Authors: Mary Roach
In 1958, the head of the Biology Branch of the Office of Naval Research, Sidney R. Galler, set out in pursuit of answers. He funded a shark research panel (the Shark Research Panel) and helped establish the Shark Attack File, a database of global incidents that continues today as the International Shark Attack File. David Baldridge’s statistical analysis of nine years of Shark Attack File data gave the world—I’m quoting a 2013 National Marine Fisheries Service paper here—“most of what we know today about shark attacks.” Much of the rest comes from studies the Office of Naval Research funded in the 1950s on shark predation, olfaction, and feeding behavior. “If you had a good idea for research on sharks,” Baldridge told the author of a historical account of shark research, published in
Marine Fisheries Review,
“you went to Sid.”
A
LBERT L.
Tester went to Sid. He had a good idea, he had three species of shark in the ocean outside his door, and he had a pair of fifty-foot-long seawater tanks for experimenting. Tester worked at the Eniwetok Marine Biological Laboratory in the Marshall Islands. (Eniwetok was one of the atolls, along with Bikini,
‡
upon which the US had tested nuclear bombs; the lab provided data on the effects of radioactive fallout on sea life—and, if anyone tracked the obituary pages over the ensuing decades, Eniwetok staff.) Tester set out to determine what, specifically, draws a shark to its prey. Do sharks hunt mainly by sight or smell? If it’s smell, which smells? Whose smells? If repelling sharks wasn’t a reasonable option, a sailor or aviator’s best bet was not attracting them in the first place.
Let’s start with the good news. Human urine does not attract sharks. When presented with anywhere from a half teaspoon to a third of a cup, blacktip sharks in Tester’s tanks took no interest. Neither excited nor repelled, the sharks simply noted the substance, as evinced by a quick turn, or “swirl,” which is, I guess, how one acknowledges pee in the pool when one has no eyebrows to raise or shoulders to shrug.
Human perspiration is likewise uninteresting to the shark. It was sufficiently hot and humid in the shark house that Tester and his grad students were able to collect what they needed by sponging each other’s bodies and wringing the sponge into a bucket of seawater that was then quietly siphoned into the shark tank. In general, the sharks, and who can blame them, were mildly put off. The perspiration of Albert L. Tester was particularly repulsive to them. At concentrations as low as one part per million, Tester’s sweat caused a captive blacktip shark to shake its head and make “a rapid exit from the area.”
All-over body sweat—the cooling waters of the eccrine glands—is different from flop sweat. Had Tester done what my friends at the Monell Chemical Senses Center did to me—gathered the pungent armpit exudations of a human under stress—his results might have been different. The sharks might have detected the scent of distress, of easy pickings, and gone into attack mode.
That is precisely what happens when a shark’s preferred prey falls under stress. The shark senses a no-hassle meal and closes in to attack. Tester harassed a bucket of groupers by “threatening them with a moving stick” (elsewhere referenced as “poking”). Pumping water from the bucket—scientific nomenclature: “distressed grouper water”—into the shark tank provoked a “violent hunting response.” Since the prey were outside the tank, we know it wasn’t the sight or sound of grouper pandemonium that set off the sharks’ predatory moves. It had to be some chemical exuded through the groupers’ skin or gills. And not just any grouper scent would do the trick. When “quiescent grouper water” was introduced into the tank, the sharks paid little heed.
Fish blood and fish guts—two blaring sensory trumpeters of piscine distress—also trigger vigorous hunting moves. So powerful is the chemical signal, Baldridge found, that sharks could be roused to devour a rat—not normally an item of gustatory interest—if its fur were coated with “mullet blend” (whole mullets blenderized with a little water). In a different study, sharks were inspired to attack a kitchen sponge that had been dipped in a bowl of fish body fluids. “Sharks,” wrote Baldridge, “will strike essentially anything that has been treated with fish ‘juice.’”
That includes spearfishers. In particular peril are those who swim around with the day’s catch hanging from their belts or trailing from lines. At the time Baldridge ran his analysis, the Shark Attack File had logged 225 incidents that mentioned the presence of wounded fish and/or fish blood or guts. “Sharks,” marveled Tester, “are able to track down and converge on a distressed fish (such as a live fish suspended from a hook through the jawbone but otherwise uninjured) with uncanny speed and accuracy.”
