The zebra, which is believed to have evolved from horses more than 2 million years ago, has such an unusual look that it just cries out for explanations and there have been no shortage of attempts to supply them. The one that has stuck in my mind is that the stripes provide camouflage in the long grass. Now another team of researchers have taken a shot at coming up with a different explanation.
A team of life scientists led by Univ. of California, Los Angeles (UCLA)’s Brenda Larison has found at least part of the answer: The amount and intensity of striping can be best predicted by the temperature of the environment in which zebras live.
In Open Science, the researchers make the case that the association between striping and temperature likely points to multiple benefits—including controlling zebras’ body temperature and protecting them from diseases carried by biting flies.
…Larison, a researcher in UCLA’s department of ecology and evolutionary biology and the study’s lead author, and her colleagues examined the plains zebra, which is the most common of three zebra species and has a wide variety of stripe patterns. On zebras in warmer climes, the stripes are bold and cover the entire body. On others—particularly those in regions with colder winters such as South Africa and Namibia—the stripes are fewer in number and are lighter and narrower. In some cases, the legs or other body parts have virtually no striping.
…Analyzing zebras at 16 locations in Africa and considering more two dozen environmental factors, the researchers found that temperature was the strongest predictor of zebras’ striping. The finding provides the first evidence that controlling body temperature, or thermoregulation, is the main reason for the stripes and the patterns they form.
You can read the paper here.
I am of course not a biologist of any stripe (ha!) but I am cognizant of the danger that in our desire to understand why a particular species has some distinguishing characteristic, we can fall prey to the lure of concocting Kiplingesque ‘Just So’ stories. The problem with associating one feature with one evolutionary benefit is why all the other species did not take advantage of that same benefit.
This may be because the benefits of that feature may not be strong enough, or its absence deleterious enough, to drive convergent evolution where every species ends up having it because those that did not just could not compete. The explanation for why only one or a few species have this seemingly desirable trait may be because the mutation ended up being useful enough that they passed it on to their descendants and it became enhanced with time, but those that did not have it went on their own way with their own advantageous mutations.
I think the authors are confusing the current utility (yes, if there’s a correlation, it suggests that the stripes do function in that manner) with the origin of the trait. For that matter, the idea that there’s just one function for a trait is an easy mistake to make. I don’t see any reason why predation might not drive the evolution of stripes, and thermoregulation could drive the further modification. (It would be interesting to see if the differences in pattern are genetically based, or if there’s some environmental input during development.)
I also suspect that the driving force vis-à-vis predation might have more with confusing the eye when looking at an entire herd than with actual camouflage.
Are they suggesting genetic variety or developmental pathways initiated by temperature?
astromash,
I have added a link to the actual paper so you can check it out. As far as I can tell, this is a purely correlational study.
And again, nobody considers drift.
Studies have shown a strong genetic input to striping.
Most of the anti-predation mechanisms I’ve read depend on the widespread adoption of strong, high definition stripes. How can that explain their initial development?
I saw another study somewhere that found stripey zebras were attacked less by biting flies. If the biting flies are less common in colder climates, thermoregulation might still not be the horse they’re looking for.
Great American Satan,
The authors look at that hypothesis as a competing one and find that the evidence for it is mixed and not as compelling. I suspect that supporters of the flies theory are going to see if they can find holes in this paper.
The variation due to temperature may have no utility whatsoever. It may just be an artifact of how a zebra’s body generates its striping.
As I recall, the specific stripe pattern is not genetically programmed. Rather, there’s in effect a rule that each hair (or patch of skin?) uses to decide whether it should be black or white depending upon what is going on around it, and striping is a result of that. It would not be surprising if this process is affected by temperature, so that there’s more striping when the skin is consistently warmer. The dependency on temperature might simply be a consequence of whatever biochemical mechanism evolution hit upon to generate the stripes. Since the zebras end up striped (and thus gaining whatever advantage striping gives them) everywhere over their range, there’s no selective advantage to making the striping mechanism temperature-insensitive.