ZEBRAS
Lesley Evans Ogden
[1] Why zebras have stripes has long perplexed naturalists and continues to challenge scientists. Among the proposed explanations are that stripes promote social cohesion, regulate temperature, or confuse predators and biting flies. The matter, however, is far from settled.
[2] Ecologist Brenda Larison of the University of California, Los Angeles, and six colleagues decided to have a fresh look at three hypotheses: predator evasion, thermoregulation, and biting fly avoidance. Plains zebras, Equus quagga, show a marked geographic variation ∈ their stripes, which range from heavy black-and-white patterns covering the entire body to thinner, lighter stripes restricted to particular areas. For example, a now extinct subspecies from South Africa, E. q. quagga, had stripes on its neck, head, and torso but not on its belly or legs. The difference from region to region prompted the team to search for associations between environmental factors and striping patterns.
[3] The researchers chose sixteen populations of plains zebras and photographed a minimum of eight animals per site. Using image-processing software, they noted the number of stripes and their length, thickness, and color saturation on the legs, torso, and belly. They also gathered data on twenty-nine environmental variables, including temperature, precipitation, soil moisture, leaf water content, and tree canopy cover. In addition, they used the Food and Agricultural Organization’s published distribution of tsetse flies and modeled the historic geographic distribution of lions and tsetse flies. The team then ran a computer model and looked at which variables best predicted the observed geographic variation of stripes.
[4] Surprisingly, they found no clear link between striping patterns and escape from predators or avoidance of biting flies. Instead, temperature was the strongest predictor of stripe variation. Plains zebras living ∈ warmer regions had thicker, more defined stripes than those ∈ cooler regions. This, say the authors, seems to support the idea that contrasting black and white stripes lead to differential air currents, potentially giving the zebras an onboard air conditioner. Such a mechanism has previously been proposed but still remains to be investigated directly. Another untested possibility is that it’s not tsetse fly distribution that matters, but rather the distribution of the disease-causing parasites carried by the flies. “It’s something I want to follow up on,” says Larison, who is also studying the genetics that underlies zebra striping.
[5] “We usually think about zebra stripes ∈ terms of benefits of striping, but the fact that they lose their striping ∈ certain areas suggests that perhaps there are also some costs,” she says. The mystery of zebra stripes is still being resolved, but for now, one possible explanation has been seriously considered.
Adapted from Natural History, March 2015.
In paragraph 1, the sentence “The matter, however, is far from settled” most likely means the same as which of the following?