Spearfishing probably serves to explain why 17 percent of the Shark Attack File victims were wearing wetsuits. The original theory put forth was that the sharks mistook people in black wetsuits for seals. Perhaps that happens too, but where spearfishing was involved, it’s more likely that the wetsuit’s accessories—the spear and the belt of oozing fish—drew the shark.
Dead fish also ring the dinner bell. Tester exposed blacktip and gray sharks to a sushi bar of fish flesh: tuna, eel, grouper, snapper, parrot fish, giant clam, octopus, squid, and lobster. All of them he classed as attractants. Sharks prefer to take no risks. They prefer to go after a meal that’s not going to put up a fight. Injured is good. Dead is better.
Which makes you wonder about the alleged shark-repellent qualities of decomposed shark flesh. Tester wondered, too. He secured some “alleged shark repellent” from a fisherman, another sample from a fisheries lab, and a sample his team prepared on their own by leaving hammerhead and tiger shark flesh outside in the tropical heat for a week. No repellent effects were observed. On the contrary, it sometimes functioned as an attractant. “Our results . . . seem to be at variance with those of Springer. . . . No convincing explanation can be made.” Tester perhaps unaware of the powerful attractant effect of kickbacks from shark-processing plants.
As with fish, so with humans. Over and over, in the shark attack reports of World War II, corpses took the hit. A floating sailor could dispatch a curious shark by hitting it or churning the water with his legs. (Baldridge observed that even a kick to a shark’s nose from the rear leg
of a swimming rat
was enough to cause a “startled response and rapid departure from the vicinity.”) “The sharks were going after dead men,” said a survivor quoted in a popular book about the 1945 sinking of the USS
Indianapolis
, an event that often comes up in discussions of military shark attacks. “Honestly, in the entire 110 hours I was in the water,” recalls Navy Captain Lewis L. Haynes, in an oral history conducted by the US Navy Bureau of Medicine and Surgery, “I did not see a man attacked by a shark. . . .” They seemed to have been, he said, “satisfied with the dead.” Haynes says fifty-six mutilated bodies were recovered, but there’s nothing to suggest that any more than a few of them were bitten into while alive.
Why, then, do sharks hang around life rafts? For what’s underneath. Schools of fish loiter there, either for the shade or to feed on smaller marine life that gathers to take the shade on the raft’s underside. Recalled one World War II sailor: “Larger fish came to feed on those minnows, then larger ones to get them; finally the boys with the peculiar dorsal fins arrived to see what the fuss was about.” Here’s one more, just because I like it: “The shark submerged and swam directly under the raft. . . . We all sat very quiet, . . . and the radar man abandoned the idea of defecating over the side for fear of capsizing. The shark repeated this behavior several times but at no time seemed concerned with us.”
And so it continues to be. I know of only one recorded instance in recent history of a shark’s biting Navy personnel. In 2009, a bull shark took off the hand and foot—in one bite—of an Australian clearance diver during a counter-terrorism exercise in Sydney Harbor. I asked Naval Special Warfare Command communications specialist Joe Kane about sharks attacking Navy SEALs. “You’re coming at this the wrong way,” he said. “The question is not, Do Navy SEALs need shark repellent? The question is, Do sharks need Navy SEAL repellent?”
The modern US Navy has no formal shark-attack curriculum. One diver recalls being told to descend slowly and take cover on the bottom should he sense a threat. A 1964 Air Force training film called
Shark Defense
advises downed aviators to blow a stream of bubbles or yell into the water. I asked veteran shark videographer Robert Cantrell what he thought of this advice. Cantrell has swum among sharks, cageless, for three decades. This is a man who will apply the adjective “nippy” to a group of excited blue sharks. His answer, an answer Baldridge and Tester often came up with, is that it depends on the kind of shark. Screaming into the water may briefly deflect a bull shark, Cantrell notes, but not a tiger shark. Bubbles scare blue sharks, but other species ignore them.
The last Air Force suggestion was a puzzler: “Tearing up paper into small pieces and scattering them all around.” I suppose it was meant as a means of distracting the shark—or maybe just the sailor, now absorbed in the challenge of locating sheets of paper while afloat at sea. On one of Cantrell’s expeditions, he threw some stale bagels overboard. Tiger sharks swam over immediately; bull sharks ignored them. Cantrell’s main advice to the diver who encounters a shark? “Enjoy the experience.”
