FRANK FISH, PHD, WAS PERPLEXED. LOOKING AT A SCULPTURE of a humpback whale, he noticed a series of bumps on the forward edges of the whale’s flippers. As a professor of biology at West Chester University of Pennsylvania, he is a widely regarded expert in the dynamics of locomotion in aquatic mammals, but he didn’t know why those bumps were there. (Dr. Fish enjoys the irony of his name.) He understands well that nature is far from frivolous with its designs. Those bumps were there for a reason; he just didn’t know why. In 2007, after confirming that the artist’s rendition was accurate, Fish set about trying to figure out what purpose the bumps serve.
Backed by a grant from the National Science Foundation, he and a small team investigated the whale’s pectoral flippers, gigantic 15-foot-long wings equal to one-third of the whale’s body. These wings, they discovered, serve multiple purposes. Aiding the giant whale’s ability to turn laterally, or side to side, pectoral flippers also allow the whale to swim up and down, increase stall speed, and perform a host of other maneuvers that allow the whale to thrive in the oceans. To measure how these bumps work, Fish collected flippers from deceased humpbacks and took a computed tomography scan (CT scan) of them, which he then used to create a three-dimensional model of the flippers, bumps included. Next, Fish placed the models in a water tunnel. Measuring lift and drag, he adjusted and tested the model at various angles in different currents. His study—the first hydrodynamic analysis of a whale’s evolutionary design—proved that the curious bumps he first spotted on a sculptor’s rendition of a humpback greatly enhanced the whale’s ability to move freely and quickly underwater, despite its massive size. Performing rolls and loops, swimming 14,000 miles per year—all are possible thanks to the bumps. And when the 30-metric-ton leviathan wants to soar out of the water, its bumpy flippers help it to do so. Fish called the bumps “tubercles” after the Latin word for “protuberance,” and he quickly realized that these previously unknown bumps greatly improve both hydrodynamic and aerodynamic performance. Fish started integrating tubercles into energy-efficient wind-turbine and fan blade design at WhalePower, his Toronto-based, for-profit venture.
I met with Fish at the American Museum of Natural History in New York, where we discussed the scalloped hammerhead shark, which like the humpback also has tubercles. “Tubercles evolved as a way to improve pitch, or up and down, performance,” Fish told me. “They add more lift, and conversely, they can also help the shark when it is pitching down.” Such freedom of movement lets the hammerhead hunt and catch prey on a par with its fellow Big Four cartilaginous kin. More than any other shark, though, questions circle around the hammerhead, unanswerable mysteries that extend far beyond the shark’s trademark T-shaped head, called the “cephalofoil.” Getting to know the hammerhead, I quickly learned, brings one into contact with the mysterious.
In 2010, curious about how hammerheads evolved over time, scientists at the University of Colorado at Boulder set about to construct the shark’s phylogenetic, or evolutionary, family tree. They collected mitochondrial and nuclear DNA from dead hammerheads at fish markets around the world. What they found is that all nine species of hammerheads share the same ancestral father, literally the same massive shark. The study’s lead author, Andrew Martin, determined that this 20-foot-long behemoth broke away from other shark species about 20 million years ago, undergoing a divergent evolution during the Miocene epoch, when the global climate was warmer.1 Some new species branched out into different developmental directions, separating into today’s nine hammerhead species, which vary in size and behavior and are found all over the world. The great hammerhead shark, the largest and most recognizable of the nine, kept the size of its ancestor. One of the larger sharks in the ocean, great hammerheads grow up to 20 feet in length and, on average, can weigh more than 1,000 pounds. Most of the other eight hammerhead species measure in at much smaller sizes, a process known as neoteny.2 In neoteny, adult sharks retain juvenile traits, and the species invests its energies in reproducing instead of growth. For instance, the tiny bonnethead, which looks like a great hammerhead doll, maxes out at 2 or 3 feet in length. Despite the variety in size and behavior, all nine species feature brownish-gray-and-white coloring on their underside.
