Why do zebras have stripes?

Why do zebras have stripes? Did you know that a zebra embryo is all black, and only acquires white stripes late in development?

• Evolutionary biologists have long pondered this. Wallace suggested that stripes provide camouflage in tall grass, but Darwin (1867) cleverly pointed out that zebras roam in open habitats where the grass is short. (Happy Birthday, Mr. Darwin!) Other theories suggest that moving stripes dazzle predators, offer visual communication in courtship or bonding, or thermoregulation (apparently stripes work better in cooling the animal than solid color). Oh, yes, stripes are fashion forward too! (Thank you Fake Science!)

• New research points to an unexpected advantage: protection from horsefly bites. Blood sucking horseflies are more than a nuisance. They spread disease and have negative fitness impact. Horseflies are strongly attracted to horizontally polarized light (waves aligned horizontally), like light reflected from bodies of water, where they lay their eggs and reproduce. Researchers showed that white and black polarize light differently, such that zebra stripes confuse the flies. Using horse models painted in different colors they showed that the best effects in keeping away flies were with natural stripe patterns found in zebras. Leopard spots are so passé.

Sources: http://jeb.biologists.org/content/215/5/iii

http://fakescience.tumblr.com/post/1517534535/why-do-zebras-have-stripes #ScienceSunday curated by Allison Sekuler and Robby Bowles .

This entry was posted in Rajini Rao. Bookmark the permalink.

34 Responses to Why do zebras have stripes?

  1. Rich Pollett says:

    Love the fake science, always get a laugh.

  2. Rajini Rao says:

    Yes, they’ve had some zingers in the past 🙂

  3. Tom Lee says:

    How about ladies who wear zebra stripe like outfits. What do they try to stay away from? 🙂

  4. Dan Bennett says:

    Fake science is my new religion.

  5. Rajini Rao says:

    Social amoebas, Tom Lee (see comment on my last post) 🙂

  6. Interesting studies. However, I am a bit bemused by the clause, “white and black polarize light differently”, considering black is about light being absorbed, not reflected. Could it be about some geometrical feature of the pigment molecules and how they are aligned in the hair? Or the flies are seeing some other frequencies, and black is not black to them? Or, hmm, maybe that simply means that the white is polarized and the black is not?

  7. I saw/heard/read that it also served to confuse individuals into the herd, so that it’s difficult to single one out for attack.

  8. Rajini Rao says:

    stefan jeffers , my understanding was that black does polarize light and the brightness of the white reflects. The combination, particularly with narrow stripes, is difficult for the horsefly to see. The black stripes can reflect highly polarized light which might be attractive to polarotactic tabanids if the polarizing black surface were not fragmented by unpolarizing white stripes. I’ll be happy to send the pdf to anyone who offers to decipher this better for us!

  9. Rajini Rao says:

    That makes more sense, Daniela Huguet Taylor , than a simple camouflage effect. Confuses the predator, I’m guessing.

  10. Exactly, if you see them all together, it is difficult to distinguish each one, with so many stripes breaking up the contours. 🙂

  11. black with white stripes or white with black stripes?? I guess that’s finally answered at least, LOL

  12. So to confuse their (micro-)predators. What about okapis? (en.wikipedia.org/wiki/Okapi) Do they also have stripes for the same reason? then why only in their legs and rumps? Don’t horseflies (or tse-tse flies) bite on their backs? perhaps a tradeoff between camouflage (above tall grass) and protection against bites (bellow the surface of tall grass)?

    Also, if other potential preys don’t have this stripe pattern, does it mean that for them it’s more useful to hide their bodies than avoid micro-predation? Perhaps zebras are so large that they wouldn’t be capable to hide from predators anyway (specially behind the short grass).

    And the different stripe pattern of each zebra species and subspecies? are all those as efficient protecting against fly bites? And why the stripes are horizontally arranged in the legs and rumps and vertically in the rest of their body?

