The development of striping patterns

Few patterns are more obvious than the alternating black-and-white stripes of the zebra (Figure 1). There are actually three extant species of zebra, and each has a different pattern of stripes. The imperial zebra (Equus grevyi) has some eighty stripes perpendicular to the long axis of its body. The common zebra (E. burchelli) has 26 wide caudal stripes, some of which extend towards the belly in the rear of the animal. The mountain zebra (E. zebra) has some 55 stripes, with three horizontal bands near the hindlegs. Each of these three species are members of the horse genus and can interbreed among themselves and other horses to produce infertile offspring.

Figure 1. Different species of zebra. (a) The imerial zebra (Equus grevyi), (b) the mountain zebra (Equus zebra), (c) the common zebra (Equus burchelli), and (d) the quagga (Equus quagga) from a drawing. Photograph from Bard, 1977, with permission of the author.

How did the zebra get its stripes? Ultimate (evolutionary) mechanism:

It is generally believed that zebras are dark animals, with white stripes where the pigmentation is inhibited. The pigment of the hair is found solely in the hair and not in the skin. The reasons for thinking that they were originally pigmented animals are that (1) white horses would not survive well in the African plains or forests; (2) there used to be a fourth species of zebra, the quagga (which was overeaten to extinction in the eighteen hundreds). The quagga had the zebra striping pattern in the front of the animal, but had a dark rump; (3) when the region between the pigmented bands becomes too wide, secondary stripes emerge, as if suppression was weakening.

Zebra stripes have often been thought to be an adaptation that prevents zebras from being seen by predators such as lions or hyenas. (This hypothesis goes back at least to Rudyard Kipling [1908]). The alternating stripes obscure the outline of the zebra. This may serve as camoflage, allowing the zebra to blend in with its backgound (Thayer, 1909; Marler and Hamilton, 1968) and/or it may serve to confuse a predator as to the distance of the fleeing animal (Cott, 1957; Kruuk, 1972). However, neither of these hypotheses can be easily confirmed. A different hypothesis (Waage, 1981) contends that the stripes serve to obliterate a large single-colored region that is favored by biting insects such as the tsetse fly. These flies prefer large, dark, moving animals (Vale, 1974).

How did the zebra get its stripes? Proximate (developmental) mechanism:

Jonathan Bard of Edinburgh has hypothesized a mechanism for the production of zebra stripes in the three species of extant zebra. His model claims that while neural crest cells begin migration at week two of gestation (in the horse), the zebra striping patterns are generated between weeks three and five, depending upon the species. Moreover, Bard asserts that the three patterns of striping are precisely those predicted if the original pattern was the same in each zebra, but was established at different times within this three week period. In the case of the imperial zebra, all the stripes are perpendicular to the dorsal axis, but are thicker towards the neck. This would be expected if the striping pattern originated at week five (Figure 2A). At week five, most of the differential body growth has ceased, except for the neck region, which becomes extended, and the rump, which is slightly shortened. Thus, if the stripes were formed at week 5, they should all be parallel, but slightly wider at the neck and slimmer at the rump.

The stripes of the mountain zebra probably form towards the end of week 4. If the stripes were originally parallel, those in the rear of the embryo would be pulled back towards the rump by the growth of the hindparts of the horse (Figure 2B). Similarly, if the stripes of the common zebra were generated during the third week of zebra gestation, the differential growth rate of the rump between weeks three and four would also pull the stripes posteriorly (Figure 2C).

Bard's hypothesis that all the stripes originally are the same width and are generated at different times in the three species also explains the numbers of stripes in each species. The common zebra has 26 stripes per side, and the three week Equus embryo is generally 11 mm long. This gives a spacing of about 0.42 mm per stripe. If the 43 stripes of the mountain zebra were generated in the 17 mm embryo of the 3.75 week zebra, the spacing is also 0.40 mm per stripe. At week 5, the embryo is 32 mm long, and the 80 stripes would yield the spacing of 0.40 mm per stripe. Therefore, the striping patterns of the common zebra, mountain zebra, and imperial zebra can be explained if the stripes are generated 0.4 mm apart in the 3, 4, and 5 week embryos, respectively.

FIGURE 2. Bard's hypothesis for the generation ofstripes in three species of zebras. The spacing and size of the stripes are the same. What differs is the time at which the stripes were generated. If generated during week 3, the stripes begin perpendicular to the anterior-posterior body axis, but become parallel to this axis in the rump, since the rear of the zebra is still growing. This generates the pattern of common zebra. If the striping pattern is generated on week 4, most of the rump has grown, and the hind stripes are more perpedicular to the body axis. This generates the pattern seen in the mountain zebra. If the striping pattern is generated on week 5, there is space for many more stripes, all of which are perpendicular to the body axis. This generates the striping pattern of the imperial zebra. (After Bard, 1977.)

It is not known how the pattern is initiated or what activators or inhibitors are being generated. It is difficult to imagine how such a pattern can be generated by preformed maternal instructions, responses to gradients, or regional inductions. It has been proposed that the Turing reaction-diffusion models could produce these alternative pigmented and non-pigmented bands. Murray (1981) has shown that the chevrons at the base of the zebra's limbs is the shape expected by the overlapping of two Turing-type reaction-diffusion systems.

References:

Bard, J. B. L. 1977. A unity underlying the different zebra striping patterns. J. Zool. (London) 183: 527 - 539.

Bard, J. B. L. 1981. A model for generating aspects of zebra and other mammalian coat patterns. J. Theoret. Biol. 19: 363 - 385.

Cott, H. B. 1957. Adaptive Colouration in Animals. John Dickens. Northampton.

Kipling, R. 1908. Just So Stories. Macmillan, London.

Kruuk, H. 1972. The Spotted Hyena. University of Chicago Press, Chicago.

Marler, P. and Hamilton, W. J. 1968. Mechanisms of Animal Behavior. Wiley, New York.

Murray, J. D. 1981. A pre-pattern formation mechanism for animal coat markings. J. Theoret. Biol. 88: 161 - 199.

Thayer, A. H. 1909. Concealing Coloration in the Animal Kingdom. Macmillan, New York.

Vale, G. A. 1974. The response of tsetse flies (Diptera, Glossinidae) to mobile and stationary baits. Bull. Entom. Res. 64: 545 - 588.

Waage, J. K. 1981. How the zebra got its stripes: biting flies as selective agents in the evolution of zebra coloration. J. Entom. Soc. South Afric. 44: 351 - 358.

Source: Zygote