Unlike any other fish in the ocean, the hammerhead surveys its underwater environment with its trademark head, which is crammed full of flesh-and-blood sensors. Many have wondered why its head is T-shaped. One theory is that the shape of the head improves the animal’s vision. Perched at the extreme ends of its head, a hammerhead’s eyes afford it a 360-degree panoramic view of its environs. Michelle McComb of Florida Atlantic University planted electrodes into the eyes of three hammerheads—scalloped hammerhead, bonnethead, and winghead—and moved a light beam across them to measure the eyes’ field of vision. She found that the degree of overlap between the eyes increased as the head of the hammerhead species widened. The wider the head, the better the depth perception. Previously people had thought that the T-shaped head was to improve the shark’s sense of smell, but McComb’s research shows that the shape of the head improves the shark’s vision.3
And it’s not just its eyes that are unique. In addition to its unique ability to survey its environs, the hammerhead has two noses. Its double-barreled olfactory system enhances the shark’s ability to assess what’s going on around it. Jonathan Cox, a senior lecturer at the University of Bath and a noted expert in fish olfaction, has been working to understand how the olfactory organs work inside a hammerhead. Using a scale model of a hammerhead shark in a flow tank, he figured out how water flows through the shark’s nasal cavity. The model, Cox told ScienceDaily, was complete with internal cavities accurate to less than eight-thousandths of an inch. He and his team submerged the three-dimensional replica into a flow tank and observed the flow of water. “Whereas humans use their lungs like a bellows to inhale air through their noses to smell, the hammerhead shark smells as it swims forwards, propelling water through its nose,” he said. To simulate the shark’s style of swimming, he went on, “we change the angle of the head model in the tank and observe the flow at each angle.”4
A hammerhead’s nasal cavity, which is located at the outer edge of the cephalofoil, is a labyrinth of pipes, with a central U-shaped channel and several smaller channels leading off it. The smaller channels contain the olfactory receptors. As the water flows through these channels, the hammerhead is able to sample the water for telltale odors. When the hammerhead swims forward, it sweeps its head side to side. Picking up on an odor plume, the shark can use the scent’s different arrival times in each nose to track it to its source. Because the distance between the shark’s polar noses is significant, the arrival times of the odor plume are staggered, which lets the hammerhead more accurately figure out where the scent is coming from. Like other sharks, hammerheads have embedded-in-the-head ampullae of Lorenzini, the electroreceptor sensory pores that detect electrical disturbances in the water. Coupled with their well-spaced noses, the similarly spread-out ampullae improve the hammerhead’s ability to sweep for prey effectively, detecting creatures buried in the sand. They do not have to see them to know that they are there.
Another distinctive aspect of the hammerhead is its dorsal fin, which is ramrod straight and can reach a height of almost 2 feet. A shark’s dorsal fin acts like a rudder on a boat to stabilize the shark’s turn. However, in the case of the hammerhead, the dorsal fin is longer than the pectoral fin. So the hammerhead swims sideways to take advantage of the lift from the dorsal fin, like a wing. The scientists at the University of Roehampton in London tracked several great hammerheads and noted that 90 percent of the time they were swimming at roll angles of between 50 and 75 degrees. When the group performed wind tunnel studies, they found that sharks would use 10 percent less energy when swimming sideways.5
The hammerhead’s arc-shaped mouth lies on the underside of the head and is small compared to that of other sharks. Some underwater photographers relish taking relatively close-up photos of hammerheads because they know no real danger exists. But make no mistake: the hammerhead is a masterful apex hunter, one that employs all of its highly evolved equipment in pursuit of prey.
HAMMERHEADS AND STINGRAYS, THE HUNTER AND THE hunted. Over millions of years, the drama between these two arch enemies has played out, a competition every bit as intense as the legendary contest between the lion and the gazelle, or the mako and the tuna. The hammerhead evolved into a supreme hunter capable of detecting stingrays invisible to the naked eye. Yet the stingray is a formidable adversary. One stingray, the spotted eagle ray, can race fast enough underwater to reach the speed necessary to fly completely out of the water. Once the ray is airborne, flapping wings replete with dripping water, it can sometimes fly over enough distance to escape the hammerhead. If the stingray can’t escape the shark, then it can stand and fight using its deadly stinger, a hard, thin spear filled with venom located a foot from the tail’s base. If something comes too close, the ray will fling its stinger into the victim. If a human is stung by a stingray, it is painful but usually not fatal unless the victim is stung in the chest or abdomen.6 When hunting a stingray, a hammerhead must work quickly to neutralize the stinger. A skillful hammerhead can disable the stingray by biting its wings, completely avoiding the stinger. However, many sharks do catch a stinger to the mouth. Some hammerheads have been spotted with more than a dozen barbs around the mouth.
