Etosha National Park, Namibia — I’m sitting in a truck in Etosha National Park, dart rifle in hand, watching a herd of zebra when 9-year-old Olivia, already a keen observer of nature, pipes up. “Is that a melanistic zebra over there?” A melanistic zebra is one whose stripes haven’t developed correctly. It’s not fully black like most melanistic animals you might think of. It looks more like the stripes arose mid-body and were trying to migrate out to the head and feet, but got stuck.
Melanistic zebra are not extremely common, and may not survive long, but each trip I find a couple more to sample. I quickly darted this one, hopped out of my vehicle (after peering around for lion, elephants and other potential dangers) and walked out to pick up the dart. The dart was fitted with a needle to take a small skin sample from the zebra, which had already returned to grazing. Melanistic zebra are one potential key to the question of how the zebra got its stripes, a question I am studying from two angles – the how and the why.
The question of how the zebra got its stripes really grabs my interest because stripes make the zebra seem so conspicuous. Zebra are one of the lion’s favorite prey so you might expect them to be less conspicuous to avoid predation.
Do Stripes Confuse Lions?
There are many ideas floating around about what the advantage of striping might be, but none are proven yet in spite of all the just-so stories you may have heard. Stripes might actually help prevent predation by confusing the lion, creating a blur of stripes in a herd running this way and that, or by making it hard to tell where one zebra stops and the other begins. They might even make the zebra less conspicuous in brush or at dawn and dusk, prime lion hunting times. Another tantalizing idea is that they help zebra avoid being bitten by tsetse flies and thus avoid sleeping sickness!
What is interesting, and what is the main focus of my research, is that even among zebra with normally developed stripes, not all are as stripy as others. In eastern Africa they are fully striped, head to toe, while in southern Africa they are less stripy, often with no stripes at all on their legs. I am using this variation to test ideas about why the zebra is striped by correlating variation in striping with climatic and habitat variation. These correlations can provide clues about why zebras are (or aren’t) striped, giving us leads that we can follow up on with more direct tests. I’m also using this variation to discover the gene or genes that cause and control striping. The genes themselves can also tell us something about whether striping is adaptive, as selection leaves its mark on the genome.
My next trip is to Uganda to sample some very stripy zebra. Ultimately, I plan to sample from six populations for the genetic work and am collecting photos from all over Africa (mine, those of friends, from people on Flickr etc.) to do the environmental correlations.
So why are zebra striped? It’s still an open question. For every hypothesis out there you can come up with good reasons it could be the answer and good reasons why it’s not! Maybe it’s something no one has thought of yet, maybe it’s a combination of things, or maybe it’s historical. With all the new tools at our disposal these days, I think we’re getting close to the time when we’ll have an answer.
Zebra stripes are among the most striking mammalian coat patterns. How these dramatic patterns are produced remains mysterious, as does their adaptive value. Brenda Larison of the University of California at Los Angeles received a grant from the National Geographic Society’s Committee for Research and Exploration to investigate the genetic basis and adaptive significance of stripe pattern variation in plains zebra (Equus burchelli). The study represents a unique opportunity to gain new insights about the evolution of zebra stripes. You can also help Brenda fund her field work by making a donation on her site.