The Dolphin Strategy

By all rights, life in the sea should leave a dolphin baked, crushed, and sterile. This graceful mammal avoids such a fate only by slipping through loopholes in the laws of physiology.

By Carl Zimmer|Saturday, March 01, 1997
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Dawn Noren hoists oxygen tanks onto her back, places an air regulator in her mouth, grabs a plastic box and slate, and falls out of the motorboat into the ocean. Through 50 feet of pale blue Bahamas water she can see the ten other divers kneeling in a circle on the sandy, ribbed seafloor. She swims down to the group and positions herself at its edge. Two more divers arrive and glide into the middle of the circle, carrying with them long white drums full of dead herring. They are followed by the animals that brought everyone here: two Atlantic bottle-nosed dolphins named Bimini and Stripe.

The circled divers are tourists who have paid dearly--about $100- -to spend half an hour in the dolphins’ element. The two divers with the dead fish are Patrick Berry and Eden Butler, trainers who work with a company called the Dolphin Experience, which keeps 13 animals in a nine- acre lagoon outside Freeport. Berry and Butler have spent years getting the dolphins to interact with customers on command. Berry points to a diver, who puts out his palm, and Bimini swims to it, curving his path so the diver can touch his rubbery skin. The dolphins snatch rings and spin divers like turnstiles by pushing against their outstretched arms. The favorite is the kiss: a diver takes the regulator out of his mouth and the dolphin puts its beak-shaped mouth to the human’s lips.

It is easy to forget that the dolphins also need an air supply; but every few minutes they casually stop what they’re doing, make a few quick upward kicks, and ascend to the surface. Each time they do, they make clear that the real performance lies not in a dolphin’s stunts but in its astonishing grace. When dolphins swim, they don’t look like they’re doing anything. Berry twists his hand and Stripe twists his whole body with as much effort as a thought. Sometimes on their ascent the dolphins disappear beyond the water’s ceiling, leaping into the air and then plunging down again. With a corkscrewing flourish, they come to a stop upside down in front of Berry or Butler, hoping for fish.

For the tourists this half hour is paradise. For Noren it is hell. Noren, a graduate student in marine biology at the University of California at Santa Cruz, is the one person in this group for whom the dolphins’ stunts are peripheral. She is here to take measurements of dolphin physiology, and neither the animals nor her equipment is being particularly cooperative. Kneeling on the sand, she slams her plastic box around in underwater slow motion. The machine inside the box is supposed to register the flow of heat coming off the warm-blooded dolphins. She needs to get her probe, a flat plastic disk on the end of a spring, onto the dorsal fin and tail of the dolphins for about a minute at a time. At best, she can hope for a mere handful of readings on each dive--the trainers help her only when there’s a break in the paying action. I can’t say, ‘This is what I want, and I want it now,’ she says. Taking notes is the only thing I can control.

To measure the heat of a dolphin, either Berry or Butler must first hold out a hand. The dolphin places the tip of its mouth in the cupped palm and holds itself still. Then the trainer works his way down to the dorsal fin and holds out his free hand like a surgeon waiting for a scalpel. Noren hands him the probe, then settles back down to watch her meter. If all goes well, the dolphin holds still until the reading is done.

This time all does not go well. A bad wire in the probe is screwing up the readings, and soap opera squabbles among the dolphins back at the Experience headquarters have left Bimini and Stripe cranky and skittish. When Berry cups his palm, Bimini pulls away. He is willing instead to deliver the ring to a few divers. After a moment, Berry tries to use the probe again, and for a few seconds Bimini goes along. But then he decides he’s had enough and snaps away. The summer is headed for hurricane season and Noren needs data badly, but today, as the tourists bring home stories and videotapes, all she brings home is a blank slate.

This is how science typically goes for those who study how cetaceans--whales, porpoises, and dolphins--do what they do. Despite our long familiarity with these creatures and our common mammalian heritage, we really know little about them. We barely even understand how they move through the watery environment they returned to some 50 million years ago. The tools we use for understanding the movement of land animals--the treadmills, the sealed atmospheric chambers, the high-speed videos--are hard or impossible to use on an animal that breathes by surfacing every few minutes to blast open its blowhole, then disappears underwater.