In the Bahamas, underwater photographers have captured hammerheads in action as they prowl along the sandy ocean bottom for stingrays. One dramatic chase was captured on camera near Bimini, which has some of the best visibility in the world for scuba diving. Here the visibility is 100 feet, which is the equivalent of seeing an entire baseball diamond underwater. The life-and-death chase started with a hammerhead shark patrolling in 25 feet of clear turquoise water. The hammerhead swam less than a foot above the seafloor, which was pure white sand pushed into neat furrows by the tides. Occasionally his pectoral fin inadvertently clipped a bit of furrowed white sand as he swept the area. His head and body twisted side to side, and his eyes flashed back and forth on the lookout. His head scanned the bottom as he moved forward, and with all this information, particularly the electrical signals from the seafloor, his senses told him a creature was buried in the sand. To human eyes, there was nothing here. His body twitched and jerked with anticipation. The shark started to shrink his search area, and the circle he was making began to tighten. With each sweep of his head, like a radar antenna, the shark divined the exact location of the hidden stingray. He focused on one area. The ray could sense it was only a matter of time before he was discovered. He unfurled his wings and, with all his might, pushed out from the sand, leaving a small cloud behind him. The chase was on. The hammerhead had flushed out his prey, and the stingray hoped to outrun him. The stingray surged ahead, twisting and turning to throw off his attacker, but the hammerhead matched every move and slowly gained ground. He drove down on top of the stingray and used his head to pin his prey to the bottom and immobilize him. The stingray squeezed out from under the head and spurted again into the open. The hammerhead flicked his tail and, with his dorsal fin providing stabilization, again closed the gap. At last, he lunged and took a bite out of the wing, sending the stingray into shock. The hammerhead began feeding on his prey. He was fortunate in that he had avoided the stinger. But in the underwater world, commotion brings attention—many times, unwanted attention, and there was something else watching the hammerhead besides the cameraman. A bull shark was watching the proceedings, and he wanted a piece of the prize. The hammerhead took off with the stingray in his mouth and the bull shark followed, nipping at the remains of the stingray hanging out of the hammerhead’s mouth. Both disappeared from the camera’s view into the misty turquoise waters.
SCIENCE HAS UNCOVERED HOW HAMMERHEADS HUNT, BUT that is just one part of their lives. I often wonder about the daily lives of hammerheads: Do they sleep? What do they do during the day? And where do they travel? Research has emerged revealing the why and where of hammerhead behavior beyond feeding.
Hammerheads are found throughout the world’s oceans, although most hammerhead species prefer warm waters, usually congregating along the coastline in shallow water. Not the great hammerhead shark, though, which lingers in deep water. Scientists have known for a while that hammerheads migrate in the summer to warmer waters, but until recently the details of their migratory patterns remained a bit sketchy. In 2006, a multi-institutional group of scientists affiliated with the Charles Darwin Foundation, Galápagos National Park, and the University of California, Davis, conducted a four-year study in the Galápagos. Charles Darwin came to this archipelago to study the development of animals, which led to his theory of evolution. He found the islands depressing and desolate, inhospitably dry, with little vegetation. But because the islands are scattered among 200 miles in the ocean, species-specific adaptations occurred in isolation. New species could develop based on adaptations necessary for survival. A mix of cold water from the south and warm currents coming from the equator brings abundant sea life to the Galápagos, an obvious draw for sharks. Being located 350 miles off the coast of Ecuador helps, too, because the islands are far enough away from the mainland for sharks to swim freely, unmolested by humans. This makes the islands the ideal place to study the behavior of hammerheads.7
In the study, scientists caught and tagged with special acoustic and satellite telemetry devices more than 130 sharks in the northern Wolf and Darwin Islands in the Galápagos Marine Reserve (GMR). Scientists also used photo identification and laser photogrammetry to monitor the hammerheads’ movements and to document their daily behavior. The daily behavior of lions, another apex predator, has been well documented. Notorious nappers, they lounge during the day, sleeping as much as 18 hours, coming alive only at night to hunt. But do hammerheads behave like these land-based predators? What does their daily routine look like? Similar to lions, hammerheads spend their daytime hours passively. One of their favorite pastimes is resting. But scientists haven’t yet determined whether hammerheads actually sleep like lions.
