Monday, February 13, 2012

Polarized light, horsefly delight. Zebra stripes, fewer bites

I've been wondering for a while about the evolution of ringed tails. Why does a raccoon's tail have rings? Why do Red Pandas? Or Ring Tailed Lemurs? The Ringtail Cat? What kind of convergent evolution is this ring thing all about? Is it just camouflage? To look like shadows cast by branches in the trees? It's not easy to get the answer to this question with regular Google, so I kind of keep putting off finding out. Then today I read this neat article about zebra stripes. Somebody finally did empirical tests with horseflies. It turns out that dark colored horses reflect polarized light similar to a pool of water. White horses don't have that invisible-to-the-human-eye resemblance to the fly breeding site. It's well known (apparently, to horse people) that flies bother white horses less. So the researchers tested variations of pattern and found that the wavelength disruption of black and white patterns is even less appealing to flies than plain white. Zebras for the win! (I like how they reveal their true scientist nature by conceding that tsetse flies, a horsefly relative, don't bother any animal out on the open plain, only around water.)

I suppose I could find out just what I want to know about ringed tails from a introductory biology textbook. I used to have one but I didn't have room to keep it anymore. From reading online college papers from people who do have these textbooks I gather that most of the ring tailed creatures, particularly raccoons and the red panda, seem to have evolved from the same distant relative, common to all in the superfamily Musteloidea. The ring-tailed cat is the same family as the raccoon, so it's easy to see how they have similar tails. Ring tailed lemurs? They're a wet-nosed primate, so I don't know. I'll just have to wait for some biology major to write a paper about it, somebody with library access and good search engine optimization.


  1. Nice topic. Could be a doctoral thesis in the making here (I know, you need one of those like you need dengue fever). I wondered if the hypothesis of zebra stripes causing predator confusion had been experimentally tested (or how one would manage that), but not enough to look it up. So glad you did.

    One thing I often think about is, given the short time humans have been cognizant of the world around them, that we may overlook something about certain mutations and characteristics. That is, that there are untold number of benefits brought on by particular mutations, and that some of those benefits may no longer even exist.

    Maybe a certain pattern of color in a coat, or a certain musk gland scent imparted a particular survival advantage in a prey species over a species of predator. But say that predator became extinct. 4000 years later, humans are trying to figure out why that prey animal stinks like hell, or has a certain pattern on its coat. We guess some of the advantages correctly, or partially correctly, but don't know about others because they no longer exist.

    That's just me thinking out loud. Of course this type of thing does happen, but is not even necessarily relevant to your post. It's just that your post reminded me of that. Maybe Raccoon tails offered several advantages that benefitted them: sexual signaling, predator confusion (shaking that booty can do lots for you if you have a ringed-tail!), etc.

    Anyway, rambling on when I'm supposed to be working.

    Oh, by the way, iTunes University will almost certainly have a course relevant to this. Did you look for one in there yet? I just found a nice course in Stellar Evolution that I'm going to start in there. Good times!

  2. Over here at the Slater Museum of Natural History, we were asking ourselves this very same question. This "ringed tail" business is far more widely distributed than you'd imagine. Excluding the bears (Ursidae) and the seals (Pinnipedia), more than half of the families in the order Carnivora have ringed tails. That includes things like the Red Panda, raccoons, and ring-tailed cats (not actually a cat), as well as most true cats, civets, linsangs, and genets. Ringed tails are strikingly absent from the dog family (Canidae) as well as the weasel family (Mustelidae).

    In spite of this relatively high rate of ringed-tailed-ness in Carnivorans, evolutionary biologists believe that the first Carnivores actually had uniform tails. The fancy tail patternings evolved later in arboreal, nocturnal species as a means to visually communicate with other animals at night (the contrasting bands are easy to identify at night). But why only in arboreal species? How could a striped tail make you a better tree-climber? Well, it doesn't, but having a long tail does. Animals that live in the trees use their long tails for balance and support as they move along branches. These long tails are rather conspicuous - sometimes they make up more than half of the animal's body-length - the perfect place to put up a billboard! A long bushy tail is essentially a blank canvas on which an animal can place valuable visual cues and signs for other individuals. For most Carnivorans, these visual cues take the form of ringed tails.

    There are, of course, some exceptions and odd balls. The Cheetah for example, has a partially striped tail despite it's very terrestrial lifestyle. And so do Tigers. In some of these cases, terrestrial species have evolved a spotted or striped coat for camouflage, and these stripes/spots simply continue down the length of the tail. But according to the evolutionary biologists, most ringed tails in Carnivores evolved entirely independently from patterns on the rest of the body.

    As for the ringed tail of the Ringtailed Lemur (a Primate, not a Carnivore), the same rules probably apply. A long tail for arboreal locomotion is a great place to put some valuable visual cues. And we know that lemurs are very social, so it makes sense that they would utilize their tail to communicate (apparently Ringtailed Lemurs use their beautiful tails in scent displays and aggressive interactions between rival individuals).

    If you'd like more information regarding color patterns in the Carnivora order, check out Alessia Ortolani's paper from 1998 titled "Spots, stripes, tail tips, and dark eyes: Predicting the function of carnivore color patterns using the comparative method."

    Stay curious!

    Education and Outreach Coordinator
    Slater Museum of Natural History
    University of Puget Sound
    Tacoma, WA, USA