By Museum für Naturkunde Berlin
Museum für Naturkunde, Berlin
Black and White
Are zebra stripes just a random creation of nature? If not: What is their function? Which evolutionary advantage do they confer?
Charles Darwin and Alfred Russel Wallace had both suggested, as early as the 1870s, that zebra stripes might act as a form of camouflage, especially in high grass in the presence of hot, hazy air. However, not everything that is plausible is necessarily true. Such untested scientific explanations are called hypotheses. Science must test these by searching for evidence that either supports a hypothesis, or proves it false.
This hypothesis has since been tested and could not be confirmed. Zebras are often caught by lions. Also, most mammalian predators see at least as well as humans, and can smell and hear much better. Good supporting evidence was not found. A camouflage effect does not seem to be a likely explanation for the zebra's stripes.
Tsetse fly in the Evolution in Action exhibition at the Museum für Naturkunde Berlin by Hwa Ja Götz (MfN)Museum für Naturkunde Berlin
In the meantime, a new hypothesis, a new target for research, has been proposed ...
Tsetse fly, detail by Hwa Ja Götz (MfN)Museum für Naturkunde Berlin
Tsetse flies and horseflies!
In 1981, almost 110 years after Darwin's und Wallace's hypothesis, another scientist (Dr. Waage) developed a new hypothesis: Tsetse flies and horseflies are vectors of dangerous diseases, e.g. the nagana pest which is often deadly for zebras. Flies have very different eyes than mammalian predators and fewer neurons to process the visual information. Could zebra’s stripes be effective in evading these disease-carrying flies? And indeed: In the past 35 years a number of experiments have confirmed this. Animal models painted with zebra stripes attracted far fewer horseflies than those painted uniformly black, brown, gray or white. Furthermore, it could be shown that the spacing of zebra stripes was in a range optimally confusing the horseflies.
The Slightly Striped Quagga
The extinct quagga, a subspecies of the zebra from the Cape Province in South Africa, provides another piece of evidence in support of the fly-hypothesis. Coming from tsetse-fly infested areas further North, the fully striped predecessors of the quagga migrated to an area free from tsetse fly infestations. It seems that the animals now had no more need for stripes and that these gradually faded over many generations. By the time the quagga was driven to extinction around 1870, only the head, neck, and tail were still faintly striped. The quagga is displayed in the Museum für Naturkunde in the same room as the zebra.
Not quite. Some scientists doubt the evidence in favour of flies and found evidence that the spacing and density of zebra stripes is correlated with average temperatures of their habitats, supporting a thermoregulation hypothesis. Although protection against tse-tse and horseflies seems to be the best explanation at the moment, the case is not closed. Scientists must continuously review, doubt, and – where necessary – revoke existing answers. There is more than enough work for the next generation of young scientists. ––– PS: Not all zebra camouflage experiments are published – see this image of zebra paintings on a street in Portugal.