Hammerheads cannot stop moving because of the way they get their oxygen. The oldest species of sharks pumped water through their mouths and over their gills. This method is known as “buccal pumping,” named for the buccal, or cheek, muscles that pull the water into the mouth and over the gills. Many sharks continue to practice buccal pumping today, including nurse and angel sharks, both of which lie on the bottom of the ocean floor. However, as sharks evolved, they found a better way to breath: ram ventilation. As a shark swims, it “rams” water into its mouth, letting it flow out through the slit of its gills. This method of breathing is more efficient, though it does require near-constant movement. Ram ventilators like hammerheads can’t stop moving, or they die from a lack of oxygen. Since the hammerheads can’t stop, it begs the question, can they sleep? Dolphins have to keep moving as well and still seem to sleep by shutting down one side of their brain while swimming. Perhaps hammerheads do the same; however, scientists have yet to confirm their theory.
The Galápagos study revealed that hammerheads live surprisingly like humans. We have daily routines, getting our morning coffee from the same place and commuting to work over familiar terrain. Like humans, hammerheads have their favorite places to hang out. The scientific term is “site fidelity,” although a more colloquial term is “home base.” Acoustic signals showed that the sharks linger in the same places around Wolf and Darwin Islands during the daylight hours. The hammerheads meander through the same reefs, gliding over the same yellow brain coral and maneuvering at low speeds along the colorful purple sea fans attached to the reef’s steep slopes. As they hug the east side of these islands, they interact with other underwater denizens. Schools of red squirrelfish scamper into the reef’s crevices and with big round eyes peer around back into the open water. The sharks also scatter the shoals of jackfish, who keep a wary eye on them. The sharks are not always lunging at prey, though they may come across the signal of a sick fish and surge forward to pick it off. That effort serves to stop the spread of disease on the reef, similar to preventing a sick human from boarding an airplane at the airport. But by and large, the hammerheads’ day is spent passively on the reefs and slopes.
Then there is the night, when the underwater world transforms. A dramatic vertical migration takes place in the temperate regions of the world—a spectacle humans rarely see. Creatures like zooplankton rise up from the ocean depth under the starry skies. On the surface of the water, they feed on the phytoplankton, which convert the sun’s energy into food. The tiny plankton that live among the waves dance on the surface and shimmer with bioluminescence.
In a single day, this vertical movement can range from 1,000 to more than 3,000 feet, depending on the size of the animal involved. Following the zooplankton are predators. At night, the biomass of the surface waters increases by as much as 30 percent. Squid and other fish prey on the zooplankton, and they all rise to the surface to feast, where the hammerheads greet them. Upon the dawn of a new day, the zooplankton retreat back to the depths for safety. As the zooplankton leave, the surviving squid beat a hasty retreat into the depths along with them. The hammerheads also return home after a night’s work. A cycle that has been repeated for eons will play out again in the undersea theater the next night.
Another discovery is how hammerheads use the Galápagos Islands to separate adult from young hammerheads. Just as humans have set up areas for children to play in schools and for adults to work, the sharks have similar arrangements. Adult hammerheads live and work on the northern islands of the Galápagos. The southern islands are where the sharks have their birthing area. After a gestation period of ten to twelve months, females give birth to as many as twelve pups, although the great hammerhead can produce as many as forty. It is not known how soon the female hammerheads become pregnant again after giving birth. After the sharks are born, the researchers found, the baby sharks live in groups among the mangroves of the southern islands, feeding off the fish there. As they mature, they move north to join their adult peers, eventually venturing out into the open sea. On some of these recorded journeys, the sharks traveled 50 miles round-trip in a day. Moreover, the sharks traversed the entire way on a direct path. The scientists were puzzled about how they accomplished that feat. One theorized that hammerheads use the earth’s magnetic field for navigation. When basalt hardens on seamounts, magnetic ridges and valleys form. By detecting the geomagnetic fields on the seafloor, the hammerheads can orient themselves. Whenever they need to reorient themselves, they take a deep open-water dive. One shark dove down to a depth of 3,000 feet and, using his head’s electrosensory organs, was able to detect his position and head back home among the familiar rock outcroppings and reefs at his favorite home base on the island.
