fredag 3 februari 2012
How the zebra got its stripes
Just like Rudyard Kipling wrote a Just So Story about why the leopard got its spots, the camel got its hump, Kipling would equally well have chosen to write about why the zebra got its characteristic white and dark striped coat pattern. In its appearance and outline the zebra’s coat pattern is formed by narrow high contrast stripes which forms individually unique patterns and covers most of its body surface. It is not surprising that there have been much speculations about why the zebra has evolved this coat pattern and what would be the benefit from having a striped coat. Even Alfred Russel Wallace and Charles Darwin being one the first to get involved in these speculations suggested adaptive reasons for the development. Wallace proposed the striped pattern would serve well as camouflage against predators in tall grass, and this was the reason for the evolution of the pattern. Charles Darwin on the other side criticized this idea by questioning the fact that zebras do not occur in areas with tall grass, but in open plain savannas. And so Darwin meant the visual protection would not serve its function, and there must be another reason. Since then many more ideas as to why the zebra got its stripes have been proposed, such as the appearance of stripes on many nearby animals in a dense flock would make it more difficult for a predator to single out an individual prey. The stripes might be involved in social interactions, or stimulate grooming, being involved in thermoregulation by generating turbulence near the body surface, or as we think along more recent suggestions be involved in parasite-host interactions. Cartoon made by: Dr Gyorgy Kriska.
We have studied the optical physical characteristics of zebra stripes, as dark and especially black/dark brown body surfaces are reflecting linearly polarized light just like a water surface, while white surfaces do not. We have reported this phenomenon in a previous paper on horses ( see older blog post). This signal is used by tabanid flies and other insects associated with water in order to locate a water source where they can lay eggs and drink. The tabanid flies (horse flies) are able to detect this optical signal from reflected linearly polarized light as females lay their eggs in water and mud and have evolved a sensory system to register polarized light. Furthermore, the female horse flies are well known to attack both humans and grazing mammals including cattle and wild ungulates. They for instance are important vectors for serious blood diseases such as trypanosomiasis, and they may irritate cattle and ungulates such that they cannot grace, and thus may drastically reduce the fitness of the hosts.
In a recent paper in Journal of Experimental Biology we report that the zebra has an evolutionary benefit from having a striped coat pattern, as this pattern drastically reduces the degree by which linearly polarized light is reflected on the body surface. The reduction is even stronger than for completely white coats, suggesting that the introduction of while narrow stripes to the initially dark coat in zebras were strongly beneficial in terms of avoiding attacks by blood-sucking tabanid flies. In our paper we have presented experimental evidence as to why the physical optical signal is changed, and the reflection of linearly polarized light is reduced, but we have also demonstrated that the attractiveness of tabanid flies is also drastically reduced by introducing stripes. More narrow stripes result in reduced attractiveness to tabanid flies. We have also noted that zebras have stripes of a width that reduces attraction to tabanides to a minimum, and so we propose that the evolution of a striped coat pattern in zebras might have been evolutionary beneficial as it would reduce attraction and disturbance from horse flies (and other biting insects with the same optical sensitivity as the tabanide flies) while grazing, but also reduce the chance of being exposed to lethal blood diseases transmitted by the blood-sucking tabanid flies. We believe this coat pattern will be especially beneficial for animals inhabiting the African savannah, where the tabanid fauna is especially rich.