How did the zebra and the squirrel get their stripes? The answer depends on who you ask or what you read. There are several charming stories about this. On a more serious note, this question has intrigued biologists for long and a consensual answer appears on the horizon.
As children, we were told that when the little squirrel brought little pebbles to help Lord Rama’s army in building the bridge to Lanka, Lord Rama fondly petted the squirrel in appreciation with his fingers and that is how its stripes came to be. As for the zebra in Africa, Rudyard Kipling in his charming “Just so stories” wrote (www.boop.org/jan/justso/leopard.htm) that two ancestral zebras decided to “melt” into the landscape as camouflage to escape the leopard and the Ethiopian hunters. A Ugandan parable has it that the dark donkey and white horse were fused into one another by an angry god in order to stop them from fighting, and out came the zebra. While these are charming folk tales and legends, the evolutionary biological explanation is slowly emerging. Scientists have been toying with a few broad set of answers. One of them is similar to what Kipling wrote, namely, that the stripes allow them to merge into the woodland background, so as to escape predators. Another says that stripes help in cooling the body from overheating. The third is based on the fact that hunters find moving striped objects difficult to target accurately – the so called “motion dazzle confusion effect.” A fourth one points out that parasites - mosquitoes and flies - find it impossible to attack striped animals, while they can feast on uniformly coloured animals such as the “dull donkey.”
It is the last one, escape from parasites attack, which has gathered much support for the advantage of stripes on animals in the African environment. Dr Tim Caro of the University of California Davis, and Dr Susanne Akesson from Lund University, Sweden, have been the proponents of this theory. Dr Akesson and her colleagues point out that horse flies and similar parasites, which feed on the blood of animals, are attracted to light that is oriented in a particular direction (polarisation) or glare. This glare attracts and helps these insects zone in on the target. Dark skin polarises light better than brown or white. But if the skin were striped dark and white, it becomes less attractive to the bloodsucker bugs. Akesson has argued that the black and white pattern “is ideal in its functions of disrupting this signal of reflected polarized light” and, in effect, is camouflaged to flies as well as big cats.” (A science writer, who covered this research paper, has suggested that we use Zebra striped wall paper to shoo away mosquitoes and flies, worth trying!)
Taking on this and studying the skin colouration and striping of a whole set of animals of the donkey/horse family (called the equids), Dr Caro and colleagues studied in detail seven different members of the equid family across the Old World (largely tropical regions of Africa and Asia), on one hand, studying their skin colouration and striping, and the location and prevalence of blood-sucking flies such as the horsefly and the tsetse across this broad region of Sub-Saharan Africa on the other. Their paper, “The function of zebra stripes,” which appeared in the journal Nature Communications (April 2014, DOI:10.1038/ncomms 4535), finds the strongest connection between the biting fly annoyance and the number, spacing and intensity of stripes in the face, neck, belly, rump, flank and legs of the striped equids.
In effect then, stripes evolved in African equids largely to protect them from the blood sucker insects which abound and thrive in the African tropics. Stripes fool these bugs, perhaps by the disruption of the polarisation of light falling on the striped pattern on the hair and skin. Is this why zebras or striped squirrels are not found in the U.S., Northern Europe or Russia? Neither is the climate there welcome to such bloodsucker insects.
The biological mechanism behind how these stripes are written on the body of these animals (squirrels, African striped mice and zebra) is also becoming clear, thanks to a recent paper by R.Mallarino and coworkers, which has very recently appeared in the journal Nature (doi:10.1038/nature20109). The gene MITF is known to be a master regulator of the cells known as melanocytes or pigment cells, which generate colour in hair cells and thus the skin. And another called ALX3, through its protein, appears to repress the regulator MITF. In yet another twist, so common in the molecular biology of development, the gene ALX3 is involved in normal facial development and a deficiency in its function leads to malformed noses in humans. Likewise, mutations in the other gene, MITF, leads to small eyes, deafness and some related disorders. How nature has recruited these two genes in order to draw lines on their bodies and thus protect some of these tropical mammals is one of those surprises that evolutionary biology often throws at us.
D. Balasubramanian
dbala@lvpei.org