Occasionally, the hammerheads made longer trips from the north of the Galápagos Islands to other areas in the eastern Pacific. Some traveled more than 300 miles to the Cocos Islands. Other hammerheads were recorded making return migrations of more than 1,800 miles. Since hammerhead sharks are highly migratory and travel great distances, they are vulnerable on their travels to fishing fleets plowing the waves, which is why these migratory studies are important for determining areas that need protection, even in open waters, where the hammerheads are in transit.
The scalloped hammerhead, which is almost as large as the great hammerhead, developed mate selection through schooling, a highly unusual trait among other hammerhead species. Scalloped hammerheads can often be spotted swimming in large schools of a hundred or more. For many scuba divers, witnessing the splendor of schooling scalloped hammerheads is the holy grail. While this phenomenon takes place in the Galápagos and along the Great Barrier Reef, the best place to witness this event is around Costa Rica’s Cocos Islands, which Jacques Cousteau called the best diving spot in the world. Around these islands, hammerheads swell into schools of well over a thousand, visiting the pinnacles like Dirty Rock, an outcrop of jagged rocks that drops down about 120 feet below the surface. Sea life bursts there. Among the staghorn and fire coral, divers cleave to the reef at 30 feet and stare out at a watery, deep-blue canvas, where sea life moves in and out to create an ever-changing seascape of brilliant colors and varying shapes of different fish and coral species. With their trailing stinger, spotted eagle rays cruise along the bottom of the ocean on graceful, undulating wings. Thousands of silvery jacks, whose scales flash in the sunlight, coalesce into a swirling globe so dense that divers completely disappear in them. Dolphins love to chase the jacks, but the fish are fast and keep eluding their pursuers. A hawksbill turtle with some green algae encrusted on her carapace skims the reef with her flippers. Hunting yellowfin tuna with their scimitar fins cruise in from the blue miasma. Sharks are everywhere on this reef. Half a dozen whitetip reef sharks nap on the seafloor. Galápagos sharks arrive in small groups and prospect for prey. Though these sharks live around the islands, they are found worldwide on reefs of oceanic islands, where they are usually the most abundant species of shark. Sleek and shaped like torpedoes, they can grow to 10 feet. They have come here for their appointment with the barberfish, a specialized fish that cleans migrating fish and many species of shark including hammerheads. These yellow fish are 8 inches long and shaped like a pancake with a black stripe running along the top. Their eyes are large, and their lips are tiny, puckered, and protruding, which allows the aptly named fish to nab parasites living on fish. A pencil-thin stripe between the eyes and lips completes their resemblance to a natty Italian barber. One hammerhead slows to a stop, and a dozen barberfish get to work on their client. They work the hammerhead’s body and pick and stab at the crustaceans and other ectoparasites clinging to the shark. Eventually, the hammerhead moves on, and the next client comes into the cleaning station.
Hundreds of scalloped hammerheads slowly coalesce and sweep forward, their distinctive silhouettes tattooed against the rich background of endless blue. A living, breathing wall made of hammerheads creates a giant shoal of sharks. Peter A. Klimley, a behaviorist at the Bodega Marine Laboratory at UC Davis, has studied scalloped hammerheads for two decades at El Bajo, a seamount, or underwater mountain, in the Gulf of California. Klimley believes the hammerheads school for mate selection. In midday, after the sharks aggregate, a structural hierarchy forms according to size and age. The largest, fecund females force the smaller females out of the center of the school. Males fight their way to the center of the school to attract the prime females. Once the dominant females make their selection, the pairs separate from the group to mate.8 The sexual encounters can be violent. The females get raked, and their skin can be in tatters after these encounters with the males. A year later, females give live birth to the next generation of hammerheads.
