The Seven Mysteries of Life

by Guy Murchie

  In addition to the upper size limit of earthly flight, moreover, there is a lower limit where tiny insects too small to feel gravity, begin to be buffeted out of control by the so-called Brownian movement of random molecular motion that turns flying into swimming and ultimately, on the microscopic scale, swimming into digging. Competition with free molecules is not the only disadvantage in being small either, I notice, for the task of keeping warm and getting enough to eat becomes cntical even sooner to the small. This is because heat is radiated only from a body's surface area, not from its total volume, and little creatures have relatively huge surfaces So the food supply must be proportional to skin size, not weight - which explains why men may eat less than 2 percent of their weight daily on the average, while mice (despite the insulation of their fur coats) must tuck away a good 25 percent of theirs.

  An extreme case in point is that of the tiniest and most numerous mammal of all the shrew, who comes out at night and burrows largely unseen in every forest floor and through practically every garden in every continent but Australia and Antarctica, and is represented by more than 30 species in North America alone. Although some shrews are so tiny they weigh less than a dime and you, seldom notice the traces of their diggings, their appetites are relatively enormous - inevitably so because such a minuscule creature, with so little mass per square inch of skin, metabolizes four times as fast as the smallest mouse (per gram of tissue) and therefore must eat up to three times his own weight in food every day, and not just vegetable matter (which is good enough for a mouse) but also worms, grubs, insects, fish, frogs, and even mammal meat. The fact that he feels hungry enough to hunt almost every waking minute, a compulsion ultimately enforced by the lurking threat of starvation should he ever fast as long as three or four hours, gives him a ferocious disposition befitting the most terrible mammalian predator (gram for gram) on Earth. A water shrew has been known to kill a fish sixty times heavier than himself by biting out its eyes and brain, which is equivalent to a man killing an elephant barehanded. And all while holding his breath underwater which, with shrew metabolism demanding hundreds of breaths per minute, is impressive if it lasts five seconds. Imprisoned with another shrew and no other food within reach, a shrew has little compunction as to cannibalism either. In fact, when three of the beasties were left alone under a glass tumbler, in a typical example, two of them promptly killed and ate the third down to its last bone and hair, emitting a shrill batlike twitter the while. Then, a couple of hours later, the hungrier of the survivors suddenly attacked and polished off his remaining companion, whereupon he took time out to clean his whiskers, apparently feeling more than delighted with himself - for the moment, that is - after having so neatly converted two worthy colleagues into breakfast, lunch, some scattered droppings and a few unavoidably wasted calories of heat. The last act in this raw drama followed in about three more hours when the sole survivor's appetite had renewed itself to such a pitch that he finally seized the most accessible flesh still in sight, his own tail, and, working up from there, literally devoured himself to death - a dramatic demonstration that at least some creatures, driven to the extreme, actually would rather be eaten alive than starve.

  It is reported that some shrews can dig themselves out of sight into firm soil in only one second. But a second, come to think of it, must be quite a spell for a shrew - perhaps equivalent to a minute for a whale - for there is evidence of a relativity in the consciousness of time in proportion to an animal's size. And the rapidity of life processes in such tiny beings includes not only metabolism but sense perception and the rates of everything from vibration and radiation to the conservation of angular momentum. If so, a second may be an even longer spell in the life of a hummingbird, since, of the more than four hundred known species (more than in any other family of bird), some weigh appreciably less than the smallest shrew and have the highest metabolism rate of any bird or mammal. According to the rule of thumb that a warm-blooded animal's metabolic pace must be in inverse ratio to its length, a hummingbird's metabolism is about a dozen times faster than that of a pigeon or 100 times that of an elephant. Which may make you wonder why a hummingbird doesn't starve to death at night, since it cannot see to buzz among flowers for nectar in the dark or catch the minute insects it must have for protein - not to mention its need for rest.

