You’d think we’d know by now but… From ScienceDaily:
The data revealed a temperature difference between the black and white stripes that increases as the day heats up. Whilst this difference stabilises on living zebras during the middle seven hours of the day, with the black stripes 12-15ºC hotter than the white, the stripes on a lifeless zebra hide continue to heat up, by as much as another 16ºC. This indicates there is an underlying mechanism to suppress heating in living zebras. It is therefore the way the zebra stripes are harnessed as one part of their cooling system, rather than just their contrasting coat colour, that is key to understanding why these animals have their unique patterning. …
The researchers propose that the differential temperatures and air activity on the black and white stripes set up small-scale convective air movements within and just above the stripes, which destabilise the air and the water vapour at the tips of the hairs.
During the field research, the authors also observed — probably for the first time — that zebras have an unexpected ability to raise the hair on their black stripes (like velvet) while the white ones remain flat. The authors propose that the raising of black hairs during the heat of the day, when the stripes are at different temperatures, assists with the transfer of heat from the skin to the hair surface and conversely, when the stripes are at the same temperature in the early morning, and there is no air movement, the raised black hairs will help trap air to reduce heat loss at that time. Paper. (open access) – Alison Cobb, Stephen Cobb. Do zebra stripes influence thermoregulation? Journal of Natural History, 2019; 53 (13-14): 863 DOI: 10.1080/00222933.2019.1607600 More.
Interesting. They’re onto something with the hair standing on end. But here are some of the other theories:
The Real Reason Why Zebras Have Stripes: To Shoo Away Flies
Stripes Confuse People But They Do Not Cool Zebras
Study: Zebra stripes neither hide nor flaunt
Stripes offer no advantage to zebra?
How the zebra got its stripes, this time really
How the zebra got its stripes, maybe
How the zebra did NOT get its stripes?
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