The Power of Shark Bites

A posting came across SHARK-L: "I have read several times that the American alligator possesses the strongest jaws of any animal. How does the biting pressure of a great white shark compare to that of an alligator or crocodile?" I made the below theoretically-based prediction.

Human fascination with the biting power of large, spectacular predators especially those known or supposed to have attacked human beings dates back to antiquity. Just about every fan of such predatory superstars has his or her favorite candidate for the title of World's Most Powerful Bite. Unfortunately, actually measuring the bite power of animals is logistically quite difficult. In the absence of hard data, most discussions about which animal has the most powerful bite run aground or degenerate into heated debate.

Let's begin by clarifying a few terms. "Strength" may be defined as resistance to deformation. "Force" may be defined as a 'push' or 'pull' in a particular direction; according to Newton's second law of motion, force (N) is proportional to the product of mass (g) and acceleration (m/s^2). "Pressure" may be defined as force per unit area. And "power" may be defined as force per unit time.

By the above definitions, shark jaws are much less strong than those of either humans or crocodiles, as (based on my experience, at least) it takes considerably less force to warp or distort the jaws of sharks than those of either humans or crocodiles (On the other hand, the jaws of humans and crocodiles are probably more prone to being shattered by a sharp blow, say with a hammer but that's more a function of power than sheer force ... not to mention being a rather short-sighted way to treat a friend or valuable specimen). Due to the often complex, interconnected ways in which muscles, connective tissue, and skeletal elements such as bone or cartilage move together, it often extremely difficult in practice to measure the mass of tissue actually accelerated during jaw closure (even if one defines force of a bite in a given direction as many head components may have a parallel vector component, which may or may not result solely from muscular contraction of jaw adductors). In addition, it can be extremely difficult to measure the duration of maximum force during a bite without altering the bite action itself (for example, sharks often seem to nip onto unfamiliar objects gingerly, as though testing it for tactile cues; as such, a shark biting an unfamiliar object such as a diver wearing a Neptonic chain-mail suit may not clamp down with all the force or power it can muster). It is therefore extremely difficult to measure either the force or power of a shark's bite.

As a consequence of these and other inherent difficulties, most scientific studies of shark bite kinematics and dynamics have been descriptive rather than quantitative (Frazetta and Prange 1987; Maisey 1980; Moss 1972, 1977; Motta and Wilga 1995; Powlik 1995; Tricas 1985; Tricas and McCosker 1984). Many shark enthusiasts are no doubt familiar with the experiments on shark bite pressure carried out by James Snodgrass and Perry Gilbert in 1965 (the paper was published in Sharks, Skates and Rays in 1967). Snodgrass designed a shark 'gnathodynamometer' (bite-meter) consisting of a soft aluminum core in the form of a 3 by 32-centimetre cylinder with four longitudinal V-shaped groves cut into its surface, into which were partially inserted a total of 12 steel ball-bearings, overtop of which were placed four curved plates of steel, and the whole lot was sandwiched in four plates of polyvinyl chloride (PVC). Wrapped in mackerel (in order to make it more palatable, no doubt), this novelty snack was offered to Tiger (Galeocerdo cuvier), Lemon (Negaprion brevirostris), and Dusky (Carcharhinus obscurus) Sharks at the (now defunct) Lerner Marine Laboratory in Bimini. By correlating the depth of indentations the ball-bearings made into the aluminum core, Snodgrass and Gilbert were able to estimate bite pressure. The maximum pressure obtained for a single tooth tip (2 mm^2) is 60 kg for a 2-metre Dusky shark. This converts to about 3 tonnes (metric tons) per square centimetre. This has often been reported in the popular press as about 18 tons per square inch. But the figure is often misinterpreted, many inferring incorrectly that a shark with, say, a 10-inch mouth could exert a total force of 180 tons. Recall that the initial reading was for the pressure at the tooth tips, and thus cannot validly be multiplied over the total area of the mouth. After all, those nifty Ginsu knives can cut tin cans and tomatoes not because the blade has a large area or the wrist of the user is Sumo-wrestler-strong but because the pressure is concentrated along the very fine edge of the blade. To put this another way Psycho Norman Bates would have inflicted considerably less damage on his victims (although maybe made a bigger though easier to clean up mess) had he stabbed them with a watermelon rather than the tip of a knife.

