[Fact Or Fiction] The Tooth Of The Rat

October 16, 2004

T his is the story about one of the most pervasive and compelling legends in urban biology — the awesome force exerted by a biting rat. It was back in 1977 when I first encountered this concept in the hallowed pages of National Geographic magazine. (The article, in the July issue, was titled "The Rat: Lapdog of the Devil.") Among an assortment of "gee whiz" facts about rat abilities was the statement that their teeth exerted "an incredible 24,000 pounds per square inch."

Huh? Twelve tons? Hey, the whole darn animal only weighs about a pound and you’re telling me he’s got the chops to bite down with a force equal to the weight of a tractor trailer? But I let it drop, and over the years the usual figure that I’ve seen quoted in various pest management publications is a mere 7,000 psi. That’s right, 3½ tons to those fangs of fury. I still smelled a rat, but nobody wanted to talk about the subject at parties so I continued to ignore it.

Then, quite recently, animal bite pressure surfaced in the news. In an effort to calculate the chomping force of both Tyrannosaurus rex and an enormous extinct reptile dubbed "SuperCroc," Dr. Gregory Erickson at Florida State University took careful measurements of alligator bites with a padded pressure gauge on the end of a pole. As reported in the March 2003 issue of National Geographic (I’m obviously their biggest fan), large adult gators "can snap their jaws shut with as much as 3,000 pounds of force — the strongest bite known among living creatures."

Wondering if I was the only one who mulled over apparent inconsistencies in animal biting lore, I resolved to finally get to the bottom of this pressing issue.

A MEASURE OF CONFUSION. The prevailing misimpression over how hard a rat bites centers around an unexpectedly deceptive unit of measure: pounds per square inch. Psi is a way of measuring force that’s acting uniformly over a relatively large expanse. You’d properly employ it to describe how much weight a concrete floor slab can support, the operating pressure of a steam boiler or the force of water against a submarine’s hull. Used correctly, psi expresses what is happening at any given square inch of surface area.

Unfortunately, even relatively sober scientists apparently cannot resist "playing the psi card" to spice up their numbers for popular consumption. In this regard, using psi to describe the bite force of a small animal is kind of like me telling you my body weight in pounds per square yard. The bathroom scale says I weigh a mundane 160 pounds, which the bottoms of my feet exert over an area of about 66 square inches. That means, after a few quick calculations, I can authoritatively state that I stand with a force of 3,142 pounds per square yard! But so what? It’s a theoretical extrapolation, an illusory figure that exists only as an algebraic sleight-of-hand.

Confusion about this sort of calculation arises because it’s easy to mistake that impressive number as representing something real, i.e. what a given force would actually be if it was simply expanded over a larger area. But, if I strap on large plywood cutouts and go to a party as Bigfoot, my weight is still 160 pounds. It’s just distributed over a greater expanse than when I’m standing barefoot on the scale.

A FORGOTTEN CLASSIC. Let’s get back to rodent biting. How did all that monster psi craziness start? Thirty years ago, an engineer from the old Bell Telephone Laboratories teamed up with two biologists from the U.S. Fish and Wildlife Service to study rodent gnawing mechanics so that more efficient sheathing materials could be designed to protect electrical wires and cable. Their report is an absolute gem, a forgotten masterpiece of pragmatic, collaborative research along the lines of what we now call an IPM approach. Unfortunately, even most university libraries do not have a copy (Cogelia, N.J., G.K. LaVoie, and J.F. Glahn. 1976. Rodent biting pressure and chewing action and their effects on wire and cable sheath. Proceedings of the 25th International Wire and Cable Symposium, Cherry Hill, N.J.: 117-124).

Using an instrument called a force transducer (the same type of tool as in the gator study, just a lot smaller), the investigators measured the biting force of the three primary rodent enemies of cabling: gray squirrel, pocket gopher and Norway rat. And (drum roll, please) what did they find? They discovered that rats bite with an average pressure of about 1 pound. You read right — 1 pound of pressure. Gophers were almost identical, but squirrels were relative brutes, weighing in at nearly 3 pounds of bite force.

The authors didn’t end there. They measured gnawing rates (the average was one-and-a-half bites per second) and concluded that typical chewing behavior is kind of a scraping action that applies only one-half or less of the maximum bite force. They studied damage to various types of cable sheathing, hypothesized two types of material failure depending on its hardness, and recommended guidelines for selecting the most reliable rodent-resistant cable armor. As an applied science aficionado, I was enthralled.

And then — well dash it all, they just couldn’t resist pressing the critters’ incisors onto a thin plastic film, measuring the actual knife-edge biting surface under a microscope, and doing the math to come up with those wacky extrapolated figures. Why, in the very first paragraph of their paper, we find out that rats can bite with 7,000 psi and squirrels can do it at 22,000 psi! The rest is history. These are undeniably sexy numbers, and have thus been passed down through the decades as bona fide rodent attributes. What isn’t mentioned is that the figures are valid only for rodents with an incisor edge the width of your thumb, which would be a rat roughly the size of a rhinoceros.