    1. Quaga: http://en.wikipedia.org/wiki/Quagga

    2. Burchell’s Zebra: http://en.wikipedia.org/wiki/Burchell%27s_Zebra

    3. Grant’s Zebra: http://en.wikipedia.org/wiki/Grant%27s_Zebra


    4. Chapman’s Zebra: http://en.wikipedia.org/wiki/Chapman%27s_Zebra




    5. Grévy’s Zebra: http://en.wikipedia.org/wiki/Gr%C3%A9vy%27s_Zebra

    (1 to 5 the most distinct stripe patterns)

  13. Jeff Stump says:

    Wow, that is interesting. However, it is hard to fathom that horseflies (and the disease they carry) are the primary selective pressure responsible for pushing the Zebra’s gene pool towards the development of stripes on the body. I don’t know much about horseflies, so forgive my ignorance; but how much do horseflies impact fitness? Does the disease often lead to death of the host?

    If this is the case, then it is very interesting to note which defense the population of Zebras evolved first–i.e. why didn’t this selective pressure lead to a more profound immune response to the particular disease agent? Why stripes? Was the striped mutant gene already present in the population by chance and the immune response wasn’t?

  14. Rajini Rao says:

    Very good questions, Zephyr López Cervilla ! Forgive me for being lazy and posting this paragraph from the JEB paper that answers some of your questions:” All three zebra species have the narrowest stripes and the thinnest skin on their head and legs (Figs 1, 2, supplementary material Fig. S4), where the stripe widths are so small that they effectively do not attract tabanid flies (Fig. 3). This phenomenon may reflect an evolutionary adaptation. In the head, there are several sensory organs (eyes, ears, tongue, muzzle), the efficient functioning of which is most important for survival. The legs also are indispensable to escape from predators. Consequently, head and legs must be protected in the best possible way from blood-sucking parasites (e.g. tabanid and tsetse flies), since any injury to these body parts due to aggressive biting insects might result in their insufficient functioning, undermining the escape and survival of the animal. Furthermore, in the head and legs, the blood vessels can more easily be reached through the thin hide, and a more efficient protection is therefore urgently needed for these body parts. We suggest that the numerous narrow stripes on the head and legs of zebras may serve such a visual protection. ”

  15. Rajini Rao says:

    Hi Jeff Stump , quoting from the paper: Horseflies, or tabanids (Tabanidae), are vectors of several dangerous pathogens (Foil, 1989; Hall et al., 1998) and, if irritated by them, horses and cattle cannot graze, the consequence of which is the reduction of their body mass and milk production (Hunter and Moorhouse, 1976; Harris et al., 1987; Lehane, 2005). This suggests negative fitness consequences for animals that attract tabanid flies. Depending on the geographical distribution of different tabanid species in the vicinity of zebras in Africa (Uscher, 1972), these blood-sucking flies can also cause serious health problems for such equines (Leclercq, 1954; Kingdon, 1979; Moss, 1982; Leclercq and Maldes, 1987; Churcher, 1993; Leclercq, 2000; Tegegne, 2004).

    But you raise a good point: is this sufficient advantage to select for stripes? The authors conclude that other explanations are valid too. They also wonder why eurasian counterparts of zebras, i.e., horses have lost their stripes (apparently they once had stripes!) despite being in fly infested areas too.

  16. Did you know that a zebra embryo is all black, and only acquires white stripes late in development?

    well I didn’t but it seems traffic engineers knew it all along.. explains why they apply white paint over asphalt surface


  17. I’ve remembered something that may help explain the differences in color pattern between zebras and other herbivores that most probably will also suffer from fly bites (horseflies or tse-tse flies). Horses (also camels, and in some extent also cows) are among the few groups of mammals that need to sweat to cool down their bodies (and also humans even more efficiently), probably because their body is too voluminous to prevent from overheating without any efficient cooling mechanism when they need to run a long distance.

    So we have a large animal that sweats a lot with a stripe pattern as protection against micropredators, and other smaller and thinner herbivores with a camouflage pattern against macro-predators, all living together. And,

    “Furthermore, in the head and legs, the blood vessels can more easily be reached through the thin hide, and a more efficient protection is therefore urgently needed for these body parts.”

