HOW ANIMALS RUN Author(S): Milton Hildebrand Source: Scientific American , Vol

HOW ANIMALS RUN Author(s): Milton Hildebrand Source: Scientific American , Vol. 202, No. 5 (May 1960), pp. 148-160 Published by: Scientific American, a division of Nature America, Inc. Stable URL: https://www.jstor.org/stable/10.2307/24940484

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This content downloaded from 132.174.250.220 on Tue, 01 Oct 2019 22:32:46 UTC All use subject to https://about.jstor.org/terms HOW ANIMALS RUN

Many animals, both predators and prey, have evolved the ability to run two or three tinles faster than a man can. What are the adaptations that make these impressive performances possible?

by Milton Hildebrand

man (but not necessarily you or from skeletons and muscles. The move- Iyzed. The main problem is to get pic A I!) can run 220 yards at the rate of ments of the running animal are so fast tures from the side of animals running 22.3 miles per hour, and a mile at and so complex that they cannot be at top speed over open ground. With 15.1 miles per hour. The cheetah, how analyzed by the unaided eye. an electric camera that exposes 200 ever, can sprint at an estimated 70 miles In my study I have related compara frames per second I have succeeded in per hour. And the horse has been known tive anatomy to the analysis of motion photographing the movements of a chee to maintain a speed of 15 miles per hour pictures of animals in action. The meth tah that had been trained by John Ham not just for one mile but for 35 miles. od is simple: Successive frames of the let of Ocala, Fla., to chase a paper bag Other animals are capable of spectac motion picture are projected onto trac in an enclosure 65 yards long. However, ular demonstrations of speed and endur ing paper, where the movements of the the animal never demonstrated its top ance. Jack rabbits have been clocked at parts of the body with respect to one speed, but merely loped along at about 40 miles per hour. The Mongolian ass is another and to the ground can be ana- 35 miles per hour. I have used the same reported to have run 16 miles at the im pressive rate of 30 miles per hour. Ante- lopes apparently enjoy running beside a moving vehicle; they have been reliably timed at 60 miles per hour. The camel has been known to travel 115 miles in 12 hours. Nearly all carnivorous mammals are good runners: the whippet can run 34 miles per hour; the coyote, 43 miles per hour; the red fox, 45 miles per hour. One red fox, running before hounds, covered 150 miles in a day and a half. A fox terrier rewarded with candy turned a treadmill at the rate of 5,000 feet per hour for 17 hours. I have been attracted by such per formances as these to undertake an in vestigation of how the living running machine works. The subject has not been thoroughly explored. One study was un dertaken by the American photographer Eadweard Muybridge in 1872. Working before the motion-picture camera was invented, Muybridge set up a battery of still cameras to make photographs in rapid sequence. His pictures' are still standard references. A. Brazier Howell's work on speed in mammals and Sir James Gray's studies on posture and movement are well known to zoologists. Many in vestigators have added to our knowledge of the anatomy of running vertebrates, but the analysis of function has for the STRIDE OF A CANTERING HORSE is shown in these photographs from Eadweard Muy most part been limited to deductions bridge's The Horse in Motion, published in 1878. The sequence runs right to left across the