WHILE THIS SCHOOLING PRACTICE IS GREAT FOR MATING PURPOSES, it also exposes scalloped hammerheads to death in some parts of the world. Costa Rica, in particular, is one of the world’s most important participants in the shark-fin trade.9 Finning is a common practice in the Cocos Islands, despite their designation as a national park. In 2012, the former president of Costa Rica, Laura Chinchilla Miranda, signed a law prohibiting shark-finning. But this law remains difficult to enforce because the Taiwanese mafia and, to a lesser extent, the Indonesian mafia maintain a strong—and intimidating—presence in the country, and they continue to export shark fins with impunity throughout the Pacific Rim. Mafia-controlled docks like Inversiones, Harezan, and others in the country’s port of Puntarenas continue to export approximately 95 percent of Costa Rica’s catches to Hong Kong.10 The Chinese demand for shark-fin soup is insatiable. An estimated 1.7 million tons of sharks are captured worldwide annually just to keep up with this demand.11 The sale of shark fins to the Chinese generates significant revenue for Costa Rica. The state’s rich bounty of hammerhead sharks is worth significant dollars. Because of their high fin ray count, coupled with their towering dorsal fins and long pectoral fins, hammerheads are more highly valued than other shark species. Hammerhead sharks represent at least 4 percent of the fins auctioned in Hong Kong, the world’s largest shark-fin trading center. Hauled onto commercial fishing boats, hammerheads are finned alive and then discarded overboard. Without a fin, the sharks struggle to stay afloat, eventually sinking to their death. Corpses litter the sea bottom.
It’s not just finning, however, that puts sharks at risk. Various shark cartilage industries exist in Costa Rica. The country is a leading processer of raw cartilage, which they export around the world, most notably to the United States, for use by the medical supplement industry, which erroneously claims that shark cartilage can prevent cancer. Costa Rica processes up to 2.8 million sharks a year, with much of this carnage going to supply the shark cartilage companies. The idea that eating shark cartilage stops cancer started when 60 Minutes aired a segment on the subject over twenty years ago. After the show, the myth sprang up that sharks do not get cancer, a ridiculous and scientifically unfounded claim.12 Even as early as 1908, a captured blue shark was found upon dissection to have been riddled with cancerous tumors. In addition, eighteen other species of sharks have also been shown to have benign and malignant tumors. The medical literature is replete with studies that prove sharks do get cancer.13 The University of Texas conducted a controlled, double-blind placebo study to determine whether shark cartilage is an effective treatment against cancer. All 379 patients in the study received radiation and chemotherapy in addition to a shark cartilage supplement or a placebo pill. The team found that those taking the supplement did not live any longer than those taking the placebo.14 Previous studies, funded by the National Center for Complementary and Alternative Medicine, also found that shark cartilage did not benefit patients with lung or advanced breast or colon cancer.15 And noted shark researcher David Shiffman in Scientific American remarked that eating shark cartilage is useless. “Sharks get cancer,” he said. “Even if they didn’t get cancer, eating shark products won’t cure cancer any more than me eating Michael Jordan would make me better at basketball.”16 That same article shows a picture of a great white shark with a large tumor protruding from its lower jaw.
In summary, the evidence is clear: eating shark cartilage had no benefit at all. The disturbing myth that sharks don’t get cancer has done great harm to both sharks and people.17 The danger of this absurd belief is that cancer patients won’t receive effective treatments. Moreover, as a cancer preventative, shark cartilage provides only a false sense of security. Other documented approaches18—such as getting screening tests, eating a plant-based diet, and exercising—actually lower the chances of getting cancer and may also help people with cancer control their disease. Sadly, the carnage continues. Sharks die unnecessarily for their cartilage, and with a $30 million industry at stake, some parties have every reason to see it continue.
Recreational fishing also poses a threat to hammerheads. More than five hundred species of sharks exist in the world, with some weighing a few pounds while others weigh a few thousand pounds. The largest ones are the most attractive to fishermen seeking a trophy. While hammerheads cannot compare in size and strength to tigers and great whites, they can weigh over 1,000 pounds. Given hammerheads’ size and imposing dorsal fin, fishermen relish catching them, and charter companies like Mark the Shark’s give them the opportunity to do so.
