Why do zebras have stripes? Theories have ranged from deterring predators and social interactions to keeping cool and avoiding flies. A new paper published on 13 June in the Journal of Natural History, the scientific publication of the British Natural History Museum, suggests zebra stripes are, in fact, used to control body temperature (1). And for the first time, the study also reveals a new mechanism for how this may be achieved.
The new findings could finally be the key to understanding why zebras have evolved their striking coat. This may also explain why zebra stripes seem to be remarkably more pronounced on animals living in the hottest climates near the equator.
The researchers, amateur naturalist and former biology technician, Alison Cobb and her zoologist husband, Dr Stephen Cobb, observed zebras in their natural habitat in sub-Saharan Africa — a stallion and a mare, plus a zebra hide draped over a clothes-horse as a control.
Based on their field research, the duo discovered a temperature difference between the white and black stripes that increases as the day goes on and the air temperature increases. The temperature difference levels off with the black stripes 12-15 degrees Celcius hotter than the white and remains relatively stable for the middle seven hours of the day. But interestingly, the stripes on the zebra hide continued to heat up, by as much as additional 16 degrees.
So, how does this work? The researchers propose that the temperature difference causes small-scale convective air flow just above the stripes that destabilise the air and the water vapour at the tips of the hairs.
Three ways to keep cool
Like horses, zebras also sweat to stay cool. The sweat travels from the skin to the tips of the hairs aided by a protein called latherin, that is present in both horses and zebras.
And another interesting finding the researchers made was that zebras have the ability to raise the hairs of their black stripes while the white ones remain flat. This could also assist with heat transfer from the skin to the tip of the hair, the authors suggest. In addition, raised black hairs could help trap air to reduce heat loss when the stripes are at the same temperature in the early morning.
Therefore, three mechanisms — air movements, sweating, and raised hairs — may work together to keep zebras cool in the intense heat of sub-Saharan Africa.
A secondary advantage?
Although the authors believe thermoregulation or temperature control may be the primary reason for zebra stripes, they also speculate that the unstable air associated with the stripes may play a secondary role in deterring biting flies from landing on them, thereby conferring an additional advantage. Caro and colleagues proposed a similar theory in their recent study (2).
Explaining how the researcher came about, Alison Cobb says “Ever since I read ‘How the Leopard Got His Spots’ in Kipling’s Just So Stories at bedtime when I was about four, I have wondered what zebra stripes are for. In the many years we spent living in Africa, we were always struck by how much time zebras spent grazing in the blazing heat of the day and felt the stripes might be helping them to control their temperature in some way.”
“The solution to the zebra’s heat-balance challenge is cleverer, more complex and beautiful than we’d imagined. Of course, there is much more work to be done to gather evidence and fully understand how the stripes help zebras control temperature, but I am 85 now, so that’s for others to do.”
(1) Cobb, A. and Cobb, S. Do zebra stripes influence thermoregulation? Journal of Natural History (2019). DOI: 10.1080/00222933.2019.1607600
(2) Caro, T. et al. Benefits of zebra stripes: Behaviour of tabanid flies around zebras and horses. PLoS ONE (2019). DOI: 10.1371/journal.pone.0210831
Image: Lead author Alison Cobb grooming a wild zebra in the Animal Orphanage at Nairobi National Park in 1991 (Photo by Stephen Cobb).