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This content downloaded from 132.174.250.220 on Tue, 01 Oct 2019 22:32:46 UTC All use subject to https://about.jstor.org/terms camera to make pictures of horses run swift as their pursuers, but sometimes a long stride and a slow rate of stride; ning on race tracks, and I am presently they have superior endurance or agility. the wart hog matches this speed with a collecting motion-picture sequences of Speed and endurance are the capaci short stride and a rapid rate. High speed other running animals from commercial ties that characterize all cursorial verte requires that long strides be taken at a and private sources. brates. But one could not make a defini rapid rate, and endurance requires that tive list of the cursorial species without speed be sustained with economy of �l cursorial animals (those that can deciding quite arbitrarily how fast is fast effort. run far, fast and easily) have and how far is far. Even then the list Although longer legs take longer evolved from good walkers, and in doing would be incomplete, because there are strides, speed is not increased simply by so have gained important selective ad reliable data on speed for only a few the enlargement of the animal. A larger vantages. They are able to forage over animals; in most cases authors quote au animal is likely to have a lower rate of wide areas. A pack of African hunting thors who cite the guesses of laymen. stride. Natural selection produced fast dogs, for example, can range over 1,500 Many cursors are extinct. On the basis runners by making their legs long in re square miles; the American mountain of fossils, however, we can surmise that lation to other parts of the body. In lion works a circuit some 100 miles long; many dinosaurs were excellent runners; cursorial animals the effective length of individual arctic foxes have on occasion that some extinct rhinoceroses, having the leg-the part that contributes to wandered 800 miles. Cursorial animals had long and slender legs, were very fast; length of stride-is especially enhanced. can seek new sources of food and water and that certain extinct South American The segments of the leg that are away when their usual supplies fail. The camel grazing animals, having evolved a horse from the body (the foot, shank and fore moves from oasis to oasis, and in years like form, probably had horselike speed. arm) are elongated with respect to the of drought the big-game animals of Af In order to run, an animal must over segments close to the body (the thigh rica travel impressive distances. The mo come the inertia of its body and set it and upper arm). In this evolutionary bility of cursorial animals enables them into motion; it must overcome the inertia lengthening process the bones equiva to overcome seasonal variations in cli of its legs with every reversal in the di lent to the human palm and instep have mate or in food supply. Some herds of rection of their travel; it must compen become the most elongated. caribou migrate 1,600 miles each year. sate for forces of deceleration, including Man's foot does not contribute to the According to their habit, the predators the action of the ground against its de length of his leg, except when he rises among the cursorial animals exploit su scending feet. A full cycle of motion is on his toes. The bear, the opossum, the perior speed, relay tactics, relentless en called a stride. Speed is the product of raccoon and most other vertebrates that durance or surprise to overtake their length of stride times rate of stride. The walk but seldom run have similar planti prey. The prey species are commonly as giraffe achieves a moderate speed with grade ("sole-walking") feet. Carnivo-

J top row and continues across the bottom row. With these and or not a horse "even at the height of his speed (hasl all four of similar photographs Muybddge settled the controversy of whether his feet .. . simultaneously free from contact with the ground."

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This content downloaded from 132.174.250.220 on Tue, 01 Oct 2019 22:32:46 UTC All use subject to https://about.jstor.org/terms IOUS mammals, birds, running dinosaurs the coracoid bone. Because mammals do when the back is extended than when and some extinct hoofed mammals, on not have a coracoid bone their shoulder it is flexed. By extending and flexing its the other hand, stand on what corre blade has some freedom of movement. back as its legs swing back and forth the sponds to the ball of the human foot; In the carnivores this freedom is in animal adds the increase in its body these animals have digitigrade ("finger creased by the reduction of the collar length to its stride. Timing is important walking") feet. Other hoofed mammals bone to a vestige; in the ungulates the in this maneuver. If the animal were to owe an even further increase in the ef collarbone is eliminated. In both carni extend its back while its body was in fective length of their legs to their un vores and ungulates the shoulder blade mid-air, its hindquarters would move guligrade ("hoof-walking") posture, re is oriented so that it lies against the side backward as its forequarters moved for sembling that of a ballet dancer standing of a narrow but deep chest rather than ward, with no net addition to the for on the tips of her toes. Where foot pos against the back of a broad but shallow ward motion of the center of mass of its ture and limb proportions have been chest, as it does in man. Thus mounted, body. In actuality the running animal modified for the cursorial habit, the in the shoulder blade pivots from a point extends its back only when its hind feet creased length and slenderness of the about midway in its length, and the are pushing against the ground. The leg is striking [see illustration on page shoulder joint at its lower end is free cheetah executes this maneuver so adept 155]. to move forward and backward with the ly that it could run about six miles per swing of the leg. The exact motion is ex hour without any legs. he effective length of the front limb ceedingly difficult to ascertain in a run With the extra rotation of its hip and T of many runners is also increased by ning animal, but I have found that it shoulder girdles and the measuring the modification of the structure and adds about 4.5 inches to the stride of the worm action of its back, the legs of the function of the shoulder. The shoulder walking cheetah. running cursor swing through longer joint of amphibians, reptiles and birds is The supple spine of the cat and the arcs, reaching out farther forward and virtually immobilized by the collarbone, dog increases the length of stride of backward and striking and leaving the which runs from the breast bone to each these animals still further. The body of ground at a more acute angle than they shoulder blade, and by a second bone, such an animal is several inches longer would if the back were rigid. This clear-