  The surprising answer is that the hummingbird's metabolism shifts into low gear at sundown, dropping to only one fifteenth the day rate. Thus, in effect, this tiny bird hibernates (or "noctivates") each night. It prepares for this by an hour of literally feverish feeding at top speed as the sun nears the horizon, its heart beating more than twelve times a second. Then, after roosting in the gathering gloaming and dropping daintily off to sleep, its temperature rapidly descends from perhaps 115 dg F. to nearly the temperature of the air, often 60 degrees cooler, its heart slows way down and it becomes torpid - so insensate it can be picked off its perch in your fingers like a ripe berry. In the high Andes, hummingbirds retreat into caves at night to avoid freezing in the chill mountain air, freezing obviously being a hazard to any such insect-sized creature. Its ease of flying, however, makes up for several of the wee hummer's other problems, for, most unpedestrian of birds, it has never been known to hop or walk a step, but flies on its shortest journeys, even down to an unpresumptuous jaunt of half an inch.

  LOCOMOTION

  Efficiency in running and swimming as well as flying is influenced by much more than size or weight in relation to gravity. For although it is true that a large animal tends to move faster than a similar small one in proportion to the square roots of its linear dimensions (Froude's Law), there is no end to the ways in which the complex lever system of limbs and muscles can be put together. The badger, skunk and porcupine, for instance, not needing speed, have short legs suited to digging, scratching and climbing. But a hare or hound of equal weight has a much longer metatarsal joint or instep-lever, geared for fast running - and so does the horse, ostrich, kangaroo or other speedy animal, whether it specializes in sprinting like the cheetah (credited with 70 mph for a short distance) or loping like the camel (115 miles in 12 hours). Keeping the heavier leg muscles close to the body (where they need not swing so far nor fast) plainly increases efficiency, as does the simplified and light hoof-shank system of the horse with its marvelously elastic ligaments that save energy by automatically restraightening the fetlock joint and snapping the foot backward like a released spring every time it leaves the ground. Another speed factor derives from the flexibility of the spine which not only greatly increases an animal's length of stride but can enable its foot actually to move in a smooth elliptical orbit in respect to the rest of its body - not so different from the cyclic path described by a planet or any point on the rim of a rolling wheel.

  The wing motion of birds is somewhat circular too - mainly a propellerlike rotation which lifts as well as propels even during its upward stroke. Although only a minority of birds have succeeded in crossing the larger oceans, the way they achieved their flying efficiency is worth a minute's consideration. Through the evolution of wings, feathers, warm blood, powerful breast muscles, hollow bones and a remarkable respiratory and cooling system, they have worked out a difficult and delicate compromise between high power and low weight. Their excess baggage jettisoned by evolution has included such seeming necessities as teeth and heavy jaws, bone marrow, leg muscles, sweat glands, and much of their reproductive system, especially during migratory seasons. Did you know that female birds make do with only one ovary (the left one) and that the sex organs of starlings, for instance, have been found to weigh 1500 times as much during the breeding season as in the rest of the year? That a flying pigeon uses one fourth of its air intake for breathing and three fourths for cooling? That bird air cooling systems include many air sacs and hollow bones (in some cases clear to the tips of wings and toes) through which their breath flows by efficient forced draft, the stale exhalation going out passages that are largely separate from those entered by the fres
h inhalation? That the bird is literally flying into breath so fast it can never run out of breath? This brand of supercharging of course is vital to the high-speed activity of flight, which also demands such "high octane" protein fuel as worms, insects, rodents, fish and seeds rather than the grass and leaves that suffice for cattle and elephants. And yet the skeleton of a frigate bird with a seven-foot wingspread has been weighed at only four ounces - actually less than the same bird's feathers!

  Insects of course are Earth's primary flyers, since they were the first to take to the air, some 300 million years ago, probably in order to escape the "fish" whose fins were already turning into legs in their efforts to catch them on land. Birds could not have appeared for nearly another 100 million years, they having to await their lizard ancestors (of fish descent) whose front legs still had to evolve into wings as they leaped after the flying insects. Naturally it was relatively easy for the insects to fly, since the air's viscosity in relation to their negligible weight gave them a fair grip in it, a factor explaining how the smallest insects today can float on air with motionless wings, flapping only when they want to advance through it. Flying was so natural to them in fact that their wings evolved from their body wall, leaving all six legs intact to run with, at least four of which are normally on the ground at any instant. Twisting their wings like variable-pitch propellers as they beat them in figure-eight orbits at almost uniform speed (950 times a second in the case of the midge), they acquired an extraordinarily high muscular efficiency (calculated at 20 percent) which takes swarming locusts 300 miles nonstop at an average air speed of 8 mph, black flies almost as far, and enables even the gentle aphid to cross the North Sea. Most surprising of all is the monarch butterfly who, in stringent laboratory tests, has flown better than 6 mph for a continuous four days and four nights, a performance which, with the aid of a moderate tail wind, should easily get him across the Atlantic and no doubt is the explanation for his occasional appearance in England.