In any event, some problems with the experimental approach of Snodgrass and Gilbert include: 1) uncertainty about to what extent the pressure of the sharks' bites was absorbed or distributed by either the fish or PVC wrapping, or distributed lengthwise along the gnathodynamometer by the steel plates rather than indenting the aluminum core; 2) uncertainty about how much of the energy of the sharks' bites was lost in sound or heat production as the animals' teeth scraped against the apparatus (probably very minimal, but a source of energy dissipation nonetheless); or 3) uncertainty about to what extent the sharks were inhibited from biting as powerfully as they could by either the novelty of the apparatus or the proximity of the researchers. Of these, the first and third seem likely to be major sources of uncertainty. Valerie Taylor, who gained a certain Cover Girl notoriety by being photographed allowing blue sharks (Prionace glauca) to bite her while she wore a Neptonic chain-mail suit, has boldly stated that she "[doesn't] believe that shark bites are as powerful as they are reputed to be". Having been bitten by sharks on several occasions (always my fault, I hasten to add), I can attest that shark bites on humans are typically not very forceful, seeming threatening or exploratory rather than sincere attempts to inflict damage. Thus, sharks may well be capable of exerting even greater bite pressures than have been recorded for them.

To the best of my (admittedly incomplete) knowledge, no scientist has yet measured the bite pressure of either alligators, crocodiles or white sharks. The jaws of vertebrate animals are essentially class 3 levers with the fulcrum at one end, the load (object being bitten) near the other, and the force (muscles) applied between these two key points. In general, short jaws produce greater bite pressure at the tip than do long jaws, in much the same way as the greatest force exerted by the blades of a pair of shears or tin snips occurs near the pivot point rather than near the tips. It is well established that the total force of muscle contraction is directly proportional to its cross-sectional area. Although crocodilians have fairly thick masseter muscles (the principle jaw-closing adductor in vertebrates), they also have relatively long jaws. Therefore, I suspect that the bite of a crocodile is not particularly powerful. The horrific tissue damage that crocodilians inflict on their prey seems to be due largely to the conical teeth tearing flesh as the predator thrashes and rolls, not from the power of the bite itself. Some elasmobranchs such as Horn Shark (Heterodontus francisci), the Nurse Shark (Ginglymostoma cirratum), or the Southern stingray (Dasyatis americana) have very short, heavily calcified jaws relative to the White Shark (Carcharias carcharodon); it would be interesting to know how much bite pressure or force any of these crestiaconts could muster. Since muscle tissue generally contracts more efficiently at higher temperatures than at lower (within limitations of muscle proteins being able to resist denaturization at high temperatures, of course), it is interesting to speculate on the bite power of warm-bodied sharks like the great white.

Portrait of the Shortfin Mako (Isurus oxyrhinchus) The only attempt to measure the bite pressure of a lamnid shark of which I am aware occurred in the late 1970's, when University of Illinois graduate student Clarice Prange offered a Snodgrass-type gnathodynamometer to a 2-metre Shortfin Mako (Isurus oxyrinchus) some 100 kilometres off the coast of San Diego. When the data were later analyzed, the Mako's bite pressure was estimated to be about 4 tons per square inch less than one-quarter that obtained for a similar-sized dusky shark. But, prior to conducting the test, the subject Mako was several times fended away from the cage and gnathodynamometer until the cameramen were properly set up, and thereafter appeared reluctant to bite the gnathodynamometer concealed in a tuna bait (footage of this experiment was featured in a Survival Anglia film entitled cleverly enough Shark, copyrighted in 1979). Therefore, it is quite possible perhaps likely that the Mako did not bite with all the force it could have. Given the difficulties of knowing what's going on in the mind of other creatures even members of our own species we'll probably never know for sure whether this Mako gave its test bite everything it had.

One of the very few perhaps the only creatures whose bite pressure can be measured fairly accurately is human beings. The average biting force of an adult male human (male because, in general, we lugs are bigger and have proportionately thicker masseter muscles than females) varies between 45 and 68 kg although forces as great as 159 kg have been recorded for Inuit males (squeaky-voiced, ear-biting tough guy Mike Tyson, please take note). The Dental Science Handbook published by the American Dental Association (1970) gives bite pressures by humans as great as 15 tons per square inch 83% as strong as the greatest pressure recorded thus far for any shark. No wonder human bites are often so devastating we have short jaws, powered by thick masseter muscles and armed with relatively blunt, chisel-shaped incisors. So play nice, kiddies! ;>

Sorry I couldn't be more helpful, but I hope that at least some of the foregoing is of interest.

Cheers,

     R. Aidan Martin

Originally posted to SHARK-L December 24, 1997

 
 

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Text and illustrations R. Aidan Martin
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