    So perhaps the other herbivores don’t need stripes because they have a thicker skin and longer hair that partially protect them from the bites, whereas the zebras need a thinner skin, shorter hair and a extensive superficial network of capillaries under their skin to facilitate cooling their bodies efficiently by sweating.

    Additionally, it’s likely the sweat will “help” attract those flies.

    So this could help explain why horseflies are called like that, i.e., why they prefer to bite not only zebras, but also other equids such as horses. Because they all have a thinner and more vascularized skin to help them cool down by sweating, although unlike humans most of their sweat doesn’t come directly from blood. Sweat glands of equids are apocrine glands whereas most human sweat glands are eccrine glands.

    This cooling mechanism also means zebras are more dependent on water (they usually need to drink water once a day) whereas other smaller herbivores such as small antilopes can survive without drinking water for longer periods of time or even never.

  18. Rajini Rao says:

    Fascinating facts, Zephyr López Cervilla ! Thanks for bringing all this together for us.

    I like the visual on the zebra stripes in your comment, Bhargav Joshi 🙂

  19. Jeff Stump, the immune system can do little against eukaryotic parasites. For instance, all these herbivores are parasitized by Trypanosoma in endemic areas. The protection of the immune system in these cases relies on mechanisms of the innate immune system and the humoral reponse activated by immunoglobulins E (IgE), the same that causes allergies. The innate reponse is the most ancient defense mechanisms shared by all vertebrates that can’t be easilly improved by a few mutations but rather the other way round, these have been highly preserved along evolution. In these cases the innate immune system have little specificity, they recognize general patterns associated to a wide array of different pathogens. In the case of eukaryotic parasites the humoral repone of their hosts can limit the extension of the parasitization but it won’t get rid of them. That’s why the African wild animals won’t usually die of sleeping sickness whereas the domestic cattle brought by the Europeans are much more susceptible and will eventually die if they’re raised in an endemic area and tse-tse flies haven’t been eradicated from that area. Even so, the heavier the parasitization in the wild animals, the greater the negative effects of the parasites on their biological fitness.

    Edit: actually, in the case of Trypanosoma adaptive response of the adaptive immune system (cellular response: T cells and humoral response: B cells releasing immunoglobulins, and the content released by basophills and mast cells) plays a role limiting the extension, but Trypanosoma has evolved a great capacity to present different antigenic determinants on its surface, not by mutation but by simple expression of different set of molecules on its surface. A great fraction of its genome encodes for surface proteins that can be periodically replaced by others on the parasite, thus avoiding its complete clearance by the immune system. On the other hand, the humoral response of the immune system has a more important role in other kind of eukaryotic parasites such as ecto- and/or endoparaitic round worms, trematodes, and some endoparasitic protozoans.

    The basic idea is the same, the immune system can’t clear the parasitization, but in the case of local wild mammals, these are sufficiently adapted to live with the parasites, whereas domestic animals and humans can’t and will probably die if parasitized (it also depends on the particular sub-species of Trypanosoma brucei, one of them has worse prognosis than the other).

  20. J Stasko says:

    It goes against what we’ve surmised from the Venus de Willendorf artifact. Slim being chic is a consequence of the industrial age. Didn’t these people read Candide?

  21. J Stasko says:

    +Rajini Rao Send the pdf!

    Did you know that a zebra embryo is all black, and only acquires white stripes late in development? (Is this due to selective apoptosis of melanin-producing cells, similar to the way our fingers develop?)

  22. J Stasko, that’s an interesting question. I’d also like to know how their stripe pattern is specified and generated. Is it controlled in some way by the segmentation process during the embryonary development, or otherwise it doesn’t keep any correspondence at all? What is what makes the boundaries between stripes become so neatly defined and traced along a curved path so regular?

    However, I doubt that the embryonary development of zebras has been well studied if anything at all. Those are large and wild animals, what will make it complex and expensive to work with them. I guess that they could discover that their hair coat is initially black from studies with miscarried embryos and fetuses obtained from pregnant females living in the wild or kept in captivity (e.g., from zoos). A similar case will probably occur with horses even though horses are domestic animals, that is, easier to breed and keep in captivity. How did they learned that horse embryos also present at some point a similar stripe pattern? I guess that from sporadic findings in miscarried embryos.

