Referring to the above, it may be flies; we still don’t know for sure. What I wrote above was clickbait inspired by James Carville.
Four years ago (has it really been that long?), I reported about a paper in the Journal of Experimental Biology showing that, on Hungarian horse farms, striped patterns are repugnant to horseflies (tabanids), for the flies prefer to land on solid colors than on stripes. That in turn suggested that maybe zebras are striped to reduce the number of fly bites they get, bites that can not only produce a serious loss of blood, but also spread disease (see below). There were, however, problems with that paper, and so the results were very tentative.
A few days ago I reported on a paper in PLoS ONE testing an alternative theory for zebra striping: the popular idea the pattern breaks up the outline of the animals, making them less visible to predators like lions and hyenas. That theory wasn’t supported by the data, but other theories for stripe evolution remain, including confusing predators in a herd by presenting them with a mass of strange patterns, “warning coloration” (a pattern that tells animals like hyenas to “stay away because I can bite and kick you”), thermoregulation, recognition of individuals of your herd or members of your species, and, of course, reduction of fly bites.
A 2014 paper by Tim Caro and colleagues in Nature Communications seems to eliminate most explanations in favor of the “fly hypothesis”. The study I mentioned earlier, and two that have also appeared, give experimental evidnce that both horseflies and tsetse flies are averse to landing on striped surfaces. The Caro et al. paper doesn’t give direct experimental evidence, but supportive correlational evidence.
What the authors did was examine the historical Eurasian ranges of zebra species and subspecies, as well as those of other equids, and then match those up with the ranges of horseflies, tsetse flies, temperature (for the thermoregulation hypothesis) and the historical ranges of predators (lions, hyenas, tigers, and wolves). They were looking for a selective factor whose historical ranges would correspond to the ranges of the zebra before humans changed range sizes. (Stripes evolved before humans changed the face of the planet.) The factor Caro et al. looked at was not the ranges of the species themselves, but of the ranges of striping on various parts of the equid body, for it’s the stripes themselves that are thought to result from selection.
The major results were these:
a. The striping patterns of zebra subspecies and species correspond more closely by far to the ranges and climate preferences of tabanid and tsetse flies than to any other factor, although lion ranges are also associated with a few measures of striping, like leg stripes.
Here’s the association between the historical (not present!) ranges of equids and of tabanids and tsetse flies; equids at top (zebra ranges striped!) and flies at bottom. Note that tsetse flies (Glossina) aren’t found outside Africa. E. kiang is an unstriped wild ass, E. africanus is the African wild ass, having thin stripes on its legs, E. hemionus is the onager, an unstriped wild ass, and E. ferus przewalskii is Prezewalski’s horse, a rare wild horse thought to be the closest living relative of the domestic horse.
The correspondence is pretty good, although not perfect, since flies live in some areas where zebras don’t. The crucial observation, though, is that biting flies always occurred in areas where zebras lived.
Note, too, that unstriped equids don’t generally coexist with either kind of fly, though the African wild ass, which does have thin striping on its legs, does live in areas with horseflies.
Here’s another figure showing the degree of association of leg striping among the equid species with activity of tabanids in the species range. The outer circles show the intensity of tabanid fly activity (see key), the next row in gives the intensity of leg striping, and the lines show the evolutionary relationship of the species. The quagga is extinct, but is part of the same clade as zebras, showing that full body striping evolved only once (this means that the correlation between striping of the species and presence of flies may not reflect independent evolution in each group, which is a problem for the authors’ conclusion, though one they admit). But the key observation here is that stripe intensity is highest in species that experience more tabanid activity.
b. No other factor supposedly associated with striping, including group size, thermal highs, or presence of predators, was as consistently associated with striping as was the presence of flies. The authors thus rule out the species and individual recognition hypotheses, the thermoregulation hypothesis, and the predator confusion or camouflage hypotheses as forces promoting the evolution of stripes. The predator hypothesis was also largely ruled out by the paper I posted about last week (Caro was also an author of that one). This points to the fly hypothesis as the most viable one. But that raises an immediate question:
c. Can flies really be a significant selective factor in the evolution of striping? Apparently yes. The authors note that tabanids can take significant amounts of blood from horses and cows, and that flies can also carry diseases that kill equids:
At an ultimate level, blood loss from biting flies can be considerable. Calculations show that blood loss from tabanids alone can reach 200–500 cc per cow per day in the United States. For example, in Pennsylvania, mean weight gain per cow was 37.2 lbs lower over an 8-week period in the absence of insecticide that prevented horn fly (Siphona), stable fly (Stomoxys) and horse fly (Tabanus) attack, and in New Jersey, milk production increased by 35.5 lbs over a 5-week period with insecticide. Milk loss to stable flies was calculated at 139 kg per cow per annum in the United States. Similarly, blood-sucking insects have been shown to negatively affect performance in draft horses.
That’s a lot of blood and a lot of milk loss (which could, of course, reduce offspring survival). But the authors favor the disease hypothesis, although there’s no reason that both blood loss and disease could act as joint selective factors:
Alternatively or additionally, striped equids might be particularly susceptible to certain diseases that are carried by fly vectors in sub-Saharan Africa. We collated literature on diseases carried by biting flies that attack equids in Africa (Supplementary Table 1) and note that equine influenza, African horse sickness, equine infectious anaemia, and trypanosomiasis and are restricted to equids, all are fatal and all are carried by tabanids. Currently, we are unable to distinguish whether zebras are particularly vulnerable or susceptible to biting flies because they carry dangerous diseases or because of excessive blood loss, but we are inclined towards the former because Eurasian equids are not striped, yet demonstrably subject to biting fly annoyance.
Here are those nasty flies, with a blood-engorged tsetse fly at the top and a tabanid below (a video of a biting tabanid is here):
d. Stripe width in zebras, especially the thin stripes on the legs, is small enough to deter flies. The graph below shoes the limits of stripe width in zebras (colored vertical lines) versus the preference of three kinds of flies for difference stripe width. Flies like to bite less when the striped pattern is thinner, and zebras are all in the stripe range that flies don’t like. Note that the thinnest stripes are on the face and legs, and that flies like to bite on the animal’s legs, as it’s shaded underneath the belly. (Remember that the African wild ass has thin stripes on the legs.):
This is getting long, so one more point:
e. Why, among the many African grazing mammals, are only the zebras so stripey? After all, other species of horses and asses, as well as many antelopes and artiodactyls, also have to deal with tabanids too, but only zebras have stripes. The answer may have to do with hair length and density. The authors show that among grazing animals, zebras have the shortest hairs and smallest hair depth, and that enables the flies to bite more easily through the coat to the flesh. That could mean either that the zebras evolved stripes because their short coats exposed them to more biting, or they evolved their short coats because it’s advantageous to have such short coats for other reasons (perhaps thermoregulation?), and the earlier evolution of zebra stripes sufficiently deterred flies to enable the evolution of shorter coats.
That’s the upshot. I think the fly-bite hypothesis is a good one, and now has both experimental and correlational evidence to support it, but there may be other advantages to being striped, like helping you find other zebras when you’re lost (granted, the authors found no correlation between stripiness and zebra group size). One crucial experiment, which can’t be done, is to release equids dyed with stripes into zebra territory, along with unstriped controls, and see if the former suffer fewer bites. Or dye zebras gray and see if they get bitten more often. For the time being, though, we’ll just have to say that we’re approaching the explanation for zebra striping slowly and asymptotically.
h/t: Diane Morgan
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