(Image credit: Flickr users Clinton & Charles Robertson)I have solved a longstanding mystery: why skunks have their characteristic stripes.
Striped skunks (Mephitis mephitis) are among the most familiar yet least understood of North American mammals. For example, people commonly think of these vividly marked animals as denizens of forests, prairie, swamps, and virtually any other terrestrial habitat, and many scientists argue that the skunk’s contrasting black and white dorsal stripes purposely advertise the skunk’s presence but simultaneously “warn” potential predators not to tangle with it. If a potential predator approaches a skunk too closely the skunk typically sprays an obnoxious musk that is commonly believed to deter further attack, but since there is no guarantee that the “musking” will actually protect the skunk some kind of advance warning might benefit the skunk (and the predator).
All of this sounds good but is not supported by the facts. I’m prepared to show that in reality, striped skunks are virtually confined to an entirely different type of habitat than woods and fields and that far from being an advertisement the vivid white stripes enable skunks to occupy this habitat in almost total secrecy.
How it Works
Figure 1. Good and not so good highway behavior in the striped skunk. Figure prepared by Emily Barry.
This paper rests on a few simple, easily verified facts. First, if the number of live and dead striped skunks we see on highways versus those we see elsewhere is any indication, it’s clear that most striped skunks actually spend their entire lives on the highways, not in forests, prairies, or swamps. And I don’t mean along the edges of the highways or on the medians, I mean right on the highways. Now and again you might see a skunk in a public park or housing development, but these skunks are just dispersing and always find a road sooner or later.
Second, the highway is obviously a difficult place to live unless you (the skunk) are well adapted to survive in the midst of pervasive hazards. Such adaptation might
include great speed or agility, neither of which the skunk has, or it might involve concealment while on the highway through cryptic coloration. A simple comparison of skunks and highways shows that cryptic coloration is the correct interpretation. Skunks survive and reach great abundance on the highways because the white dorsal stripe is an incredibly precise “mimic” of the painted highway lane divider stripes (Fig. 1A). Rather than advertise the skunks’ presence, the skunks’ stripes blend right in with the highway stripes and no one sees the millions of skunks that populate the lane dividers. All a skunk has to do is to stay on the white lane divider line and it becomes extraordinarily difficult for any predator to detect.
What else factors into skunk/highway adaptation?
Well, for one thing, skunk survival on the highways is linked directly to skunk behavior. As long as the skunk stays longways on the painted lines the animal is relatively protected from any kind of attack (Fig 1A) but if the skunk moves off of the stripe (Fig. 1B) its visibility and vulnerability increase. If the skunk turns sideways (Fig 1C) the entire crypsis system breaks down and then the skunk is extremely vulnerable to any of a number of predators. In fact it is most often taken instantly by the most efficient skunk predator of all, the automobile. As common as these casualties are they are still several orders of magnitude below the millions of skunks that thrive for years on the highways just by staying longways on the stripes.
Likewise, life on the highway is in itself an efficient selective force that maintains striped skunk color pattern phenotypes within very narrow limits. Normally patterned striped skunks (Fig 2A) are, as previously noted, cryptic when positioned longways against the highway stripes (Fig 1A). However, crosswise striped (Fig 2B) and offset striped (Fig 2C) skunks are easily detected especially when positioned longways on the highway stripes. Consequently, these aberrant crosswise and offset color patterns are so rare in the striped skunk population that the only place I’ve ever seen them is in Fig. 2. Thus, only normally patterned, normally behaving striped skunks benefit maximally from life on the highway.
Figure 2. Adaptive and maladaptive striped skunk pattern phenotypes in the Wainwrightian context. Figure prepared by Emily Barry.
Earlier Progress on This Question
The only reference I could find regarding a cause and effect relationship between skunks and highways is Wainwright (1972),1 but although the fundamental observations that Wainwright reported were correct as far as they went, that author overlooked the cryptic coloration that is the foundation of this new paper. Nevertheless, in recognition of Wainwright’s seminal contribution to this research I propose the term “Wainwrightian Adaptation” for the newly discovered phenomenon reported here. Further investigation will probably show that other animals such as magpies and garter snakes also demonstrate Wainwrightian Adaptation and may even interact trophically with skunks in the same type of habitat. Research into Wainwrightian Adaptation shows great promise for investigators with the requisite speed, agility, and insurance.
Emily Barry prepared the two figures. I also thank Loudon Wainwright III, who likely has no idea why.
1. “Dead Skunk (in the Middle of the Road),” Loudon Wainwright III, Sound recording on Album III, Columbia KC 31462, 1972. A first edition copy of Wainwright’s 1972 study of skunks.
_____________________This article is republished with permission from the September-October 2010 issue of the Annals of Improbable Research. You can download or purchase back issues of the magazine, or subscribe to receive future issues. Or get a subscription for someone as a gift!
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