  23. Rajini Rao says:

    J Stasko , thanks for the offer to read the original paper and decipher the polarization aspects for us! I’ve sent you a limited post with my email ID, email me so I can reply with the pdf file.

  24. Rajini Rao says:

    Interesting that there is a lot of research on the development of striped pigmentation in zebra_fish_, though! There was even a paper titled “How the zebrafish gets its stripes” 🙂

    See http://www.ncbi.nlm.nih.gov/pubmed?term=pigment%20stripes%20development

  25. Perhaps the mechanism that determines boundaries between stripes operates in a similiar fashion in zebra and zebrafish, but it hasn’t to be necessarily the case unless in both species such mechanism is regulated in some unexpected way by homologous processes of their embryonary development.

  26. J Stasko says:

    How many stripes are there? Is this related to the number of segments in the animal, is it associated with dermatomes?

  27. Each segment could generate more than one stripe, however, the stripes on the limbs and face would require to be determined in a different fashion.

    Alternatively, the stripe pattern could be following the migration pattern of the cells of the neural crest that give rise to the melanocytes. In that case, the different stripes would be generated by the advance of the wavefront of those cells at certain periods of time, by some means their inactivation (and future apoptosis) would become synchronized with all the other cells that had migrated a similar distance. Another interesting point to be determined is how those melanocytes that have been inactivated are rescued in horses to remove the original stripe pattern. Or perhaps there’s another later migration wave?

    In another vein, I ‘ve had a look at some pics of tigers, and the arrangement of the stripe pattern is rather similar to the zebra’s (except for the rump). I wouldn’t be surprised if both used the same mechanism of determination to arrange the stripes along their body.

  28. J Stasko says:

    Gosh, and I was just thinking of putting my embryology book up for sale.

  29. Rajini Rao says:

    Good job, JEFFREY COSGROVE , for finding exactly the same research story that this post is linked to 😉 But, yes, the combination of polarizing light from the black and reflection from the white fails to attract the tabanids (horseflies).

  30. I’ve counted the number of stripes of one color that arise from the vertebral column area, and its number roughly corresponds to the number of vertebrae in each section, at least the 7 cervical vertebrae and other 17 vertebrae (12 thoracic and 5 lumbar vertebrae). The hypothetical stripes corresponding to the 5 sacral vertebrae can’t be spotted in the pics I’ve found (the sacral area doesn’t seem to be a favorite area to take in the pics).

    In any case, this correspondence doesn’t give any clue on how the thinner stripes on the face, the legs and the tail are generated, nor the wide, almost horizontal stripes on the hindquarters.

    Here you are the pic of a flank of a zebra:


    And the diagram of the skeleton of a horse (a close relative of zebra):


    Note that some stripes merge together on the flanks, and a few ones even close to the backbone (on the lumbar and sacral regions).

    It’d be interesting to find some pic showing directly the pattern on the backbone and the belly.

    If I had to propose a hypothesis, I’d say that the imprinting for the pigmentation pattern (either pigmented or depigmented/apoptotic) is already established in each segment when the precursors of the pigmentary cells are still in the neural crest. Then they migrate from there to cover all the skin. Initially all those pigmentary cells start synthesizing pigment (the hair coat of zebra fetuses is dark without any stripes). At some point, the cells that originated from the spots imprinted to be depigmented/apoptotic cells will stop synthesizing pigment or die by apoptosis. Thus, each white stripe that arises from the backbone follows the pathway that the precursors of the pigmentary cells had traveled from the imprinting spot for deactivation/apoptosis of a particular segment.

  31. Rajini Rao says:

    Simply awesome, Zephyr López Cervilla ! My embryology/developmental biology is pretty rusty (perhaps I should purchase that text from J Stasko !). Anytime you want to come to Johns Hopkins for a spot of research, let me know 🙂

  32. Ross Straud says:

    recent bio research suggests that zebras were black and acquired white stripes..gradually..

    their recent colouration only seems to produce one discernable benefit … oddly.. it protects them from flies???? true..

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s