Saturday, 30 January 2021

The Deer Family

Doe (a deer, a female deer)
A little over eight years ago (blimey...) I began a series on members of the goat subfamily, and started it off with a discussion of the difference between horns and antlers. In short, horns are permanent structures, comprised of a central bony core surrounded by a sheath of um... horny material. Antlers, on the other hand, are regrown every year, and once they reach full size and shed the velvet on their outer surface, they consist solely of dead bone, with no sheath of any kind. Antlers are also often branched, whereas 'true' horns never are.

Antlers are, of course, the key defining feature of the deer family, the Cervidae. They are found on (almost) every species in the family, although (almost) only on the males. A large stag with branching antlers is instantly identifiable as a deer, but it may be fair to say that some of the species with unbranched antlers do have a certain resemblance to some of the smaller species of antelope.

Indeed, deer and antelope, if not quite each other's closest relatives, are very nearly so and share a considerable number of common features. And, since 'antelope' is just a general term for any member of the cattle family that's not obviously a cow, goat, or sheep, that means that deer also have a lot in common with more widely domesticated species.

Deer, for example, are cloven-footed herbivores with a four-chambered stomach, structured just like that of a cow. A combination of a similar diet and a close relationship means that their teeth - often a diagnostic feature in mammal families - are remarkably similar. Like bovids, deer have what look like four clipping incisors at the front of the lower jaw, followed by a toothless gap that enables the cheeks to hold food in place while chewing, then a set of six grinding teeth on each side of each jaw that have ridges shaped like a crescent moon. 

In fact, the fourth 'incisor' is actually a canine tooth, which has lost its distinctive shape over the course of evolution, as the deer has no need to use it to stab into meaty prey. Also like bovids, there are no incisors in the upper jaw, just a horny pad that acts like a chopping board. One of the few differences between the two families is that some species of deer do have canine teeth in the upper jaw, which bovids never do.

In general, deer are long-legged animals with a sleek body and a short tail. There is some variation in the relative proportions of the legs depending on whether the animal is better adapted for running, as the larger species tend to be, or for leaping, as the smaller ones are. Both are, of course, adaptations to escaping from predators, aided by the fact that deer have excellent hearing and sense of smell. The latter is important, not just to spy out danger, but also because, like bovids, deer have a number of scent glands on their faces, limbs and below the tail, which they can use to leave signals for one another.

Their eyesight isn't thought to be quite as good overall, but is highly tuned for the detection of movement. For that matter, their night vision is better than that of humans, and there is some evidence that they may have limited colour vision.

Deer have a polygynous mating system, where a small number of males mate with a large number of females and the other males lose out; the imbalance tends to become more extreme in larger species. This means that males have to compete with one another, which in turn means that they tend to be much larger than the females. And, of course, this is also the purpose of the antlers.

Antlers are remarkable things. No other mammal carries around quite such a large structure composed of entirely dead tissue. In addition to being of use in fighting, the ability to be able to grow something so large - and do it from scratch every single year - shows the male's physical fitness and that he can afford to divert calories and mineral nutrients from elsewhere in his body and still remain healthy. Antlers grow from a 'pedicle' of permanent, living, bone at the base and do so with remarkable speed - up to 2cm (one inch) per day in some of the larger species. 

It's no wonder the females don't waste time growing their own set.

The process is driven by testosterone levels, which surge while the antlers are growing, and fall to almost zero when it is time for them to drop off. Indeed, stags castrated before adulthood never grow antlers. In a series of experiments conducted in the 1940s, researchers showed not only that injecting testosterone into castrated male deer caused them to grow antlers, but that exactly the same thing happens if you inject it into spayed females.

The fawns of most species are spotted

Besides the antlers, there's one other unique anatomical feature of deer that distinguishes them from other ruminants, although unfortunately, it's not one we can detect in fossils: deer have no gallbladder. It's not clear why, and the lack doesn't seem to bother them.

When Carl Linnaeus published the first scientific catalogue of animal species in 1758, he knew of the existence of just six species of deer. Five of these lived in Europe, and over the following decades, many more species were named from Asia and the Americas. (Deer, it turns out, never had much luck in Africa, although they haven't been entirely absent [edited, see comments]). Georg Goldfuss officially named the deer family in 1820, by which time 20 of the species we still recognise today had already been described. More, unsurprisingly, followed.