Few people know this better than Noren’s adviser, physiologist Terrie Williams. Williams has been trying to get a handle on her slippery subjects since 1990, when the U.S. Navy first offered her an opportunity to study the physiology of free-swimming dolphins. The Navy’s interest traces back to a decades-old belief that there is something almost magical in a dolphin’s swimming abilities--something in its design that, by greatly reducing drag, allows the animals to achieve a speed and maneuverability that physics says should be impossible. Around the beginning of the cold war, both the American and Soviet navies imagined that by divining dolphin secrets, they would be able to design submarines that could slip through the water with unheard-of efficiency and silence. Guesses as to what the key dolphin secret was were plentiful. Perhaps, some suggested, the heat coming from a warm-blooded animal made the surrounding water less viscous. Or maybe a dolphin’s skin had ridges that could channel the water down its flanks. The U.S. Navy even invented rubberized paints on the suspicion that a dolphin’s rubbery skin could damp out tiny waves that ultimately create drag.

By the time Williams came along, it was clear that none of these hypotheses would lead to a stealth submarine. Still, the Navy thought dolphins might prove useful if they could at least be trained to patrol underwater installations and hunt for mines. The Navy wanted to know, are the things we’re asking the dolphins to do easy or hard? says Williams. If we ask a dolphin to dive 200 meters, is that a tough thing for it to do?

Williams and her Navy co-workers tried to figure out a way to measure a dolphin’s oxygen consumption while it swam alongside a motorboat. To have a metabolic unit out there was just too difficult, she says. You’d have to have a big hood in the water and an oxygen analyzer and pumps with a ton of gear--about $50,000 dollars worth of stuff you don’t want to sink on a boat. And if the wind came up with diesel, it would throw everything off. So we tried having them breathe into a metabolic balloon. These things look like potato chip bags. The dolphins would come up, and we’d put this little cone down and they’d exhale into it, and then we could take it back to the lab and analyze it. But I didn’t like that because the dolphins thought they had to exhale as forcefully as they could, and because they’d also take in a big breath, you’d get the oxygen consumption looking abnormally high.

What finally worked, Williams discovered, was measurement by proxy. On the wall of a pool she set up a force-sensitive disk and had trained dolphins swim against it. Then, with a hood set on the water, she measured how much oxygen they breathed. At the same time, she measured their heart rate with a suction-cup electrocardiogram. She found that as a dolphin burned more oxygen, its heart rate always increased in a beautifully linear way. And since she could attach suction cups to dolphins and measure heart rate in open water, she could then calculate how much oxygen they were burning while swimming.

Williams used this data to calculate the dolphins’ cost of transport. Technically, this is milliliters of oxygen consumed per kilogram of body weight per kilometer, but essentially what it tells you is how much energy it takes to move a given weight of animal a given distance. Williams soon discovered that a dolphin’s cost of transport is the lowest ever measured for a swimming mammal, and only two or three times as high as that of a like-size fish. Seals have a cost four times as high as fish. Yet when she compared a swimming dolphin with a running terrestrial mammal-- comparing the animals as they move in their own elements--she found that it was about as impressively efficient as a zebu (an African ox). There was nothing I saw that made them look amazing, she says.

Dolphins harbor no great physiological secret, Williams has concluded, beyond a fine adjustment to a life at sea. And that adjustment consists of a host of tricks and shortcuts. Dolphins are famous, for example, for surfing on the bow waves of ships, sometimes just below the surface, or bouncing along on a wave’s crest. Though they may seem to be doing it for fun, Williams has shown that the practice is cost-cutting good sense: a dolphin surfing at eight miles an hour has about the same cost of transport as it does swimming at four miles an hour. Most likely dolphins had been grabbing cheap rides long before the first boat cut a wake. If it’s a wave off a whale, if it’s wind waves--if a dolphin has any opportunity to snag a ride, it’ll do so, says Williams.

