New idea re evolution of plains zebra
Here is a new idea about the evolutionary differentiation within the plains zebra (Equus quagga).
The problem is this: why is it that subspecies of the plains zebra differ so greatly in colouration across Africa?
This is a puzzle whether one is a lumper, regarding all forms including the extinct quagga, or a splitter, regarding the plains zebra as consisting of about five different species (quagga, chapmani, crawshayi, boehmi, and borensis).
It is difficult to think of another species, or even ‘superspecies’ of ungulate which shows as great a variation in colouration as the plains zebra complex.
This variation has often been interpreted, rather wishfully, in the literature as some sort of ‘cline’. However, this is ambivalent. True, there seems to be some sort of gradation from quagga through burchellii to chapmani; and all the northern forms are boldly black and white down to the hooves.
But so many other features break any clinal pattern. For example, shadow stripes re-occur in the northernmost form. The southernmost form (quagga) has a shorter mane than its neighbour burchellii.
The densest striping occurs in the centre, not at one extreme.
No matter how much selective breeding has been tried on burchellii, there seems minimal prospect of ‘re-breeding’ the extinct quagga from it, because the results are far too pale to fit the quagga. Etc.
What emerges when we stop ‘shoe-horning’ this variation into the concept of a ‘cline’ and look at it for the bewildering complex that it really is?
Well, what emerges is a rather capricious scenario in which it seems that evolution has played ‘artistic licence’ rather than ‘environmental adaptation’.
Contrast this with the Equus hemionus, which is even more widespread in Eurasia, from Palestine to Mongolia, but consists of subspecies with only minor variations in colouration.
The extinction of the Syrian wild ass (Equus hemionus hemippus) is not nearly the tragedy that the extinction of the quagga is; this is because the Syrian wild ass might not even be distinguished from the Mongolian wild ass – which occurs thousands of kilometres away, on the other side of Asia and under a different climate - by the average interested zoologist, in contrast to the quagga, which is so odd for a zebra that it seems like some sort of chimera between zebra and wild ass.
No matter how much we might like to pretend that the loss of the quagga was a mere subspecific loss, there remains a qualitative rather than quantitative tragedy in its extinction. A whole separate form of equid was lost with the extinction of the quagga, whether the taxonomy has acknowledged this or not.
If plains zebras have colouration that is not only as bizarre as that of zebras generally, but additionally so capricious as to strain the very concept of a zebra, could there still be some sort of logical explanation of an adaptive kind?
I suggest that there might be, as follows.
The first assumption in my rationale is that, throughout the range of Equus in Africa and Eurasia, the spotted hyena (Crocuta crocuta) has been a major predator, capable of shaping the colouration of zebras, wild asses, and wild horses.
The second assumption is that, because the spotted hyena is a cursorial hunter dependent on assessment of body condition before the chase begins, the main function of colouration in zebras is that of delaying, and disruptive, the scanning process when the spotted hyena first encounters a group of the plains zebra.
I have detailed the physiological process elsewhere, and I’ve explained elsewhere why this selective pressure did not apply to Asiatic wild asses and wild horses despite the likelihood that their main predator in many areas was the very same species, the spotted hyena.
The logic goes that any kind of zebra-striping on the neck and torso will tend to protect Equus from the spotted hyena, the exact configuration mattering less than the fact of striping.
Now please note a great biogeographical difference between plains zebras and the spotted hyena: the predator was an extremely widespread species with minimal regional/geographical variation.
The skulls of the spotted hyena in Asia seem indistinguishable from those near the southern tip of Africa, other than some differences in size which seem predictable by latitude based on Bergmann’s rule. It is not even clear that the form found in or near Mongolia differed from that found near Cape Town at the subspecific level.
Certainly nobody seems to believe that subspecies of spotted hyena can be distinguished within Africa today.
Note, then, how different prey and predator are in this relationship: the prey enormously variable and in a rather geographically capricious way, the predator showing minimal variation.
I am not arguing that there is anything particularly unusual about the spotted hyena in its widespread distribution and minimal variation across a vast range.
This is actually typical of large predators worldwide.
However, it implies gene flow over large distances, contrasting with the limited gene-flow implicit in the almost unbelievable variation among forms of plains zebras, with some remarkably abrupt transitions in the absence of any significant geographical barrier, e.g. from chapmani in the Caprivi Strip to boehmi in southwestern Zambia.
Now, the process I’m invoking is that there will tend to be some ‘counter-evolution’ on the part of the spotted hyena, to at least partly ‘disarm’ the disruptive effect of the striping on its eyes.
I am not suggesting that this kind of ‘optical arms-race’ could ever completely anull the adaptive value of the striping, because after all the optical system of the spotted hyena in Africa is under a predominant selective pressure in keeping with its mainly ruminant prey – and no species of ‘plains game’ other than zebras has significant striping of the kind that could disrupt the visual system of predators by means of ‘illusory’ optical effects (I am not referring to anything resembling camouflage here; the optical ‘illusion’ involved when predator scans zebras has nothing to do with classic disruptive colouration, the ‘illusion’ of which lies in the mental, not optical, part of the visual system).
So the idea is this:
What all this extraordinary variation in configuration of striping achieves for the plains zebra complex is the prevention of even modest degrees of counter-adaptation by the spotted hyena. If some measure of counter-adaptation were to evolve in the eyes of the spotted hyena within the range of, say, boehmi, there would be a tendency for this to spread to other parts of the distribution of this species of predator, by the same genetic processes that have ensured that after millions of years all populations of the spotted hyena have more or less remained genetically similar.
But this sort of spread would be foiled, and its adaptive process frustrated, by the spotted hyena encountering a significantly different configuration of striping in another region.
This rationale does not depend on whether we split plains zebras or lump them; because in this context it matters little whether boehmi is a different species from burchellii, or merely a subspecies.
The point is that the very fact that plains zebras vary so radically in colouration, while the spotted hyenas attacking them remain genetically uniform, could mean a kind of thwarting of the otherwise likely ‘arms-race’ between predator and prey.
And there would be little point in the spotted hyena embarking on a different genetic strategy, in which it confines genetic changes to restricted areas, because in no area have zebras ever been its most abundant, or most reliable, prey.
What this rationale invokes is a kind of incongruity between zebras as prey and the spotted hyena as predator, resulting in a major anti-hyena adaptation in zebras consisting of not one but two components.
The first component is striping, which continues to be effective because it would make no economic sense for the spotted hyena to grossly re-configure its retina given that it relies on non-striped prey in the form of wildebeests and other ruminants.
The second component is apparently capricious regional variation in the striping, which thwarts even the slight re-configuration of the retina of the spotted hyena because any evolutionary gain in one part of the range of the predator would be ultimately cancelled in adjacent parts where the plains zebra looks different.
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