This profusion of species led to various proposals as to how the deer family could be subdivided, initially for convenience, and later to try and reveal evolutionary relationships. Perhaps the first of these schemes to try to do so on a proper scientific basis was proposed by Sir Victor Brooke in 1878. He placed all known species of deer into one of two subfamilies, on the basis of the bones in their front legs.

Being cloven-footed animals, deer walk on only the third and fourth digits of each foot - in this case, the equivalent of the middle and ring fingers. But the other toes aren't entirely missing. They have no thumbs (or big toes, on the hind feet), but the other two digits do remain, in the form of "dewclaws" at the back of the feet, which don't reach the ground.

But what appear to be the ankles of a deer's forefeet are in fact the base of the digits, while what look like the knees are actually the wrists (the elbows are further up, closer to the shoulder). This means that the lowest segment of a deer's front leg is really equivalent to the palm of the hand in a human and, like a palm, contains metacarpal bones that connect the fingers to the wrist bones. With two functional toes, there are also two complete metacarpals in this part of a deer's leg, along with two bony remnants associated with the non-functional dewclaws.

But what Brooke noticed was that these remnants always take one of two different forms. In one group of deer found mostly (but not entirely) in the Americas, the remnant is attached to the dewclaw, but doesn't reach all the way to the wrists. In the other group, found mostly (but not entirely) in Eurasia, the remnant is at the other end, attached to the wrist, but not reaching the dewclaw - and sometimes is very tiny indeed. These, he said, defined two different subfamilies of deer, with separate origins. It might seem odd that he ignored the fact that not every species with a particular pattern lived on the same side of the Atlantic, but he was able to identify other features that each group seemed to have in common in the bones of the nose and the position of the scent glands near the ankles on the hind feet.

Over the following century, classifications came and went, some based on Brooke's scheme, but others not. Nonetheless, by the 1980s, there was general agreement that he'd largely been right, with all but one of the species he knew about (and many described since) placed into two subfamilies. His original naming scheme had had to be ditched, because the rules had been codified around a different principle since, but we had two main groups: the cervine deer, such as the red deer of Europe, with the bony remnant at the top, and the capreoline deer, such as the white-tailed deer of America, with the remnant at the bottom. 

Plus one exception that got its own subfamily on the grounds of being weird in ways that weren't related to its feet.

Then, in the 21st century, came the possibility of detailed genetic analysis that could give us a clearer insight into whether this was real or just parallel evolution. And we now have multiple studies confirming that, yes, back in 1878, Brooke had got it right. Despite the fact that some members of each group have evidently crossed over the Bering land bridge at some point, the cervine and capreoline deer really are distinct groups that last shared a common ancestor around 10 million years ago, during the Late Miocene. Even that one weird exception turns out to be just another cervine deer with some unusual adaptations and no longer gets to be off on its own.

We currently recognise over 50 different living species of deer across the world, a number of which have only been identified recently as genetic evidence has forced us to elevate what had previously been considered to be mere subspecies. This is too many to cover in the sort of detail I usually do, but nonetheless, over the coming year, I will try to say something about every one of them...

[Photos by Johann Jaritz and BS Thurner Hof, from Wikimedia Commons. And, yes, if you're wondering, there will be plenty of opportunity for pictures of deer that have antlers in future posts!]

2 comments:

  1. I thought the 'Barbary Stag' was native to the Atlas Mountains in North Africa? It's nearly genetically identical to the Corsican and Sicilian red deer populations, which means it's likely there was one original population that was introduced by man to the two other localities. It's distinct enough from other red deer populations to possibly warrant it's own place in Cervus. Cervus corsicanus.

    It looks like there was a Megaloceros type deer in the Atlas Mountains in the Late Pleistocene-early Holocene. Megaceroides algericus.

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  2. I'll be talking more about the Barbary stag in the next post, but I think you're right and I'll rephrase. Although it has been argued, I believe (perhaps not with strong evidence?), that C. corsicanus might have originated in mainland Italy and been introduced to the places it now lives by the Romans.

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