SECOND FRONT FIRST HIND I I

FIRST FRONT rlRST HIND I, ! I I TIME (SECONDS) .1 .2

STRIDES OF THE CHEETAH AND THE HORSE in full gallop relate to the time·scale at bottom, which is calibrated in lOths of a nre contrasted in these illustrations. The sequence and duration of second. The cheetah has two unsupported periods, which account their footfalls, indicated by the horizontal lines under each animal, for about half its stride; the horse has one unsupported period,

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This content downloaded from 132.174.250.220 on Tue, 01 Oct 2019 22:32:46 UTC All use subject to https://about.jstor.org/terms ly increases stride length, but it also ag the horse all of these anatomical and vantage of its longer length of stride. gravates a problem. The body of the functional adaptations combine to pro The familiar mechanical principle of animal tends to rise when its shoulders duce a 23-foot stride. The cheetah, al gear ratio underlies the fast runner's and hips pass over its feet, and tends though smaller, has a stride of the same more effective use of its trim muscula to fall when its feet extend to the front length. ture. In the linkage of muscle and bone or rear. Carnivores offset this bobbing the gear ratio is equal to the distance motion by flexingtheir ankles and wrists, ast runners must take their long between the pivot of the motion (the thus shortening their legs. Ungulates do F strides rapidly. The race horse com shoulder joint, for example) and the the same by sharply flexing the fetlock pletes about 2.5 strides per second and point at which the motion is applied joint at the moment that the body passes the cheetah at least 3.5. It is plain that (the foot) divided by the perpendicular over the vertical leg. The cheetah, a the higher the rate of stride, the faster distance between the pivot and the point long-striding back-flexer, supplements the runner must contract its muscles. at which the muscle is attached to the its wrist-flexing by slipping its shoulder One might infer that cursorial animals bone. Cursorial animals not only have blade up its ribs about an inch, and thus as a group would have evolved the abil longer legs; their actuating muscles are achieves a smooth forward motion. ity to contract their muscles faster than also attached to the bone closer to the Since running is in actuality a series other animals. Within limits that is true, pivot of motion. Their high-gear mus of jumps, the length of the jump must but there is a general principle limiting cles, in other words, have short lever be reckoned as another important incre the rate at which a muscle can contract. anns, and this increases the gear ratio ment in the length of the stride. Hoofed Assuming a constant load on the muscle still further. In comparison, the anatomy runners have one major unsupported pe fibers, the rate of contraction varies in of walking animals gives them consider riod, or jump, in each stride: when the versely with any of the muscle's linear ably lower gear-ratios; digging and legs are gathered beneath the body. The dimensions; the larger muscle therefore swimming animals have still lower gear galloping carnivore has two major un contracts more slowly. That is why an ratios. supported periods: when the back is animal with a larger body has a slow But while high gears enable an auto flexed, and again when it is extended. In er rate of stride and so loses the ad- mobile to reach higher speed, they do

ONE STRIDE

:OND FRONT

ONE STRIDE

which accounts for about a quarter of its stride. Although both because it takes about 3.5 strides to the horse's 2.5. The size oj the cheetah and the horse cover about 23 feet per stride, tbe cheetah the horse has been reduced disproportionately in these drawing, attains speeds on the order of 70 miles pel' hour, to the horse's 43, for the sake of uniformity in the stride·lines and time·scale.

