Zebra Stripes

Tim Caro

• Publisher: University of Chicago Press
• Year: 2016
• ISBN:
  9780226411156
• Topic: Sciences biologiques
• Subtopic: Science générale

From eminent biologists like Alfred Russel Wallace and Charles Darwin to famous authors such as Rudyard Kipling in his Just So Stories, many people have asked, "Why do zebras have stripes?" There are many explanations, but until now hardly any have been seriously addressed or even tested. In Zebra Stripes, Tim Caro takes readers through a decade of painstaking fieldwork examining the significance of black-and-white striping and, after systematically dismissing every hypothesis for these markings with new data, he arrives at a surprising conclusion: zebra markings are nature's defense against biting fly annoyance.Popular explanations for stripes range from camouflage to confusion of predators, social facilitation, and even temperature regulation. It is a serious challenge to test these proposals on large animals living in the wild, but using a combination of careful observations, simple field experiments, comparative information, and logic, Caro is able to weigh up the pros and cons of each idea. Eventually—driven by experiments showing that biting flies avoid landing on striped surfaces, observations that striping is most intense where biting flies are abundant, and knowledge of zebras' susceptibility to biting flies and vulnerability to the diseases that flies carry—Caro concludes that black-and-white stripes are an adaptation to thwart biting fly attack. Not just a tale of one scientist's quest to solve a classic mystery of biology, Zebra Stripes is also a testament to the tremendous value of longitudinal research in behavioral ecology, demonstrating how observation, experiment, and comparative research can together reshape our understanding of the natural world.

ONE

Stripes and equids

1.1 The question of stripes

Zebras are one of the most visually arresting animals in nature because they have contrasting and regular black and white striped coats. They look like no other mammal and have a strikingly different colored coat from the familiar but closely related domestic horse. To most people, they are exceptionally beautiful. As far back as 1824, Burchell, after whom the plains zebra is sometimes named, wrote, “I stopped to examine these zebras with my pocket telescope: they were the most beautifully marked animals I had ever seen: their clean sleek limbs glittered in the sun, and the brightness and regularity of their striped coat, presented a picture of extraordinary beauty, in which probably they are not surpassed by any quadruped with which we are at present acquainted. It is, indeed, equaled in this particular, by the dauw (mountain zebra) whose stripes are more defined and regular, but which do not offer to the eye so lively a colouring” (p. 315).

Outside of Africa, zebras have long been popular curiosities. For instance, Grevy’s zebras were brought to ancient Rome to draw chariots in circuses in AD 211–217, and quaggas were used to pull carriages in Hyde Park, London, in the 1800s (MacClintock 1976). Yet despite the animals’ attraction, the riddle of why zebras have stripes has never been satisfactorily solved (Cloudsley-Thompson 1999; Ruxton 2002; Caro 2011). Long debated as far back as the nineteenth century by Wallace (1867a, 1879) and Darwin (1871), it has fomented much discussion by other great biologists (Poulton 1890; Beddard 1892; G. Thayer 1909; Mottram 1916; Cott 1940) and keen observers of natural history (Kipling 1902; Selous 1908; Cloudsley-Thompson 1984; Kingdon 1979; Morris 1990). Now, there are many intriguing ideas, but few have been tested experimentally, and it is difficult to take insights from similarly colored species because repeatedly alternating black and white stripes are found in so few vertebrates. Examples include the zebra duiker (see Appendix 1 for scientific names); okapi; some snakes, such as the desert banded snake and California king snake; and fishes, including the zebra shark, zebra moray, and zebra red dorsal. Unfortunately, the adaptive significance of striping patterns in snakes and fishes is also poorly understood (e.g., Seehausen, Mayhew, and Van Alphen 1999; Allen et al. 2013; Kelley, Fitzpatrick, and Merilaita 2013). In short, the reasons that three species of equid have striped pelage have not lent themselves to easy investigation. This book tries to fill that gap.

In this chapter, I first outline the functional hypotheses and associated mechanisms that have been proposed for why zebras have evolved black and white striped pelage. This task has been conducted by others (e.g., Cloudsley-Thompson 1984; Kingdon 1984; Morris 1990; Ruxton 2002; Egri, Blaho, Kriska, et al. 2012), but my purpose here is to make this list comprehensive and recast these ideas into larger functional categories that then enable us to use ecological data to bear on the evolutionary drivers of striping. I then briefly describe the natural history and external coloration of zebras and other equids since many interspecific comparisons will be made in this book, and finally summarize what is known about zebra hair.

1.2 Hypotheses for striping in equids

1.2.a Antipredator hypotheses

By far the most renowned and popular ideas as to why zebras are striped center on stripes reducing the likelihood of zebras being hunted or killed by predators. These come in several guises.

Crypsis

The first notion as to why zebras have stripes is for concealment against predators. Close up and in daylight, zebras’ contrasting black and white livery is very conspicuous to the human eye, but at a distance and under low illumination, when predators hunt, zebras might be difficult to see, and early ideas about zebra coloration focused on this. Wallace (1896), the father of the field of animal coloration, remarked, “It may be thought that such extremely conspicuous markings as those of the zebra would be a great danger in a country abounding with lions, leopards and other beasts of prey; but it is not so. Zebras usually go in bands, and are so swift and wary that they are in little danger during the day. It is in the evening, or on moonlight nights, when they go to drink, that they are chiefly exposed to attack; and Mr Francis Galton, who has studied these animals in their native haunts, assures me, that in twilight they are not at all conspicuous, the stripes of white and black so merging together into a gray tint that it is very difficult to see them at a little distance” (p. 220). We now call this type of coloration background matching. G. Thayer (1909) argued that stripes conceal zebras in “reeds and grasses, or even bare-limbed bushes and low trees, or sand streaked with shadows of any of these plants, or quiet water striped with their reflections—its obliterative effect must be almost perfect” (p. 138). It is not clear whether Thayer’s purported mechanism was background matching or disruptive coloration where false edges break up the outline of the animal, because he stated, “The stripes . . . still play their true obliterative part ‘cutting the beast to pieces’” (p. 138). Certainly Cott (1940), in his benchmark and influential book on animal coloration, as well as subsequent authors (e.g., Matthews 1971; McLeod 1987), thought that stripes broke up the continuous surface contour of the animal and thereby masked the margin of the body or the body’s appendages; these biologists also subscribed to the idea that zebras were difficult to see at a distance (see also Hingston 1933). Cotton (1998) was of the opinion that stripes might produce a shimmering effect in a heat haze. In addition, Cott and others such as Cloudsley-Thompson (1980) proposed that narrowing of the dark flank stripes and lighter belly acted as countershading, making the body appear flat and difficult to recognize as a prey object (see also Mottram 1916). Thus, there are ac...