And I mention rats only because there hasn't been much research on other species in this particular regard). When they do, the behaviour in question is called "whisking", and consists of moving the whiskers in rapid back-and-forth motions, something they do almost constantly to allow them to rapidly scan the environment around them. And, by 'rapidly', I mean about eight times a second for rats, and rather faster for some other animals.

Not all animals with whiskers whisk in this fashion. For instance, seals, which are perfectly capable of twitching their whiskers forward to sense something, do not perform the rapid to-and-fro of whisking, probably because it would be too much effort underwater. Perhaps more surprisingly, neither do guinea pigs, in their case likely because they are diurnal, reducing the importance of such detailed touch sensation. Indeed, so far as is known, all animals that whisk are either nocturnal or arboreal.

Whiskers can be used for other purposes than sensing the general environment. They can be used to explore the precise shape and texture of a specific object, especially underwater, as can be seen in sea ottersmanatees, and (out of water) shrews. They can also be important socially, with rats touching their whiskers when they meet, and many young mammals using them to maintain contact with each other and with their mothers. 

As one might expect, one of the groups of mammal that often doesn't have whiskers are the cetaceans, not least because sonar is perfectly sufficient to sense their surroundings in the dark - not to mention having a much longer range. Nonetheless, some cetaceans do, in fact, have whiskers (and thus are not as entirely hairless as one might assume) most notably the non-echolocating baleen whales, although some river dolphins do, too. One remarkable case is that of the Guiana dolphin (Sotalia guianensis) which is born with whiskers, but soon sheds them. It is then able to use the enlarged hair follicles in which the whiskers once grew to sense, not touch, but electrical currents, presumably to enable it to find fish in muddy waters.

Since so many different species of mammal have whiskers, it seems likely that they are something that arose fairly early in mammalian evolution. But just how early? The anatomical structure of whiskers and the muscles and nerves associated with them are essentially the same not only in widely separated groups of placental mammals, such as cats and mice, but also in marsupials. This suggests that the common ancestor of placentals and marsupials must have had whiskers, and they have only subsequently been lost in those few, mostly large ground-dwelling diurnal animals (such as humans) that didn't need them.

That's a very long time ago, since the two main groups of mammal last parted company during the Jurassic, well back into the time of the dinosaurs. Those early mammals were likely nocturnal and arboreal, both features that correlate with well-developed whiskers in living mammals, so they would doubtless have been useful then, too. 

Since whiskers don't fossilise, it's hard to say much more than that. It is notable that living monotremes don't have whiskers - although they are also highly specialised animals with, among other things, electrosensory organs which, like that of the Guiana dolphin, might make whiskers less useful to them. This had led some scientists to speculate that the ancestors of monotremes, too, once had whiskers and that the origin of the structures therefore lies even further back in the mammalian family tree.

One theory even has it that whiskers are not modified hairs after all, but evolved independently from touch receptors. That seems unlikely to me, but if it is true, then it could mean that whiskers evolved even before hair did and are even more a defining feature of what it means to be a mammal than we might expect.

[Photo by David Corby and Arad, from Wikimedia Commons.]

3 comments:

  1. If the whiskers-first idea is true, is it possible that normal hairs are modified whiskers rather than vice versa?

    ReplyDelete
    Replies
    1. It has been suggested that hairs originally evolved as mechanoreceptors, only becoming thermoregulatory later. Which is essentially the same thing as you suggest. I don't know how popular the idea is currently, but I think it's fair to say that it's certainly *possible*.

      Delete