The carnivorans form a natural evolutionary group within the placental mammals, arbitrarily (if conveniently) ranked as an "order". The various living families within this order can in turn be classified as either "feliform" or "caniform", depending on whether they are more closely related to cats or dogs. The dog-like group contains a great many species, which might be more varied than we would think at first glance.
Tracing back the family tree of the dog-like carnivores, ignoring various extinct groups, we can see that the first family to split off from the others was that of the dogs themselves, with the bears coming next. The third split, however, saw one group of dog-like animals entering the water and becoming highly adapted to that environment. These are the seals, sea lions, and walruses, with the remaining group of dog-like carnivores being collectively referred to as "musteloids", and including skunks, raccoons, and the highly diverse weasel family, which itself incorporates everything from badgers and stoats to otters.
Like all other dog-like carnivores, seals and their relatives have whiskers. In fact, they have what are probably the most sensitive and highly specialised whiskers of any mammal. (Although, to be fair, there hasn't been extensive research on the matter, so it's hard to be sure.) In fact, the whiskers of seals have, collectively, ten times as many nerve endings as those of, say, polecats. Compared with their land-dwelling relatives, their whiskers are also shorter, thicker, and stiffer, with a smoother surface.
This is unsurprising, because the primary purpose of whiskers is to sense the environment through a highly developed sense of touch. It may be the most important tactile sense for many species, and that is going to be especially so for those animals that move around in dark spaces. That includes nocturnal animals, of course, as well as those that spend their lives in burrows and tunnels, but moving along the bottom in deep water also counts. The fact that water is more resistant to motion than air probably explains the enhanced stiffness of aquatic whiskers.
Indeed, the whiskers of seals have probably been studied more closely than those of any animal other than rats and mice. But if the unusual sensitivity and shape of their whiskers are an adaptation to their aquatic habits, we should see something similar in semi-aquatic mammals. This has, in fact, been checked, with some studies comparing seals and land-dwelling musteloids with their semi-aquatic kin, the otters and mink.
These have generally found that the whiskers are, as we might expect, often intermediate between those of related aquatic or terrestrial animals. By some definitions, seals themselves are semi-aquatic, since they often sleep on land and spend most of their time there during the breeding season and when they are very young. But here, we are using the term in a more typical sense that excludes seals but would include animals that spend a lot of time swimming for food, but are also agile on land, often sleeping in burrows or the like. Outside of the carnivorans, muskrats and hippos would fit this description and, further afield, we could include penguins and crocodiles.
However, it is only in the last few months that a dedicated analysis of otter whiskers and their use has been published. So let's take a look at what it uncovered.
We should start off by saying that there are at least thirteen recognised species of otter, with a fourteenth having recently been added to one of the most influential lists of such things. Of these, one, the sea otter (Enhydra lutris), is, under the definition used above, fully aquatic, since while it can move about on land, it doesn't really have to. The others are all decidedly semi-aquatic, forming a distinct branch within the weasel family, thus being comparatively close relatives of the seals, but the product of a distinct, later, evolutionary event.
In this case, the study looked at one particular species: the common or Eurasian otter (Lutra lutra). The researchers were able to acquire five deceased otters from the National Museum of Scotland for anatomical examination, and a live one from the Wildwood Trust in Kent, to analyse its behaviour.
The first thing to note is that, as with many mammals, otter whiskers are not restricted to the area we immediately think of, to either side of the snout. In addition, there are smaller clusters of whiskers above the eyes, on two patches on either cheek, and below the chin. These are, as they are in cats, smaller and finer than the primary or "mystacial" whiskers, but are doubtless just as important in giving the otter a complete tactile sense of its immediate environment.
The mystacial whiskers are, as in all mammals studied so far, arranged in a regular grid pattern. In this case, they are arranged into seven rows of eight, with the whiskers getting progressively smaller and more closely packed towards the snout, where an irregular set of tiny whiskers lies in front of the true mystacial ones. The exact pattern seems to vary between individuals, and may well be unique, in the same manner as human fingerprints. This is known to be the case for cats, including lions, although the researchers concede that it would probably be of little practical use for identifying individuals.
The whiskers themselves are, as indicated in previous studies, thicker and stiffer than those of land-dwelling animals, such as foxes, but not quite so much as in seals. However, seal whiskers are unusual in having an oval cross-section, rather than a round one, which may help them move through the water; otters, it turns out, do not.
Most mammals move their whiskers in a cyclical motion like... well, a whisk. The observations of the live animals in this study, however, showed that, like seals, otters do not. This is probably because of the greater resistance provided by water than air, which might make such motions tiring for the animal. Instead, they push their whiskers forward and back, aided by stronger and more organised muscles beneath the skin of the snout.
These muscles form regular strips between the whisker rows, running the full length of the mystacial pad that bulks up the tissue beneath them. This is not what we see in, for example, rats, but it is very similar to the pattern in seals, suggesting its role in their particular whisker motions. Like most other mammals, including rabbits and hedgehogs, otters can twitch each side of their nose independently, pushing the whiskers of one side or the other forward to get a better feel for the environment.
One test that the researchers gave to their otter involved what they describe as a "texture discrimination enrichment device". This is basically a framed plastic sheet with six small doors in it. The doors are either smooth, finely ridged, or comparatively rough, with food being hidden behind only the roughest of the doors. The otter's task was to quickly learn where the food was hidden and identify the right doors using its whiskers alone.
When presented with this task, the otter used its whiskers in the same way, and at about the same speed, as seals and sea lions had previously managed in similar tests. In fact, once it had learned the trick, it could identify the correct doors more quickly, being able to tell the difference in 0.2 seconds instead of 0.5 seconds. For comparison, it takes a human using their fingertips about 5 seconds to detect the same difference, giving some idea of the sensitivity of whiskers, at least on this particular type of task.
This inevitably leaves some questions unanswered. Perhaps most importantly, we don't know how widely these principles hold. The authors suggest extending the study to other otter species that are relatively easy to obtain from zoos, such as the Asian small-clawed otter, as well as seeing whether there are differences in the fully aquatic sea otter. But it strikes me that it would be interesting to know how other semi-aquatic animals have developed their whiskers. We know a lot about the whiskers of rats and mice, but how do those compare with beavers?
It's a small feature of animal anatomy, but one that is very important for the animals themselves and for how they navigate and interpret the world around them.
[Photo by Claudia Peters, from Wikimedia Commons.]
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