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Infrared Thermography Reveals Relationship Between Zebra Stripes and Temperature
The enigmatic role of the striking black and white stripe pattern of zebras has been the subject of vigorous discussions among researchers. Until now, as many as 18 different explanations have been proposed for the possible functions of zebra stripes which can be combined into the following four major groups: (1) anti-predation, including camouflage and various aspects of visual confusion, (2) facilitating social interactions, (3) thwarting the attack of biting flies, (4) regulating body temperature. In this study, however, we test hypothesis (4) using field experiments.
According to hypothesis (4), zebra stripes are expected to cool the body by means of convective air eddies induced by temperature gradients over alternating black and white stripes. This hypothesis seems reasonable, because in sunshine the black zebra stripes are warmer due to their stronger absorption of sunlight compared to the cooler white stripes of higher reflectance. Infrared photography of zebras showed that sunlit black stripes are warmer than sunlit white stripes and that the difference between them increases with rising air temperature. At night, however, temperature differences are reversed, with black stripes being cooler than white ones.
Figure 1. Photographs and thermograms of the three sunlit barrels covered by different hides (black cattle, white cattle, grey horse) used in our experiments.
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.
Figure 2. Photographs and thermograms of the three sunlit barrels covered by different hides (grey cattle, artificial zebra, real zebra) used in our experiments.
Due to these open questions, the hypothesis of cooling by zebra stripes is physically and physiologically challenging. Black and white zebra stripes reflect sunlight quite differently, but the questions on the existence and mechanism of an overall cooling effect remained open until now. The aim of this work is to test experimentally the hypothesis of thermoregulation by means of thermography and thermophysical models of monochrome white, black, grey horses and striped zebras.
Figure 3. In the thermograms red, yellow, white, light blue and dark blue hues code hot, warm, medium, cool and cold temperatures, respectively.
We performed field experiments and thermographic measurements to investigate whether thermoregulation might work for zebra-striped bodies. A zebra body was modelled by waterfilled metal barrels covered with horse, cattle and zebra hides and with various black, white, grey and striped patterns. The barrels were installed in the open air for four months while their core temperature was measured continuously. 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.
We found that there were no significant core temperature differences between the striped and grey barrels, even on many hot days, independent of the air temperature and wind speed. The average core temperature of the barrels increased as follows: white cattle, grey cattle, real zebra, artificial zebra, grey horse, black cattle. Consequently, we demonstrate that zebra-striped coats do not keep the body cooler than grey coats challenging the hypothesis of a thermoregulatory role of zebra stripes.
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.
Gábor Horváth, Ádám Pereszlényi, Dénes Száz, et al. Experimental evidence that stripes do not cool zebras. Scientific Reports. 8(9351):1-12, 2018.