In the next of our ‘often seen but largely under-appreciated’ group of animals, I would like to share with you something of the singularity and amazing environmental adaptation of ‘horses in striped pyjamas’.
So convinced was I as a child that zebras were indeed horses in striped pyjamas, that I recall asking my mother why they were allowed to stay in their pyjamas all day but I was not! To my five-year-old mind, this seemed grossly unfair. An answer, if there was one forthcoming, is lost in the mists of time, and the etiquette of attire for zebras and me still differs dramatically.
Whilst obviously not horses in zany apparel, zebras are, without a doubt, closely related to horses. DNA evidence has demonstrated that horses, donkeys and zebras all evolved from a common ancestor known as a “dawn horse” (Hyracotherium) about 4 million to 4.5 million years ago.
Today, they represent the only living members of the Equid family, all of which belong to the single genus, Equus. All members of this group walk on an extended toe (only one per limb) so that neither the sole nor the heel of the foot touches the ground. It is the middle toe that has developed completely to end in a hoof, with only rudimentary remains of the other toes present. This adaptation has, in effect, lengthened their limbs, with the resultant longer strides facilitating faster running speeds. It has also been suggested that a further benefit of longer limb-length (and increased height) is that it allows better visibility in a savanna environment.
Genetic similarities aside, the most striking difference between zebras and their equine cousins is, of course, their black and white stripes. Why zebras have these markings is a question that has intrigued scientists and spectators for centuries, and many hypotheses have been postulated since Alfred Russel Wallace and Charles Darwin first debated the issue 120 years ago.
The longest-standing speculations about the reason for their unique coat-pattern have included its being a form of camouflage, disrupting predatory attack by visually confusing carnivores, and being a mechanism of heat management. Research in the past few years has, however, taken the debate in a very different direction, postulating that zebras’ stripes provide an evolutionary advantage by discouraging biting flies. The research, conducted by a team from Canada’s University of Calgary under Professor Tim Caro, mapped the geographic distributions of six different species of zebras, horses and donkeys, noting the thickness, locations, and intensity of stripes on several parts of their bodies.
Their next step was to compare these animals’ geographic ranges with different variables, including woodland areas, ranges of large predators, temperature, and the geographic distribution of disease-carrying tsetse flies and horseflies. They then examined where the presence of the most heavily-striped animals (primarily zebra) and these variables overlapped.
Again and again, they found greater striping in animals in those parts of the world where they were more under attack from biting flies. It appears that the striped coats of zebras have an adaptive function to reduce the risk of parasitism by those organisms causing sleeping sickness (trypanosomiasis) and other lethal diseases. A century-old riddle solved; or so it would seem. Yet, in science, one solved puzzle often results in others − in this case, why do biting flies avoid striped surfaces? And, if it is such an effective deterrent, why are zebras the only striped animals in these areas?
Despite the fact that answers to the latter questions are still a subject of scientific research for anyone who has visited the Zambezi Valley and experienced the discomfort that Tsetse flies inflict, it may be worth making use of the initial findings and packing that Zebra-patterned ‘onesie’ for your next 4×4 trip. In fact, it may even validate my wearing pyjamas – albeit black and white striped ones − during the day; and so for a time, at least, placing my dress etiquette and that of a zebra’s in the same space!
By Lorraine Doyle