Sunday 20 September 2020

The Tale of the Tail

The defining feature of vertebrates is that they possess a vertebral column, or "backbone", a series of interlocking skeletal structures that run down the centre of the back and serve to protect the main nerve cord. Unless you're a shark or something of that sort, these structures are made of bone and, at least once we ignore fish and amphibians, there's a reasonably consistent pattern as to how these individual vertebrae are structured.

One of the distinguishing features of the mammal skeleton, however, is that the backbone can be divided into five distinct regions, based on the function and detailed structure of the vertebrae within it. At the front end (or top, if you're bipedal) are the cervical vertebrae, which together form the neck. The first two of these are further specialised, the frontmost one because it has to connect with the skull, rather than to another vertebra in front of it, and the second because it's the pivot that allows the head to turn. Including those, there are almost always seven cervical vertebrae, even in giraffes... although sloths and manatees are exceptions, because of course they are.

Behind these are a variable number of thoracic vertebrae, which are distinguished by the fact that these are the ones that are attached to the ribs. Humans have twelve of these, which is fairly typical, but there's clearly no requirement for all species to have exactly the same number of ribs - for instance, a horse has eighteen pairs. The key innovation of mammals is that this thoracic region is followed by a set of lumbar vertebrae, forming the back of the abdomen. We don't see this in either reptiles or birds, where the ribs just keep on going (even if the short body of most birds means that there aren't very many of them compared with, say, snakes). As a general rule, the more ribs a mammal has, the fewer its lumbar vertebrae; antelopes always have 19 vertebrae along the back, but the number of rib pairs varies from 12 to 14, leaving seven to five lumbar vertebrae behind them.

Next, we come to the sacrum, a set of three to five fused vertebrae that anchor the pelvis and, indirectly, the hind limbs. This is missing in animals such as whales and dolphins, but either way, the final section of the backbone consists of the caudal vertebrae, which together form that common feature of so many mammals - the tail.

Having a tail can be remarkably useful, and can be employed for all sorts of different purposes by mammal species. Especially in climbing animals, it can be used for balance and it can also be useful for similar purposes when running fast. In aquatic animals, it can serve as a rudder, or even provide some degree of propulsive power. It can be prehensile, and used to carry things about or hold onto branches. It can be used for communication, whether as a warning or a sexual signal. Some species can store fat in their tails while others can use it to swat away flies. It can even be used as a support, either while sitting up, like a meerkat on guard, or as an effective "fifth limb" while walking, as kangaroos do. 

Humans, of course, don't have a tail, but even we do have caudal vertebrae, typically four of them wholly or partially fused into a short "coccyx". The same is true of other similar mammals, such as most of the tailless bats (Anoura spp.)... although, oddly, the tailed tailless bat is not the only tailless bat to have a tail. 

And, yes, that really is what it's called.

The number of caudal vertebrae is enormously variable, as you'd probably expect. Humans are clearly near the bottom end of the scale, with pangolins apparently holding the record, with up to 50 or so. Apart from that, you might expect that the skeletal structure of the tail isn't hugely complicated, but there's perhaps more subtlety to it than you might think.

Aside from animals, such as ourselves, with very short or vestigial tails, the part of the backbone behind the sacrum has three different regions. In the first one, the vertebrae are much like those further forward in the body, including the full set of articular surfaces that form the joints between them. The last vertebra in this part of the tail, however, lacks these surfaces at the rear end, and beyond that the bones are simple cylinders, having reached beyond the end of the spinal cord. At first, the individual bones get longer the further you go down the tail, but in the third and final section, they start getting shorter again until they eventually reach the tip.

What may be more surprising is that these aren't the only bones inside the tail. While there are some exceptions, such as elephants and (usually) humans, the great majority of mammals also have a second series of bones underneath the vertebrae. These consist of tiny arches, lined up with the discs between the overlying vertebrae, and pointing downwards in a V or Y shape. 

These are termed haemal arches or chevron bones, and they're far more significant in fish. In mammals and reptiles, however, they seem to serve two functions. In most mammals, the primary one appears to be that they surround the main blood vessels of the tail and help to protect it. But in many others, they are also anchor points for muscles, something that we see, for example, in kangaroos and prehensile-tailed monkeys.

Being small and relatively obscure, however, there has been relatively little analysis of these bones. They're not even present in most museum preparations, in part because they often don't articulate with the backbone itself, and so tend to fall away once flesh is stripped from the skeleton. A 2010 study examined the tails of South American monkeys, confirming that the haemal arches are larger and better developed in species with prehensile tails and that the vertebrae themselves retain their complex structure further down the length than they do in other monkey species. Since "prehensile" is defined here as a tail strong enough for the animal to hang its entire weight from, we can reasonably assume that this has something to do with the size of the relevant muscle attachments.

A more recent analysis attempted to extend that to a wider range of mammal species, but was unable to find much in the way of a pattern, beyond the fact that the base of the arch tends to be wide and flat in kangaroos and wallabies. Since some of these species use a "five-limbed" locomotory gait, supporting their bodies with their tail and forelimbs while they swing their hindlimbs forward, that's presumably to help distribute their weight more evenly... but the fact that wallabies that don't move like that have a similar structure may just mean that it's something they've taken advantage of rather than something that evolved specifically for that purpose.

Given that humans don't have a proper tail, it's something that can be overlooked by anatomists. My copy of Gray's Anatomy devotes three pages to the scapula, but one short paragraph to the coccyx... which is likely fair enough, in our case. Obviously, it's somewhat different for zoologists, although even there, there's at least as much attention on the tail skeleton of Brontosaurus and its ilk as there is on that of mammals.

But even in something so apparently simple, there can still be secrets to discover.

[Photo by Peter Schoen, from Wikimedia Commons.]