A recent study led by UC Davis researchers helps shed light on the puzzling and rarely researched question as to why certain animals – like skunks and zebras – have evolved to maintain contrasting color patterns.
“Why are skunks so successful?” asked Jennifer Hunter, a recent Ph.D. graduate who conducted the study for her dissertation. “It’s because they have this amazing evolutionary tool that fends off predators.“
For her research, Hunter built several “mounts“- realistic-looking sculptures covered in fur – that effectively tricked the predator into thinking the mount was a skunk or a fox.
She used five different treatment types: a natural-colored skunk, a natural-colored grey fox, a grey fox dyed to mimic a skunk, a skunk dyed to mimic a grey fox and a control station where no mount was placed. She then used motion-sensitive cameras at 10 different sites across California to record the effect these deceiving sculptures had on different carnivores visiting the mount.
“For this study, I found that different coloration and shape worked and caused the predators to act differently,” Hunter said. “Predators were very nervous around the fake skunks, and it’s thought that their color patterns are memorable to predators.“
In fact, it is thought that striped coloration serves as a warning signal, much like a rattlesnake’s rattle or the bright colors of a harmful butterfly, obtrusively letting the intruder know that the individual is well equipped with a defense.
“The skunk has the ability to spray a predator with a noxious odor,” said Gail Patricelli, assistant professor of evolution and ecology. “[Skunks] must advertise this ability. It makes it easier for others to identify that you are a potentially dangerous individual.“
This evolutionary advantage is likely to keep the individual alive longer, increasing the chance of producing offspring, thus making the individual more successful than its defenseless counterparts.
“In theory, [skunks] are supposed to be as obvious as possible to their predators,” Hunter said. “And if you don’t have to avoid your predators, you’re doing something that works very well.“
But striped coloration is not only a skunk-specific advantage. Zebras, too, are striped black and white and possess an advantage much different than the skunk.
“One hypothesis is that it helps them blend into the herd,” Patricelli said. “If you’re an individual zebra you don’t want to be picked out by a predator. It’s virtually impossible [as a predator] to get one individual. This is called disruptive coloration.“
With such a large mass of animals, disruptive coloration poses a major problem for the predator. As a whole, contrasting colorations can break up the body shape of a single animal.
“When the herd is running together it may be difficult for lions or hyenas to tell where one animal ends and the next begins,” Hunter said in a later e-mail interview. This is thought to “disguise the animal and should (in theory) make it more difficult to capture by predators.”
Although evolutionary research has been studied for over a century – especially after the seminal publication of Darwin’s On the Origin of Species – research into the evolution of coloration has been a relatively recent endeavor, and there is much investigation yet to be done.
“The field has advanced so little in 100 years,” Tim Caro stated, professor of wildlife, fish and conservation biology and co-author of Hunter’s thesis, in a February 2005 publication. “Naturalists‘ anecdotes about mammalian coloration were never put to experimental test, and the generality of these ideas – most of them formulated on the basis of only one or a handful of species – remained unexplored until very recently.”
Other experts agree.
“People have been talking about how black and white colors might function for a long time, but there really has been very little science to test these hypotheses,” Patricelli said. “This coloration in particular is what we know very little about. We need careful experiments as to why these traits have evolved.“
MICHAEL MILLER can be reached at campus@theaggie.org.