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This content downloaded from 132.174.250.220 on Tue, 01 Oct 2019 22:32:46 UTC All use subject to https://about.jstor.org/terms so at the expense of power. The curso rial animal pays a similar price, but the exchange is a good one for several rea sons. Running animals do not need great power: air does not offer much resistance even when they are moving at top speed. Moreover, as the English investigators J. M. Smith and R. J. G. Savage have noted, the animal retains some relatively ANCESTRAL low-gear muscles. Probably the runner MAMMAL uses its low-gear muscles for slow mo tions, and then shifts to its high-gear muscles to increase speed. Since the speed at which a muscle can contract is limited, the velocity of the action it controls must be corre spondingly limited, even though the muscle speed is amplified by an opti mum gear-ratio. A larger muscle, or additional muscles, applied to action around the same joint can produce in creased power but not greater speed. Several men together can lift a greater weight than one can lift alone, but sev 3 eral equally skilled sprinters cannot run faster together than one of them alone. The speed of a leg can be increased, however, if different muscles simultane ously move different joints of the leg in the same direction. The total motion they produce, which is represented by the motion of the foot, will then be greater than the motion produced by any one muscle working alone. Just as the total speed of a man walking up an escalator is the sum of his own speed plus that of the escalator, so the inde pendent velocities of each segment of the leg combine additively to produce a higher total velocity. The trick is to move as many joints as possible in the same direction at the same time. The evolution of the cursorial HORSE body has produced just this effect. By abandoning the flat-footed plantigrade posture in favor of a digitigrade or un guligrade one, the cursorial leg acquired an extra limb-joint. In effect it gained still another through the altered func tioning of the shoulder blade. The flexi ble back of the cursorial carnivore adds yet another motion to the compound mo tion of its legs; the back flexes in such a 5 way that the chest and pelvis are always rotating in the direction of the swinging limbs. The supple spine of the carnivore contributes to stride rate by speeding up the motion of its body as well as of its legs. The spine is flexed when the runner's firsthind foot strikes the ground, 3 4 3 4 3 and by the time its second hind foot

leaves the ground the animal has ex MODERN CURSORIAL FOOT EVOLVED from the broad, five.digited foot of an an· tended its spine and thus lengthened its cestral mammal (top). Lateral digits were lost and metatarsal bones, the longest in the foot, body. In the brief interval when its hind were further elongated. Resultant foot is lighter and longer. Pudu is a deer of the Andes.

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This content downloaded from 132.174.250.220 on Tue, 01 Oct 2019 22:32:46 UTC All use subject to https://about.jstor.org/terms A plastic that blasts holes

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This content downloaded from 132.174.250.220 on Tue, 01 Oct 2019 22:32:46 UTC All use subject to https://about.jstor.org/terms feet are planted, the forequarters, riding cles mllst raise the body to compensate on- the extending spine, move farther for the falling that occurs during the un in ITletal and faster than the hindquarters. Simi supported phases of the stride. The load larly when the front feet are on the they must raise is proportional to the Experimentation in THIOKOL labora ground, the hindquarters move faster mass of the body, which is in turn pro tories constantly widens military and than the forequarters. So although the portional to the cube of any of its linear industrial horizons of polymer chem speed that the driving legs can impart dimensions. A twofold increase in body istry •••offers challenging opportu nities to chemists intrigued with this to the forequarters or hindquarters is length thus increases weight eightfold. area of research. limited by their rate of oscillation, the The force that a muscle can exert, on the Who but THIOKOL, originator and body as a whole is able to exceed that other hand, increases only as the square firstproducer of the wonder chemical limit. In a sense the animal moves faster of its cross section. Thus against an - liquid polysulfide polymer - should than it runs. For the cheetah the ad eightfold increase of load, bigger mus find greater interest in the newest vantage amounts to about two miles per cles can bring only a fourfold increase of family of polymers, the urethanes. hour-enough to add the margin of suc force. As body size increases, the capac One of the many intriguing ure cess in a close chase. ity of the muscles to put the body in for thane applications developed by In addition to the obvious tasks of ward motion and to cause its legs to os THioKOL scientists is molding a new propelling the animal's body and sup cillate cannot quite keep up with the kind of shaped charge which will cut porting its weight, the locomotor mus- demands placed upon them. These fac- holes in steel plate of up to 1" thick ness in a matter of microseconds. In this application, an explosive charge is set in a urethane casting. Once ignited, the force of the explo sion is directed along a sharp line a line of fire that blasts right through metal. Shaped charges for producing circular holes, stmight lines or other forms can be made from urethane castings. The resultant cut will be ac curate to within plus 01' minus .002". Currently, shaped charges are being used to cut port vents in rocket cas ings. Improving urethane compositions and finding new utility for them-both military and industrial-is one job of THIOKOL chemical research teams. Imaginative chemists with the desire to explore new and uncharted avenues will find challenging assign ments - and commensurate rewards at THIOKOL. We are interested in sci entists with experience, or academic accomplishments in: Formulation and Physical Testing of Polymers • Organic and Inorganic Chemistry • Organometallics • Metal Hydrides Synthesis • High Vacuum Techniques • Propellant Analysis and Formulation • Combustion Proc esses • Fast Reaction Kinetics · Fluo rine Synthesis • Many other areas of fundamental and advanced research. Many doors are open at THIOKOL. For full information, contact Personnel Directo1' at any plant address below.