  For sheer speed it is believed the big dragonflies and hawk moths are insect champions, who may cruise up to 24 mph, their wings beating alternately (front ones rising as the rear ones fall), reaching 36 mph with extreme exertion, while the giant paleozoic dragonfly of 300 million years ago with his 36-inch wing span probably could do 43 mph. thus although insects have since been outflown by the birds, their still unchallenged position in the kingdom rests in no small degree 24 on their ancient and honored ability without fuss or feathers just to take off in any instant during 300 million years and buzz steadily ahead at 100 times their own length per second!

  Speed underwater admittedly is neither as easy nor as important as speed in air, so one should not be too surprised that the fastest any fish can swim, even in a brief spurt, is only about ten times its length per second. Although a blue marlin has been credited with 50 mph at sea and a tuna with 47 mph, more reliable scientific measurements made in a special revolving laboratory tank give the fish speed record (by a 20-pound 4-foot barracuda) as but 27 miles an hour, while salmon, which proved unable to leap more than about 10 feet upward or 12 feet forward in the air, did so only after an extreme spurt that attained 18 mph in less than a tenth of a second.

  It is now known that probably nearly all swimming creatures, and particularly sei and killer whales (who have been clocked at 35 and 34 mph respectively) and porpoises (up to 37 mph), achieve their speed through their remarkable capacity to move in water without creating turbulence. Their perfection of laminar flow is of course impossible to land animals or human swimmers, since it depends on extreme smoothness of skin (lubricated by special oils), as well as on a rare degree of legless, armless streamlining of form. And, interestingly, the surprising truth was revealed only when a thoughtful naval officer (who had been a biologist) one night observed a porpoise swimming almost invisibly through a sea in which boats and seals unavoidably stirred up fiery wakes of microorganic luminescence. Although squids use a kind of liquid jet propulsion and can zoom many feet into the air, they are not as fast as some other leapers, such as flying fish, while the beautiful silvery blue velellas, the iridescent Portuguese man-o-war and other jellyfish with sails, some of which can tack well into the wind, have too much drag (even with retracted tentacles) to attain even one mile an hour waterspeed.

  Beneath solid earth, of course, quite different means of locomotion prevail. The skink literally swims in sand. But the common earthworm uses a kind of slow-motion ram-jet propulsion by eating in front and excreting behind as he goes, some of his species augmenting the alimentary flux by gripping the soil with retractable bristles which function in regressive waves exactly coordinated with alternate elongations and hunchings of his streamlined body. Then there is the burrowing snail's creepy advance by microscopic, rhythmic expansions and contractions of his single slimy foot, a technique that often enables him to overtake a worm and, if he is a worm-eater, lure it into his specialized worm-welcoming mouth, which turns out to be lined with one-way quills that permit no withdrawal. Still another kind of motion is the flea's ability, with the aid of a boatlike bow, clean streamlining and six powerful legs, to "swim" up to 50 times his own length per second through a "sea" of animal fur, or to jump 120 times his own height at a peak acceleration of 140 Gs (30 times that of a moon rocket).

  Below the world of worms and burrowing snails we may consider microscopic creatures like the ameba, who oozes along by pushing out lobes or ruffles of his single cell protoplasm and just flowing into them. His locomotion system (often called "ruffling") was only recently elucidated. It works in each lobe something like an advancing jet of jelly that spreads like the crown of a fountain at its forward end, turning outward in all directions, then back in the form of a surrounding cuff that condenses into a sleeve stiff enough to grip the ground until, turning once more in the rear, it liquefies and pours inward through the center. Biologists used to think that a muscle-like squeezing of the sleeve's protoplasm at the hind end pushed the jelly forward like toothpaste out of its tube, but newer evidence strongly indicates that it is the continuous contraction of the jelly at the forward "fountain zone" (where it turns and stiffens) that literally pulls the central stream steadily ahead. So the consensus of opinion now is that the ameba advances somewhat as does an oriental king, by having his carpet swiftly unrolled before him as he walks, then as nimbly rerolled behind for further unrolling ahead. In fact he may travel thus half an inch an hour and, if he could keep going steadily in one direction, he might cover a foot a day or the length of a football field in a year.

