Zebra stripes: Another theory to explain their purpose bites the dust

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The experimental setup, showing the arrangement of the six barrels and the meteorological station (centre) used to monitor weather conditions. The barrels were on wooden holders which held them 10 cm above the ground. Photo: Gábor Horváth
The experimental setup, showing the arrangement of the six barrels and the meteorological station (centre) used to monitor weather conditions. The barrels were on wooden holders which held them 10 cm above the ground. Photo: Gábor Horváth

The mysterious purpose of zebra stripes is back in the news, with a novel study exploring whether the striking black-and-white pattern has a role in regulating body temperature.

Gábor Horváth and her colleagues, writing in the journal Scientific reports, said there were up to 18 theories for the possible functions of zebra stripes, one of which is to cool the animal.

Others propose that the stripes deter predators, either through camouflage or by causing visual confusion. Further theories suggest they have a role in social interactions or thwart the attack of biting flies.

The researchers, most of them based at Eötvös Loránd University in Budapest, Hungary, conducted a field experiment to investigate whether thermoregulation might be a primary role of the stripes.

According to the hypothesis of thermoregulation, upwelling air streams may form over the warmer black stripes which are replaced by cooler air from the adjacent white stripes with downwelling air flows.

Consequently, convective air eddies might build up above periodic patterns of black and white stripes. In principle, such eddies might cool the zebra body in sunshine by transporting warm air away over the black stripes, and/or accelerating the evaporation of sweat on the zebra skin.

For their experiment, a zebra body was modelled by water-filled metal barrels, which were covered with horse, cattle and zebra hides, and with various black, white, grey and striped patterns.

The barrels were installed in the open air on a horse farm in northern Hungary for four months while their core temperature was measured continuously.

A zebra in its natural habitat.
A zebra in its natural habitat. Photo: File

Using thermography, the temperature distributions of the barrel surfaces were compared to those of living zebras.

The sunlit zebra-striped barrels reproduced well the surface temperature characteristics of sunlit zebras, the study team reported.

However, there were no significant core temperature differences between the striped and plain grey model (which had a similar average whiteness to the black-and-white striped model), even on many hot days, independent of the air temperature and wind speed.

The zebra-striped coats did not keep the body cooler than grey coats, they concluded – a finding which challenges the hypothesis of a thermoregulatory role of zebra stripes.

“Even if stripe-induced convective eddies might have formed above our striped barrels in sunshine, their cooling effect was not reflected in the core temperature, which was predominantly governed by the average whiteness of the hide covering the barrel shell, that is by the net amount of sunlight absorbed by the hide,” they said.

The core temperature was always the highest in the black-covered barrel, they reported, and was always the lowest in the white-covered barrel.

The core temperature of the barrels covered by zebra-striped and uniformly grey hides was between that of the white- and black-covered barrels.

“All these experimental findings provide evidence against the hypothesis of cooling effect of zebra stripes, because striped coats do not keep the core temperature of the body any cooler than homogeneous grey coats with a similar average whiteness.”

The study comprised Horváth, Ádám Pereszlényi, Dénes Száz and Imre M. Jánosi, all from Eötvös Loránd University; András Barta, from Estrato Research and Development Ltd; Balázs Gerics, from the University of Veterinary Medicine in Budapest; and Susanne Åkesson, from Lund University in Sweden.

Experimental evidence that stripes do not cool zebras
Gábor Horváth, Ádám Pereszlényi, Dénes Száz, András Barta, Imre M. Jánosi, Balázs Gerics & Susanne Åkesson
Scientific Reports, volume 8, Article number: 9351 (2018) https://doi.org/10.1038/s41598-018-27637-1

The study, published under a Creative Commons License, can be read here

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