CHEMICAL CORPORATION Bristol, Pennsylvania Other Plants in: TRENTON, NEW JERSEY; DENVILLE, NEW JERSEY; MOSS POINT, MISSISSIPPI; ELKTON, MARYLAND; MARSHALL, TEXAS; HUNTSVILLE, ALABAMA; BRIGHAM CITY, UTAH. ADAPTATION OF THE LEG FOR SPEED is illustrated by the hind·leg bone of the slow badger (left), moderately fast dog (middle) and highly adapted deer (right). The length. ®Registered trademark of the Thiokot Chemical Corporation for its rocket propellants,liquid polymers, plasticizers, and other chemical ened metatarsus of the latter two has yielded a longer foot and an altered ankle posture that products. is better suited to rW1l1ing. The thigh bones of all three animals have been drawn to the same scale to show that the leg segments farthest from the body have elongated the most.

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This content downloaded from 132.174.250.220 on Tue, 01 Oct 2019 22:32:46 UTC All use subject to https://about.jstor.org/terms tors in the nature of muscle explain why but also adaptations that reduce the load in the direction of travel, and the mus the largest animals can neither gallop on their locomotor structures and econo cles that manipulate the digits or rotate nor jump, why small runners such as mize the effort of motion. In satisfying the forealID have disappeared. The ulna rabbits and foxes can travel as fast as this requirement natural selection pro in the forearm and the fibulain the shank race horses without having marked struc duced a number of large and fast runners -bones involved in these former mo tural adaptations for speed and why the that are able to travel for long distances tions-are reduced in size. The ulna is re larger cursorial animals must be highly at somewhat less than their maximum tained at the point where it completes adapted in order to run at all. speed. In these animals the mass of the the elbow joint, but elsewhere becomes limbs is minimized. The muscles that in a sliver fused to its neighbor; the fibula f the bigger runners are to have en other animals draw the limbs toward or is sometimes represented only by a nub I durance as well as speed, they must away from the midline of the body (the bin of bone at the ankle. have not only those adaptations that in "hand-clapping" muscles in man) are The shape of the cursorial limb em crease the length and rate of their stride, smaller or adapted to moving the legs bodies another load-reducing principle.

I // I

POWER AND SPEED are alternatively achieved in the badger oscillation rate, coupled with a longer leg (l), yields a faster stride. (left) and the cheetah (middle) by placement of the teres major In the vicuna (right) the gluteus muscle (c) develops about five muscle. In the cheetah the small distance (b) between the muscle times the velocity but only a fifth the force of the larger semimem insertion and the joint it moves yields a higher rate of oscillation branosus muscle (d). The animal may use the latter to overcome than in the badger, in which the distance (a) is greater. The higher inertia; the former, for high speed. Legs are not in same scale.

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This content downloaded from 132.174.250.220 on Tue, 01 Oct 2019 22:32:46 UTC All use subject to https://about.jstor.org/terms IN TORRINGTON SPHERICALS ... NO COMPROMISE WITH QUALITY

The spherical design concept is the major factor in the su the roller seeks positive contact with the center flange. The perior performance characteristics and longer service life of result is extremely accurate roller guidance, a high degree Torrington Spherical Roller Bearings. It provides inherent of geometric stability, minimum friction, and truer rolling self-alignment... allows compensation within the bearing for motion. initial or dynamic misalignment. It insures the maintenance Torrington Spherical Roller Bearings are one outstanding of full radial and thrust capacity under these conditions. example of Torrington's uncompromising research, engi The center guide flange thrust surface and mating roller neering and manufacturing skills. They are your assurance ends are spherically ground to the same radius. Under load, of the ultimate in bearing performance.

THE TORRINGTON COMPANY

South Bend 21, Indiana • Torrington, Connecticut

PROGRESS THROUGH PRECISION-IN BEARING DESIGN AND PERFORMANCE

160

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