Are zebra stripes part of a fancy cooling system?

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zebra-lewisEvidence points to stripes playing a role in keeping zebras cool in the African heat, a new study suggests.

The researchers said there was evidence that stripes may also have a role in keeping disease-carrying biting insects at bay, but said the explanation for zebra striping was likely to be more complex than one straightforward reason.

Their findings, reported in the journal, Royal Society Open Science, noted that zebras in the warmer regions tended to have more stripes than those in the cooler parts of Africa.

Scientists have pondered the riddle posed by zebra stripes for well over a century, with five main theories put forward: to repel insects, provide camouflage through some form of confusing optical illusion, confuse predators, reduce body temperature, or help with social cohesion.

The new research does not declare a winner in the age-old puzzle, but does show that temperature is the factor most strongly linked to striping.

Essentially, the warmer the climate, the more stripes a zebra is likely to have.

The researchers from the United States and Germany examined 29 different environmental variables in Plains zebras across 16 different sites ranging from south to central Africa in reaching their conclusions.

zebra-lewis_0178The research, based at the University of California, Los Angeles, found that stronger stripes along zebras’ backs were most closely correlated with temperature and rainfall.

It did not correlate with the prevalence of predatory lions or tsetse flies.

Study co-author Ren Larison and her colleagues noted that Plains zebra striping patterns varied regionally, from heavy black and white striping over the entire body in some areas to reduced stripe coverage with thinner and lighter stripes in others.

Their finding that higher temperatures were linked to bolder striping added weight to the hypothesis that stripes helped zebras control body temperature.

The hypothesis is based on the idea that black and white stripes would heat up differentially, causing different airflows between black and white stripes and creating eddies of air that would have a cooling effect.

It should work most effectively on strong, contrasting stripes, so the scientists would predict good coverage with bold black-and-white striping to occur in areas in which zebras were regularly exposed to higher temperatures.

“Our finding that the two environmental variables most closely associated with variation in striping were both temperature variables lends support to the hypothesis that striping may be related to thermoregulation,” they reported.

“Given the hypothesis that stripes give rise to differential air currents that produce a cooling effect, intense black stripes would be expected to create more of a differential relative to white stripes, and stripe saturation is greatest in the tropics where animals experience sustained high temperatures.”

The authors noted that there have been no published direct tests of the thermoregulation hypothesis. However, preliminary observations using a non-contact infrared digital thermometer gun show that zebras maintain a significantly lower surface body temperature – 29.2 degrees Celsius versus 32.5 degrees Celsius – than nearby, similar-sized herbivores grazing under the same conditions.

Also, observations of differences in shade-seeking behaviour between thin-striped Grevy’s zebras and thick-striped plains zebras suggested stripe thickness could play a role in thermoregulation, they said.

They said striping on the legs was not easily explained as a mechanism for thermoregulation.

“It may simply be a result of genetic correlation as stripe characteristics on the legs and torso are highly correlated or it may be a response to a different mechanism, such as avoiding fly bites,” they wrote.

“The possibility that biting flies could be a selective agent favouring striping is supported by experimental evidence that tsetses and tabanids avoid striped surfaces.

“However, we found no relationship between tsetse flies and variation in striping across populations, which suggests the explanation for striping in zebra is more complex than simply the avoidance of biting flies.”

They said it was not particularly surprising that probabilities of lion occurrence failed to predict zebra stripe pattern, as the predators were common throughout the range of Plains zebra.

The authors said much more work was needed to find out the true functionality of stripes in zebras.

“Our work shows a correlation with temperature, but the cause of this correlation remains unknown.”

The research was supported by a National Geographic grant.

How the zebra got its stripes: a problem with too many solutions
Brenda Larison, Ryan J. Harrigan, Henri A. Thomassen, Daniel I. Rubenstein, Alec M. Chan-Golston, Elizabeth Li, Thomas B. Smith
Royal Society Open Science.20152:140452.DOI: 10.1098/rsos.140452.Published 14 January 2015

The full study can be read here

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