  Another method of microbe locomotion, up to several hundred times faster but still slow in human terms, is that of flagella or bacilli with tails. Some of them propel themselves by a recently discovered swivel motion, each flagellum lash rotating freely about its axis like the rigid propeller of a small airplane, generally pulling from the front end and changing course by reversing the direction of rotation. So far as I know, this is the only true wheel motion produced by nature before man invented the wheel about 3500 B.C. and it is powered by the equivalent of a reversible microscopic engine, something technological man has not learned how to make even today.

  A different system again is that adopted by ciliates, common in mud and ponds, whose bodies are covered with thousands of rapidly waving hairs called cilia. Up to 20 times a second each cilium (one thousandth of an inch long) makes its stroke, much like a human swimmer's arm action, first reaching gently forward edgewise for minimum resistance, then sweeping rigidly backward broadside for maximum resistance, the beats coordinated in beautiful rhythmic waves of succession, like pistons in an engine or stalks of wheat blowing in the wind. Even some visible animals use ciliated drive, notably two kinds of comb jellyfish the sizes of a gooseberry and a walnut, and often called respectively the "sea gooseberry" and "sea walnut," each of which has eight longitudinal belts of cilia (coordinated by a special balance organ) that steer and propel it like a spherical Caterpillar tractor.

  Such techniques should not seem too exotic
to us either for, little though we may be aware of it, our own bodies use similar methods, as, for example, our sperms that flagellate themselves forward with lashing tails, the cilia that line our windpipes and automatically convey unwanted fluids and dust particles out of our lungs, and not only real amebas in our watery fluids but white cells in our blood that move by the same ameboid motion.

  This brings up a significant relativity in microbic activity since these tiny creatures, particularly ciliates, often work their wills not by going thither toward their objectives but rather by stirring up liquid currents that make their objectives come hither to them. Furthermore, they work their hundreds of limbs in a wholesale fashion, perhaps disbursing their energy on a random or probability basis. Certainly the use of extra limbs beyond an insect's six or a spider's eight contributes no more to speed than do extra wheels accelerate a train, and the centipede goes not a whit faster for all his 16 to 346 legs (depending on the species) nor does the millipede for his twice as many (which, in one Panamanian species, were actually counted as 784 legs) that carry him only half as fast. These multitudinous limbs do, however, give their proprietor a certain diffuse stability, or even what might be called a Lilliputian majesty.

  The locomotion of snakes seems to be as mysterious as it is fascinating to most people but, I'd say, not totally unexpectable when you consider that a snake may have as many as 400 trunk vertebrae, each with its pair of ribs and at least a dozen bundles of muscles by means of which it may bend 25°to either side and 14°up or down. And, by a seeming miracle, young snakes never need slithering lessons, for they instinctively use a half dozen different techniques to slither their way ahead in grass, over pavement, up trees, through pipes, across sand dunes or, on the desert, even to hop along parched ground "too hot to touch." The snake's common lateral undulation method works, of course, by pushing backward or partly sideways against grass stems, pebbles, etc., with continuously moving waves of his body. But such a technique doesn't work on a smooth surface like a tennis court, so a snake there usually reverts to one or a combination of other systems, like the concertina method (used by the inchworm) or the rectilinear method of advancing in a straight line (used by the earthworm). Although, inside a pipe, he can press outward to grip the walls and advance 'a la concertina', outside a pipe he must spiral, pressing inward and pushing against any irregularity. A tree is easier, usually permitting him to undulate among its branches. In soft sand, on the other hand, he may use the oblique loop-weave method of the sidewinder rattlesnake in which he weaves ahead with most of his body looping high off the shifty grains to reduce friction and heat.