Let us turn now to the question on many a sailor’s mind: Is it true that human blood draws sharks? The results of Baldridge’s and Tester’s experiments are inconsistent. Sometimes the sharks behaved as though attracted to the blood; other times they avoided the test area. Tester wondered whether the freshness of the blood was a factor. In his own experiments, blacktip sharks and greys were strongly attracted to blood less than one or two days old—at concentrations as weak as .01 parts per million of seawater. But Baldridge’s analysis of the Shark Attack File data belie this finding. In only 19 of 1,115 cases was the victim bleeding at the time of the attack. “It is difficult,” he concluded, “to accept the concept that human blood is highly attractive and exciting to sharks in general when so many shark attack victims have been struck a single blow and then left without further assault even though they were then bleeding profusely from massive wounds.”
In Baldridge’s own tests, he presented four species of shark with the novel menu option of a swimming, bleeding lab rat. As fellow mammals, rats should possess blood that’s about as enticing (or unenticing) to a shark as our own. As he expected, the sharks showed no interest.
The bottom line is that the preponderance of shark attacks, like most animal attacks, are prey-specific. If you don’t look or smell like dinner, you are unlikely to be so treated. Predators are attuned to the scents of creatures they most want to eat. Sharks don’t relish human meat. Even though a shark can detect human blood, it has—unless starving—no motive for tracking it to its source.
That fact
should
be reassuring to women who enjoy swimming in the ocean but worry about doing so during their periods. But menstrual blood is different, in a uniquely shark-worrisome way. If you’ll permit it, a brief shore leave; the US Navy of the 1960s was not interested in menstruating women. The National Park Service, however, was. In 1967, two women, at least one of them menstruating, were killed by grizzly bears in Glacier National Park. Conjecture arose that it had been the blood that inspired the attack. Wildlife biologists didn’t buy it, and one of them, Bruce Cushing (delightfully mis-cited in subsequent bear attack/menstruation research as Bruce
Gushing
), set out to collect some data. Cushing opted to study polar bears, because they feed almost exclusively on seals, yielding a clean baseline with which to compare the animals’ zeal for menstruating women.
If you put seal blubber in a fan box and aim the aroma at the cage of a wild polar bear, that bear will exhibit what Cushing called “maximal behavioral response.” It will lift its head and sniff the air. It will begin salivating heavily. It will get up and pace. It will chuff. It will
groan
. Only one other item that Cushing placed in the fan box could make a polar bear groan: a used tampon. Chicken didn’t do the trick, nor horse manure, musk, or an unused tampon. Coming in a close second: menstruating women. The women weren’t in the fan box, but in a chair facing the polar bear cage, where they “sat passively,” perhaps marveling at the strangeness of life on Earth. Cushing also tested ordinary blood, drawn from people’s veins; this elicited no response whatsoever from any of the four participating bears.
In other words, it isn’t the blood that makes a tampon attractive to polar bears. It’s something uniquely . . . vaginal. Some kind of secretions that, please forgive me, smell like seals. This makes sense, does it not? When a feminine hygiene company hires a lab to test the efficacy of a scented menstrual product, the standardized odor employed for this purpose is known as a “fishy amine.”
So alluring is the intensely vaginal/sealy scent of a tampon that a polar bear seems not to notice that it does not also taste like seal. In 42 of 52 instances, a wild polar bear who encountered a used tampon affixed to the top of a stake (scientific nomenclature: “used tampon stake”) ate or “vigorously chewed” it. Only seal meat was more consistently pulled from the stake and consumed. Paper towels soaked with regular blood—here again, nailed to a stake like a skull warning foolhardy jungle explorers—were eaten just three times.
What does this tell us about sharks? Should women be worried? Hard to say. How crazy are sharks for seal meat? Do dead groupers smell like used tampons? Unknown. I’d stay in my deck chair, if I were menstruating you.
Cushing concluded his paper by suggesting that since polar bears enjoy used tampons, there was a strong possibility other ursids would, too. But bears, like sharks, vary by species. Forest bears aren’t connoisseurs of stinky marine life as polar bears are. Grizzlies like salmon, but they take them fresh. Black bears forage for garbage, so who knows what they’ve come to develop a taste for over the years.