Misperceptions exist around catching sharks. One is that they are so strong and tough that if you release them by cutting the line, they will swim away and be just fine. In reality, sharks are at risk anytime they are caught since the struggle can lead to their death. They can swallow the hook or get banged on the side of the boat, which damages their organs. Scientific studies show that the struggle on the line many times leads to the shark’s death, even if it is released. The University of Miami Abess Center for Ecosystem Science investigated the effects of catch-and-release fishing on shark mortality.19 The study investigated experimentally simulated catch-and-release fishing on five shark species—hammerhead, blacktip, bull, lemon, and tiger sharks—in South Florida and Bahamian waters. Researchers took blood samples to examine stress, carbon dioxide, and lactate levels. Researchers then used satellite tags to determine postrelease survival. Not surprisingly, the blood lactate levels of sharks soared after fighting on a fishing line in much the same way lactate in humans rises during intense or exhaustive physical exercise. An increase in lactate levels significantly affects the odds of survivability of many fish species. The study showed that even with minimal fighting times on a line, hammerheads exhibited the highest buildup of lactic acid. The lead author, Austin Gallagher, wrote, “Our results show that while some species, like tiger sharks, can sustain and even recover from minimal catch and release fishing, other sharks, such as hammerheads, are more sensitive.”
With the ubiquity of GoPro cameras, YouTube abounds with videos and pictures of fishermen catching hammerheads. One YouTube video clearly shows the vulnerability of the hammerhead to sports fishing.20 One hammerhead, filmed underwater, was caught on the line and was pulled up next to the boat. It was not long before a tiger shark appeared on the scene due to the hammerhead’s electrical distress signals. In the fight between the tiger and the hammerhead, the fishing line holding the hammerhead snapped. However, the hammerhead was so exhausted that it could do nothing to stop the tiger shark from continuing its attack. The tiger shark took a bite out of the hammerhead, and the water turned crimson with blood. The tiger shark attacked again at the hammerhead’s midsection and dragged the hammerhead into the depths beyond the camera’s view.
In another case, a fisherman hooked a 14-foot hammerhead shark near Corpus Christi, Texas, a feat he called a “catch of multiple lifetimes.”21 The fisherman claims to have tried to release the shark back into the ocean, but it was “too tired” and did not survive. Of course, the death of the hammerhead comes as no surprise after looking at the data from the University of Miami. While the fisherman tried to show remorse that the shark did not survive, many on Facebook expressed horror at the tragedy. As one commenter on Poco Cedillo’s Facebook page noted, “Had they not been sport fishing, that shark wouldn’t have died that day.”
In the United States, both the commercial coastal fisheries and the pelagic longline fishing industry punish the hammerhead population.22 Recreational fishing is also having an impact on the shark. Hammerheads are the third most common shark caught, according to reports from Florida recreational charter companies, and their clients consider the great hammerheads specifically to be one of the most attractive species to catch. Across the coastal states of the Carolinas, Virginia, and Florida, hammerhead populations have been decimated. But the United States is not alone in its persecution of hammerheads. The global population of the great hammerhead shark is estimated to have declined by approximately 80 percent over the past twenty-five years. Scalloped hammerhead sharks have declined by 89 percent over a fifteen-year time period, from 1986 to 2000, less than one generation.23 As a result, since 2007, the great hammerhead has been listed as endangered by the International Union for Conservation of Nature. The shark has also recently been included in the Convention on International Trade in Endangered Species Appendix II, which includes species not necessarily threatened with extinction, but in which trade must be controlled in order to maintain their survival. Yet in spite of the human onslaught on sharks, hammerheads rarely attack humans. They are considered neither aggressive nor dangerous to humans. Only seventeen unprovoked attacks by hammerhead sharks on humans have been documented. No human fatalities have been recorded over the past four centuries.24
Hammerheads are also faced with new challenges arising from global warming. The world’s seas are approximately 1.3°F (1°C) warmer than they were a century ago, mainly due to the increase in CO2. This fact is changing the ecosystem within which the sharks live in unknown ways. For instance, blacktip sharks usually migrate from the Carolinas into South Florida’s waters as part of their annual winter migration. The sharks normally spend between mid-January and mid-March off the South Florida coast in search of warmer water and food. As the water has warmed, however, the blacktips have no incentive to move into the waters off the southern tip of Florida. What this means to the hammerheads, who hunt the blacktips, and the ecosystem of South Florida is unknown.
With such a perilous present and future, the hammerhead remains vulnerable, which puts into jeopardy the opportunity to learn from the species’s many remaining mysteries. