|Mountain gazelle (probably)|
You might suppose that the answer is something along the lines of 'do they cross-breed to produce fertile offspring or not?' Horses and donkeys are different species, you might think, because, while they can interbreed, the result is a sterile mule. Similarly, tigers and lions can also cross-breed, but the results are usually infertile. (Actually, fertile mules and "ligers" do exist, but they seem to be rare). If they don't cross-breed at all, of course, the answer is even simpler. So, take the two gazelles you're wondering about, mate them, and see what happens. Right?
Well, not really. For one thing, that's much easier said than done, especially if one of the possible species is endangered. It's hard enough getting pandas to mate with other pandas to be sure that mere lack of mating proves anything at all. Not only that, but there are, believe it or not, a number of cases where animals that are clearly different species can, indeed, interbreed to produce fertile offspring. Polar bears and grizzly bears, for instance, to give one example becoming more common of late.
As a result, in practice, we have to use different means to determine if two animals belong to the same species or not. Such methods often rely on the physical appearance of species, and, increasingly in modern times, genetic and biochemical data. In a sense, we aren't so much looking for whether they can intebreed, but for whether or not they actually do. If there's no evidence that they're mixing genes, even where they live in the same area, then they may well be distinct.
There is, however, inevitably some degree of opinion involved in this. And we can see this when we try to count the 'official' number of species of some group of animal. This has already cropped up more than once in my series of posts on goat-like animals, and I'm by no means done with that yet.
So what about the gazelles? As I've said, the IUCN says there are sixteen (not counting one it lists as definitely extinct). Mammal Species of the World, one of the most commonly used reference works, says that there are fifteen - again, not counting the extinct one, which was incorrectly thought to still be alive in captivity when the curent edition was written. The latest edition of Macdonald's Encyclopaedia of Mammals says there are eleven. I've even seen a list that says there are thirty one (!)
[I am, for these purposes, considering a "gazelle" to be anything from the genera Gazella, Eudorcas, or Nanger. There are broader definitions one could use, most notably, perhaps, including Procapra. But I haven't.]
I'm not going to attempt to sort this out today. Perhaps if I ever cover gazelles in the same way I'm currently describing goats, I'll take a stab at it, but not now. Instead, I want to look at one particular point of dispute: the Arabian gazelle (Gazella arabica).
Arabian gazelles were first "discovered" by Christian Ehrenberg and Wilhelm Hemprich on an expedition to the Middle East in 1825. They brought some specimens back to their sponsor, the Museum für Naturkunde in Berlin. In 1827, the director of the museum, Martin Lichtenstein, wrote up a formal description of the specimens, describing them as a species new to science, and gave them the name Antilope arabica. They were said to live from the Sinai to Lebanon, along the eastern coast of the Red Sea, and on the Farasan Islands. It was later decided that "Antilope" wasn't really specific enough, and they were moved to their present location in Gazella.
Nobody seems to have thought much more about it for the remainder of the nineteenth century. There were unquestionably gazelles in Arabia, they had specimens of gazelles collected there, and there doesn't seem to have been much dispute that what they had was a species. In 1906, Oscar Neumann went so far as to divide the Arabian gazelles into three subspecies, although this didn't really catch on. As early as 1873, however, it had become clear that there were other gazelles living in the same area, and, in the early twentieth century, at least some scientists began to wonder if the Arabian gazelle was really a species at all.
In 1935, British zoologist Reginald Pocock took another look at the specimens and decided that they were actually just a subspecies of mountain gazelle (Gazella gazella). In 1968, A.W. Gentry instead considered them a subspecies of Dorcas gazelle (Gazella dorcas), a species found across the Sahara, but otherwise absent from Arabia. Many other scientists agreed with one or the other of those two opinions, while others considered them still to be distinct. In recognition of this confusion, and the lack of hard facts, the IUCN officially lists them as "Data Deficient", meaning they have no clue as to how well they're doing - although they also note that, if they ever existed at all, they may well be extinct.
Part of the reason for this confusion was the lack of specimens to look at. Certainly, there are gazelles in Arabia, but the ones that people had checked all seemed to be either mountain gazelles or sand gazelles (Gazella subguttarosa marica). If only we had an animal that was indisputably, definitely, an Arabian gazelle, at least we could tell what sort of creature it actually was.
This is a known problem in zoology, and we have a system in place for dealing with it. When a new species is described, the scientist naming it has to designate a type specimen, one single individual animal that everyone from then on can point at and say "this is what he meant". The problem is, back in 1827, the rules weren't so strict, and Lichtenstein had actually used two different specimens - one male, and one female - as the "type".
Fortunately, in 1906, when he was describing his purported subspecies, Oscar Neumann had identified this problem, and fixed it. When you're dealing with these really old descriptions, with multiple type specimens, you're allowed to go back and pick one of them as the "real" type. If you write it up properly, that one sticks - it's technically called a "lectotype" - and you can ignore the other one.
Neumann picked the male, because, of the two, it was the one with the most distinctive appearance. All we have of it is the skull and the skin, but the skull, in particular, does look different from that of any living gazelle. And it's still in the same museum, so we can look at it, and figure out what it really is.
The skull does not, to the expert eye, look like that of a normal mountain gazelle. But even in the 1980s, it had been suggested that maybe it was just an odd-looking individual. Maybe it was even slightly deformed. However, now we can do with confidence what they couldn't do then, and they certainly couldn't do in 1906: we can check the specimen's DNA.
Last year, doctoral student Eva Bärmann and colleagues did just that. They took tiny samples of tissue from the specimen's skin and the inside of the skull, and compared their genetics with that of blood samples taken from mountain gazelles, Dorcas gazelles, and sand gazelles. What they found was a bit of a surprise.
The hide and the skull aren't from the same animal.
Somebody in the 1820s had screwed up, and put specimens from two different animals together in one box, labelling them as if they were from the same individual. (In retrospect, this sort of makes sense: Hemrpich and Ehrenberg had said they collected four specimens, but the museum only seemed to have three. Now we may know what happened to the other one).
Comparing them with the modern blood samples, it seems that both individuals were mountain gazelles. So, case closed: the Arabian gazelle never existed. They were all just mountain gazelles all along.
Except... not quite so quick, because there's a sting in the tail.
See, in 2010, Torsten Wronski and colleagues had performed their own genetic analysis of mountain gazelles, to see how they varied across their geographic range. They found that there are two distinct genetic subtypes of mountain gazelle, one in Arabia proper, and one further north. From the new analysis, the skin of the lectotype specimen is from an "Arabian" mountain gazelle, and the skull from a "northern" one - presumably one of several that Hemprich and Ehrenberg said they'd shot in what is now Lebanon.
And these two genetic subtypes are so distinct, and so different, that, according to both Wronski and Bärmann, they should really be classified as two separate species. The type specimen of the mountain gazelle comes from Syria, so it's the northern group that's the "real" mountain gazelle. To name the other one, one of the first questions we'd have to ask is this: does there exist, somewhere, a specimen belonging to this southern group that somebody, at some point, thought was a distinct species, and has already given a name to?
If this southern group of mountain gazelles - the ones living in Arabia - genuinely is a new species, there really is only one name we're allowed to give it...
[Picture by "Bassem18" from Wikimedia Commons]