A leopard may not be able to change its spots, but some zebras change their stripes. Zebras in warmer places have more stripes, a new study shows, which might help answer an age-old question: Why stripes?There's a fifth possibility: maybe there's no reason at all and stripes are just an evolutionary accident.
The answer probably comes down to keeping zebras cool and fending off disease-causing insects that are more common in hotter climates, researchers reported Tuesday in the journal Royal Society Open Science.
All three species of zebra have bold black and white stripes that stand out among more drab-looking African grazers, like buffalo and antelope, especially against a plain savanna background. And standing out would seem to make a zebra more likely to become a lion's lunch.
This "stripe riddle" has puzzled scientists, including Darwin, for over a century. There are five main hypotheses for why zebras have the stripes: to repel insects, to provide camouflage through some optical illusion, to confuse predators, to reduce body temperature, or to help the animals recognize each other.
The last time I addressed this issue was in 2012 [How Did the Zebra Get Its Stripes?]. At that time I quoted the famous Spandrel's paper where Gould and Lewontin wrote ...
... the rejection of one adaptive story usually leads to its replacement by another, rather than to a suspicion that a different kind of explanation might be required. Since the range of adaptive stories is as wide as our minds are fertile, new stones can always be postulated. And if a story is not immediately available, one can always plead temporary ignorance and trust that it will be forthcoming ....That's exactly what's happening in the National Geographic article. After almost 100 years of speculation, nobody has come up with a good adaptive explanation of zebra stripes. They never consider the possibility that there may NOT be an adaptive explanation.
How the Zebra Gets Its Stripes]. He pointed out back then that stripes are almost certainly due to small changes in just a few genes that alter the timing of differentiation in early embryology. He rails against adaptationist thinking then says ...
For many reasons, ranging from probable neutrality of much genetic variation to the nonadaptive nature of many evolutionary trends, this strict construction [vulgar Darinwism] is breaking down, and themes of unity are receiving attention.... One old and promising theme emphasizes the correlate effects of changes in the timing of events in embryonic development. A small change in timing, perhaps the result of a minor genetic modification, may have profound effects on a suite of adult characters if the change occurs early in embryology and its effects accumulate thereafter.The point is that the prominence of stripes on zebras may be due to a relatively minor mutation and may be nonadaptive. That's a view that should at least be considered even if you don't think it's correct.
Gould was being very optimistic when he suggested that the old ways were breaking down.
Gould, S.J., and Lewontin, R.C. (1979) The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme. Proc. R. Soc. Lon. B Biol. Sci. 205: 581-598. [PubMed] [doi: 10.1098/rspb.1979.0086]