Dolphins cheat underwater as well, Williams subsequently discovered. Before they take a plunge, they gulp a few breaths of air and swiftly pull the oxygen into hemoglobin in their blood and then into a muscular equivalent called myoglobin. To conserve the oxygen, researchers have long assumed, the animals cut off circulation to their skin and their extremities, leaving only vital organs and tail muscles working. Obviously, on long dives--which can easily last several minutes at depths of up to 600 feet--these physiological responses are crucial. Yet when Williams plugged in her numbers for oxygen consumption, she found that even with such cost- saving measures, and despite blatant evidence to the contrary, it was simply physiologically impossible for dolphins to do what they do. I had all my predictions ready, telling me this is how much oxygen the animal will consume on a dive, she says. Boy, I had it wrong. By her calculations, a dolphin should run out of oxygen and drown in mid-ascent.

Her mistake was made clear when Texas A&M; biologist Randall Davis presented her with a camera that a diving dolphin could carry on its back. The instrument consists of a velocity meter, a depth gauge, a light, and a video camera, all strapped to a dolphin and pointed at the animal’s tail. She sent dolphins on deep dives and reviewed the tapes they brought back. Each recording was the same. As the depth gauge readout appears in one corner of the screen, the dolphin’s tail moves up and down. Before long the water grows dark, the tail illuminated only by the camera. You’re watching the depth gauge, and it hits the 70-meter [230-foot] mark, and the tail just stops. You think the film has stopped, but when you look at the depth gauge, it’s just rolling and rolling--100 meters, 150 meters, 200 meters, and nothing has moved in the picture.

Williams had assumed that dolphins dive by swimming, but actually, she found, they fall. At the 70-meter mark the pressure of the surrounding water becomes so great that their lungs collapse. Williams thinks that by squeezing the animal into a smaller, denser shape, the pressure makes the dolphin less buoyant and allows it to drop through the ocean like a rock. Since it doesn’t need to move a muscle, it doesn’t need to burn any oxygen and can thus slip out of Williams’s limiting calculations. At the bottom of a deep dive, a dolphin’s heart races for a few seconds as it starts kicking into an ascent. When it comes back up to a depth of 70 meters, its lungs open, its body becomes buoyant, and it glides the last leg to the surface.

But even as this revelation resolved one paradox, Williams realized, it created another. A dolphin is wrapped in a thick, insulating layer of blubber to preserve heat. However, in some cases it can provide too much insulation and make the animal overheat. Thus, for cooling purposes, a dolphin also has blood vessels that poke out of the blubber and run under the surface of its dorsal fin and tail flukes; from there heat is easily transferred to the ocean. We have a similar set of vessels in our arms and legs. When we’re warm, blood is shunted to these surface vessels, which give off their heat to the air and return the cooled blood to our body core; if we’re cold, these vessels are shut off. The surface vessels of dolphins are thought to work the same way. But if to conserve oxygen dolphins pinch off blood flow to their fin and tail flukes, then their need to stay cool, Williams realized, collides head-on with their need to avoid suffocation.

Williams attempted to resolve this conflict on her first trip to the Dolphin Experience in 1995. Although Patrick Berry had prepared the dolphins for her experiments, it was not a promising visit. We had a bad hurricane season and we couldn’t go out in deep water, she recalls. It was just horrible.

She thought she could salvage her trip by taking some measurements in shallow water. After all, a dolphin has to hold its breath whether it’s 15 feet underwater or 50 feet. So we took the dolphins to a shallow area, and we were taking all our little measurements. Patrick was holding the probe on one dolphin’s tail. You’re underwater, and as you’re working the box, you basically have to stare at the meter till it stabilizes and you can tell Patrick to stop. As she watched the meter, it stabilized at 60 watts per square meter. Then without warning it suddenly lurched up to 120 watts for a few seconds, then fell back down again.

Williams signaled to Berry to head up to the surface. I said, ‘End of dive! The thing’s busted--water got into it, I don’t know how, but it’s a total washout.’ We packed everything up and brought the animals back to the pen. On land she tested her meter to find the trouble, but instead she found it was working perfectly.

