The Three-Minute Outdoorsman: Wild Science from Magnetic Deer to Mumbling Carp

53. A New Respect for Porcupine Quills

206 53 a new respect for porcupine quills I own pointing dogs, which I let range far afield, and when we’re hunting, several major concerns loom. One is that they’ll get caught in a trap. Second, they’ll find a skunk that they can’t help investigate (been there, done that). Lastly, the dog will corner a porcupine and get a snoot full of quills (fortunately, not yet). We have all heard plenty of trip-ending stories about dogs and porcupine encounters, for good reason—the porcupine is a formidable opponent! Our local porcupine has about thirty thousand quills, which are actually modified hairs that are reinforced with keratin, a tough structural protein. Contrary to myth, the porcupine cannot throw quills at predators; they are only released upon direct contact. My attention was drawn to a 2012 article titled “Microstructured Barbs on the North American Porcupine Quill Enable Easy Tissue Penetration and Difficult Removal,” by Woo Cho and colleagues in the Proceedings of the National Academy of Sciences. My first reaction was, “Gee, haven’t we known for a long time that the quills have barbs like a fish hook that hold the quills in, and it is these barbs that result in painful extraction even if you cut the ends and ‘let the air out?’” But Cho went many steps beyond this and actually figured out how and why the quills are so effective. I read on. Cho and his group examined porcupine quills under extremely high magnification. Quills have two distinct regions: a conical black tip that contains a layer of microscopic backwardfacing barbs on its surface, and a cylindrical white base that con- A N I M A L I N T E L L I G E N C E 207 tains smooth scalelike structures. The barbs overlap slightly, and there is a one- to five-micrometer space between the tip of each barb and the quill shaft. The size of the barbs becomes larger farther from the apex of the tip. Most features of organisms, like porcupine quills, are the results of long periods of natural selection, making them better and better adapted to their particular functions. Of course, evolution via natural selection works within competing constraints, as we’ll see. If a structure gets too good at doing one thing and it has multiple functions, then it may need to balance competing demands and not be as good as it could for a particular function. Compromise, then, is an evolutionary strategy. Imagine that you wanted to make a porcupine quill from scratch. You would want it to have characteristics that would make it the optimal weapon. Your quill should provide excellent penetration, that is, maximal capacity to penetrate the skin and muscle of your adversary. Get it in deep is your number one priority . But then you would want it to stay there, and so you would want the quill to be difficult to remove. These two qualities, penetration and removal, might be oppositely opposed. For example, if the backward barbs were too big, they might impede penetration . If they were too small, the barb might be too easily removed. Somewhere in between must be just right. Now for the fun point—who says research has to be boring? How would you “test” the value of quills for these two functions, given that using them on yourself or getting lab mates to volunteer seems rather unlikely? The researchers plunged quills into pig or chicken skin and muscle and measured the “penetration force” and the “pullout force.” To have a basis for comparison, they carefully sanded off the barbs on some quills, making them smooth and needlelike. From others, they removed barbs from different sections, to see where the maximum effects were localized. I had guessed that a quill shaped like a needle the nurse uses to give vaccines would be better at penetrating than one with barbs like a porcupine quill, and that the opposing needs of pen- t h e t h r e e - m i n u t e o u t d o o r s m a n 208 etration and removal would sort of hamstring the quill’s penetrating ability. But I was wrong! Cho found, using the same force, that barbed quills penetrate deeper and better than smooth ones. Further experimentation showed that the barbs within four millimeters of the tip help most...