As for the alligators, that 3,000-pound bite was the actual pressure at the tooth tip and is thus a legitimately scary statistic.

End of story? Not at all. There is indeed an aspect to rat teeth that is truly incredible.

A REMARKABLE TOOL. Along with the gonzo bite pressure lore, there is another category of rat dental accomplishments that is about 95 percent true. This pertains to eyewitness accounts of the awesome variety of stuff they can gnaw through. Like many of you, I see a lot of this damage on a regular basis. Probably the most common example in urban areas involves chiseling through plastic trash receptacles (see Figure 1 on page 84), which I suspect is more difficult than it looks. This is just slightly less impressive than accounts of rats vanquishing tough but brittle materials like brick or cinderblock.

Upping the ante is their ability to penetrate through solid wood (see Figure 2 at left). I was not surprised to learn that prior to the introduction of European metal implements, rat teeth were the tool of choice for wood carving in the South Pacific islands. But the zenith of my personal experience with rodent gnawing prowess is on my own property in Maryland, where squirrels make a hobby of chewing through the 1/8-inch thick hardened aluminum tie wires that secure my chain link fence fabric to the horizontal rails (see Figure 3 on page 87). This is mind-boggling; I can hardly make a scratch on those ties with my steel pocketknife.

How can rodents chew through metal? To begin with, their two sets of incisors are constantly growing, and although they don’t really depend on gnawing to keep them in check (that’s done by the upper and lower pairs grinding against each other), it’s nevertheless handy to have a set of continually renewable chisels. Even more remarkable is what their teeth are made of. Dental enamel is the hardest substance of any mammal’s body, but rodent enamel is the toughest of the tough as a result of intricately crossed crystals of a mineral called calcium hydroxyapatite embedded in collagen. The composite microstructure is so strong that it has served as a model for a so-called "bio-mimetic material," a synthetic copy created in the laboratory with ceramics and polymers.

Why did I say that the spectacular reputation of these tools to inflict some serious damage was "95 percent true"? It’s because context is everything. Studies of resistance of various construction materials to rat penetration have repeatedly confirmed that hardness of a barrier is only one factor in its effectiveness. Equally important is that the rodents need to have some sort of edge on which to initiate a gnawing attack, and sufficient incentive (food comes to mind) to expend all that energy in the first place.

A GRIM REMINDER. Rats don’t just gnaw on structural materials. Thousands of people are bitten every year in the U.S., just one more awful result of poverty and urban deterioration. The worst part is that most of the victims are children younger than six years old, typically attacked on their hands or face (probably because of food residue) while they sleep.

As a matter of fact, I’ve been bitten myself, albeit under far less grim circumstances than most of the statistics. Several years ago I took possession of Casper, a wild Norway rat who was rescued from his doomed nest as a young pup and raised in my house as a family pet. From a ghostly gray furball that seemed to appear and disappear in the blink of an eye, Casper matured into a fine figure of a rodent that tipped the scales at just under a pound. His intelligence was remarkable and he seemed to enjoy human contact, but this was no laboratory rat numbed by thousands of generations of selective breeding. Despite the fact that he permitted himself to be handled on a daily basis, it was obvious he retained the intrinsic wariness and nervous temperament of his kind — something I forgot one day as I was petting him. Casper had just fed and seemed unusually docile as I stroked his fur. In a gesture of affection that my dogs had always tolerated with good humor, I lightly tapped the end of his nose.

Whoa. He didn’t take that well at all. Faster than I could follow with my eye, he seized the offending finger in his jaws, bit down hard, then scampered away to cower in his little house. In the moment of shock that followed, I wasn’t even sure what had happened. Then the blood spurted out, my brain started working again, and I realized I had felt his incisors glance off the bone.

After equal measures of elementary first aid and advanced cussing, my finger was patched up and healed just fine. Poor Casper went on to live out his life and died in his sleep at the ripe old age of three. But our little misunderstanding was when I first remembered what I had read years ago in National Geographic about the supposed 12 tons of bite pressure. Lucky for me — and all of us — that fearsome capability is just a myth. One pound’s worth is quite enough.

All photos are © Al Greene/Nancy L. Breisch

The author gratefully acknowledges the numerous contributions of Dr. Nancy Breisch in his ongoing rodent ruminations, including the calm administration of first aid following biting incidents. The opinions expressed herein are the views of the author and do not necessarily reflect the official policy or position of the U.S. General Services Administration.

The author is regional entomologist for the U.S. General Services Administration in Washington, D.C. He can be reached via e-mail at agreene@giemedia.com.