Berry could see that she was at a loss. Patrick said, ‘Didn’t you see what the animal did?’ I said, ‘No, I was watching the stupid meter.’ Well, Patrick was standing in only 15 feet of water holding the tail, and the animal is 9 feet long, and so it came up and took a breath.

Williams knew that when dolphins hit the surface, they get a lot done fast. They exhale and inhale quickly, during which time they clear out carbon dioxide, rebalance their body’s pH, and load oxygen into their blood and muscles. Dolphins spend as little time as possible on the surface, because there they have to battle surface waves and waste strokes that push air instead of water--altogether making their swimming inefficient. Perhaps, she began to wonder, they also take these brief opportunities to suddenly open up the blood gates, fill up the surface vessels in their fin and flukes, and in a torrent let loose all the heat they had built up on their dive. If that were the case, dolphins would be the camels of the sea. Camels don’t try to cool themselves by sweating, because they can’t afford to use so much water in the desert. Instead they store heat during the day and unload it in the cool of the night. If Williams was right, dolphins would have taken this strategy to its extreme, unloading heat in a matter of seconds. I think that’s what happened. It was a onetime observation, but I think it might be real.

That’s one of the things Dawn Noren tried to test last summer. Whenever Berry had time, they would take a dolphin out on a shallow dive and try to reenact the events that had triggered Williams’s weird spike. Barracudas, bad weather, and the fussiness of dolphins slowed her research, but one afternoon near the end of the summer a female named Cayla let her take half a dozen measurements in a row. And every time, the heat flow more than doubled when Cayla took a breath.

Apparently then, Williams was right, and the dolphin’s remarkable physiology had devised an elegant solution to the conflicting problems of oxygen conservation and heat buildup. The only puzzle is that the dolphin’s equally remarkable anatomy seems to argue that such a solution is, to say the least, problematic.

In a lab at the University of North Carolina at Wilmington, Ann Pabst and Bill McLellan stand by a steel table, edged with troughs, a bucket hanging at one end. Pabst snaps on powdered surgical gloves and McLellan sharpens a dissection knife with flicks along a steel rod by the table, on which lies the corpse of a little bottle-nosed dolphin, its eyes half shut, still smiling as it starts to thaw.

Six weeks earlier this baby dolphin’s corpse washed ashore at Virginia Beach. A crew from the Virginia Marine Science Museum Stranding Project arrived quickly with a truck and drove it to their facility, where it was frozen and sent on to the North Carolina lab.

By rights, Pabst and McLellan ought to be a pretty grim couple. They take apart a dead dolphin almost every week, and a fair number of whales each year. Yet they have a sparky rapport as they do their work. They talk easily about dissection workshops the stranding network has, about their first dates on beaches up and down the mid-Atlantic, slitting open bottle-nosed dolphins, spotted dolphins, beaked whales, pilot whales, and sperm and baleen whales.

This is a really cool little animal, Pabst says, rolling it onto its side.

What do you want to do? McLellan asks her. You want to do just the standard?

Yeah, and we’ll weigh it up. And get samples of blubber and liver.

McLellan executes quick long cuts down the length of the dolphin’s back, the scalpel making a faint creasing sound as it goes. With tiny nicks so fast that they sound like the whir of a cicada, he detaches long flaps of skin and blubber from connective tissue. The flesh below is pink, except for a gooey patch of red near the head, the remnant of a remarkable piece of cetacean anatomy. When a diving dolphin hits the lung- collapse depth, its rib cage collapses also. If it had the normal mammalian carotid artery running to its brain, the blood squirting out of its squeezed heart would rush through the vessel with so much pressure that it would literally blow the dolphin’s brains out. Instead the blood flows from the aorta into a mesh of capillaries called a rete mirabile--Latin for miraculous net. The effect is like water from a faucet pouring into a sponge; as the blood surges into the fine vessels, it slows down. Only then does it gently flow into an artery that dives into the spinal canal and up to the brain.

The hemorrhaging in the neck is so bad that McLellan is pretty sure he knows how this baby died. We know there was some sort of blunt trauma, but we don’t see any fishing net marks on the head, which is a good indication that this was natural. But it could have taken a hit from somebody. By now the table is slippery, so McLellan has to pull the dolphin back toward him by the flipper. He slices off the dorsal fin and amputates the flippers at the shoulder. Soon the face is a gray mask on a barrel of meat.

Skinned, even this baby dolphin flaunts the architecture of power carried by cetaceans 30 times its size. The muscles attached to the front and back of the spine--used by land mammals like ourselves simply to hold our body up--are the crankshafts of the body’s motor. Cables of muscle run from the neck to the base of the tail; when McLellan cuts loose the biggest muscle, the longissimus, it looks like a fat snake. On a blue whale, the longissimus stretches 80 feet, the longest muscle on Earth.

Although the dolphin’s gut is still frozen, rude gases escape as McLellan takes apart the viscera. Pabst weighs the esophagus, intestines, stomach, and liver. The uterus is a drab purple swath of flesh. McLellan pores over it, pointing out details to a grad student holding a video camera. McLellan and Pabst are particularly interested in dolphin wombs these days; they have discovered to their surprise that when a dolphin swims, even this organ at the core of the animal’s anatomy is affected.

Their interest in wombs was actually triggered not by dissections of female dolphins but of males. We were taking apart male animals bit by bit, says Pabst. At the end of each dissection, we would have the testes, and there would be two kilograms of stuff around it. We thought, two kilograms of anything in a hundred-kilogram animal is significant. It was vasculature. More than that, it was a particular kind of vasculature: fine veins ran closely alongside arteries branching off the aorta. As they dissected more males, Pabst and McLellan pumped milk or latex into the veins around the testes to trace their path, and the white trail led to the dorsal fin and the tail flukes--the areas where dolphins cool their blood.

Suddenly something very obvious--and totally uncontemplated in decades of dolphin research--occurred to them: swimming should make a male dolphin sterile. Sperm can grow and survive only at temperatures a few degrees below that of a mammal’s body, and so most males keep their testes in a sac that hangs away from the body. For a swimming mammal, though, this clever arrangement would be a disaster. The last thing you’d want to put on a streamlined, hydrodynamic body is a dangling bag. Instead dolphins lodge their blimp-shaped testes snugly in their bodies. Now they can swim quickly, but it would seem they couldn’t have children. The testes sit between massive muscles that work continuously as the dolphins swim, and are nourished by vessels coming off the nearby aorta, full of hot blood. Arranging things this way makes as much sense as putting a tub of ice cream on an engine block.

There was a way, Pabst and McLellan realized, that a dolphin could escape this dilemma. When blood is cooled on the surface of a dolphin’s tail and dorsal fin, it doesn’t simply cool the core of the body in general. Instead it heads to the region around the testes. There the veins split up into finer vessels that run side by side along the hot arteries. The arrangement functions as a countercurrent heat exchanger, warming blood in the veins and cooling blood in the arteries. Thus the blood that bathes the testes should, in theory, be cool enough to save the dolphin’s sperm.

Fentiel Rommel of the Marine Mammal Pathobiology Laboratory in St. Petersburg, Florida, came up with a way for Pabst and McLellan to test the idea. They built a 16-inch probe to be inserted into a willing dolphin’s colon; once in place it would give them temperature readings along its length. Rommel, Pabst, and McLellan then went to Hawaii and joined Terrie Williams at the Navy’s lab. There three very amiable male dolphins were trained to have their temperatures taken. The region around the dolphins’ testes turned out to be 1.3 degrees cooler than the surrounding body. Moreover, the refrigerator was so powerful that when the dolphins took long, fast swims--heating their bodies even more--the testicular temperature actually dropped half a degree more.

Soon afterward Pabst and McLellan realized that female dolphins had an even more desperate need than males did for a cool core. The fetus is a little furnace, explains Pabst; its metabolic rate is one and a half or two times higher than the mother’s. That heat has to be removed or it can cause damage--anything from fetal distress to terrible developmental disorders to death. In terrestrial mammals, 85 percent is conducted away through blood flow. Fifteen percent is conducted across the abdominal wall into the environment. You know that when you touch a pregnant lady’s belly and it’s hot.

A female dolphin, on the other hand, doesn’t have that window. The dolphin uterus is in the same part of the body as the testes, which means that between the fetus and the ocean is a hard-working, heat- producing layer of abdominal muscle, and beyond that a layer of insulating blubber. It seems like a dangerous place to have a kid, says Pabst.

Female dolphins, Pabst and McLellan have now shown, use the same anatomical trick that males do. McLellan pokes at the baby dolphin’s uterus to show the dark web of veins that cover the uterus. Cooled blood flows directly to the womb from the tail and fin.

Last summer, Pabst and McLellan began taking measurements of wild dolphins off the coast of North Carolina. Pabst would sit at a computer on board the boat while McLellan handled the animals, shouting to Pabst when the thermometer was in position. One of their subjects was a pregnant female in her first trimester, and she turned out to have the coldest temperature they had ever measured inside a dolphin. I couldn’t believe it, says Pabst. All I kept doing was shouting to him, ‘Is it on? Is it turned on?’

How can their results be reconciled with those of Williams? How can a dolphin both refrigerate its uterus and yet release its body heat only when it surfaces for a breath? Pabst hopes to pursue this question soon by studying blood flow in female dolphins immediately after a long dive. But she suspects that the cooling flow may not be regulated by a simple on-off switch--it may be a graded response, diverting blood to the uterus (or testes) as needed, throughout a dive.

For years Pabst and McLellan cut right past some of a dolphin’s most well-kept secrets. I would be doing dissections and peeling off the blubber and saying, ‘What is this thing underneath the blubber layer?’ says Pabst. To get the blubber off, she had to slice through a sheath of connective tissue. Anatomists knew that it had evolved from a sheet of gristle that covers the lower back of mammals (we have it as well), but no one had given it much more thought than that. To Pabst, though, the beauty of the densely woven mesh of crisscrossing fibers suggested that it must have importance beyond a glue for blubber.

She ended up spending five years mapping the sheath’s marvelous complexity. From the back of the dolphin’s head to the base of its tail, the subdermal sheath wraps completely around the muscles. It doesn’t simply encase them; the muscles attach some of their tendons to it just as they would normally attach to bone. It took Pabst a while to figure out what all the connections were for, scribbling diagrams and tugging on the tail of a dolphin skeleton with dozens of strings. She finally concluded that when dolphins swim, they first contract a set of muscles that tighten the sheath until it’s stiff. Then another set of muscles use the stiffened sheath as a skeletal anchor, almost like a second spine; other muscles use it as a tendon to transmit their forces down the length of the animal’s body.

The sheath was obviously mechanically important, but Pabst wondered if it had another, more subtle function. Basically, a dolphin is a pressurized cylinder wrapped with crisscrossing sets of helically wound fibers. This kind of structure has some valuable properties for an animal. Like a garden hose, such a cylinder won’t kink when it bends and the angle of the fibers lets it resist twisting. Squid, earthworms, and sharks are only three of the many animals that wrap their bodies in helically wound fibers.

Chalk-and-blackboard models of these objects suggest that, for a given length of fiber, the angle of the fibers can change the properties of the cylinder. To enclose the maximum volume, for example, sets of fibers should be fixed at a 55 degree angle relative to the long axis of the cylinder--any smaller than that and the cylinder will be too narrow; any bigger and it will be too squat. And at angles above 60 degrees--depending on the materials that a cylinder is made of--it becomes so springy that it bounces back when bent. In other words, the energy that goes into the bending is stored in the fibers and then released again.

Thus Pabst listened very closely when Terrie Williams told her about strange results that had come out of her work. In her lab she had studied dolphins swimming against a force sensor, pushing as hard as they could. As the force they delivered increased, so did their consumption of oxygen, which is normal for any animal. When they got up to a load of about 85 kilograms (187 pounds), their oxygen consumption leveled off, which again is perfectly unamazing--animals have limits to their aerobic metabolism, and to generate any more force, they have to resort to metabolic chains of chemical reactions that don’t use oxygen--that is, anaerobic metabolism. For humans, as with most other animals, this anaerobic energy is short-lived because it generates lactic acid as waste, which builds up in the muscles. But Williams found that her dolphins could keep pushing against the sensor--up to nearly 200 kilograms in the case of one dolphin. After these exercises, trainers would instruct the dolphins to turn upside down and present their tail so that a blood sample could be taken. Even at 200 kilograms, Williams found, they had paltry levels of lactic acid in their blood.

That, Pabst knew, puts dolphins in the company of some highly efficient land animals, the prime example of which is the kangaroo. Kangaroos, once they reach their aerobic capacity, can still double their hopping speed without burning any additional fuel in their muscles, and they do so by letting the springlike tendons in their legs take over. With each jump, they stretch the stiff strands of collagen and store much of the energy of their fall; when they spring out for the next hop, the tendons return 93 percent of the energy put into them. Williams’s dolphins were showing signs of doing the same thing in water. Perhaps at a certain frequency the upstroke of a dolphin’s tail would load energy into the subdermal sheath, which would then spring back, helping push the tail through its downstroke. At the right frequency, the dolphins would resonate as they swam, ringing like bells. If they did, their impressive swimming with the physiology of an ox would make more sense.

Pabst knew that a number of researchers had speculated that dolphins might swim with springs, but their findings have been ambiguous. Part of the trouble is that dolphins are not the simple cylinders that theoretical biologists use as models; their tails narrow and flatten, and the sheath does confusing things like anchor to the spine. Still, Pabst was tantalized by the finding that the angle of the fibers around the chest (where the dolphin’s body is rigid and large) was a volume-filling 55 degrees, while closer to the tail (where a spring would be useful) the angle went over 60 degrees.

The most direct way to test this idea, of course, is to measure the resilience of the sheath. Pabst tried clamping pieces of it into a machine that carefully stretched material, but it was a bust. I grab onto it and I pull on it, and because I’m not pulling on every fiber, I’m reorienting them because of the looseness of the weave, and I create a new material. There was no way to know if the properties she registered in her machine were the same in a dolphin.

As she was testing the sheath, however, Pabst began to think a lot about blubber. Most people think of blubber as plain fat, but it is blubber alone that creates a cetacean’s slippery shape. She decided to compare blubber with normal fat under a microscope. Under polarized light, the fat of a cow is an expanse of purple blobs. When Pabst put blubber under the scope, she saw the same purple blobs, but woven into them were gorgeous blue and gold stripes of connective tissue. It was like the most beautiful Japanese tapestry I’ve ever seen, says Pabst.

She realized that she had found a second cross-woven material encasing a dolphin. And when she had a student of hers measure the angles of the fibers in the blubber along the length of a dolphin’s body, he found that the angles were identical to those of the sheath fibers. Like the sheath, the blubber is stiff around the ribs but stretchy closer to the tail.

Now Pabst could ask the same questions of blubber that she could of the sheath: Is it a good spring? And since blubber, unlike the sheath, holds its shape when clamped and stretched, she could finally get some hard numbers on resilience. Resilience is just a measure of the amount of energy you get out of a system relative to the amount you put in, says Pabst. Collagen is the best spring--it’s got a value of 93 percent. So 93 percent of the energy you put in, you get back. Spider silk is a good shock absorber because you don’t want your fly ricocheting out of the web. And 35 percent is its value. Blubber has a value of 87 percent. So, despite the fact that it’s fat, it’s a fine spring.

Whether an entire dolphin acts like a spring is another matter, one that Pabst will have to consider by working on living dolphins rather than a patch of blubber. With Williams and other dolphin experts, she is now studying how blubber stretches in relation to the kicks of a dolphin’s tail. They may have a few ideas of how dolphins swim, but they’re trying to be careful not to assume too much. Better to let the dolphins guide them to the truth.

That’s been the fun part, says Williams, looking for the tricks. They use so many of them. You go in thinking you know so much about them, and then they say, ‘No, no, no, we’ll let you know what’s really going on.’
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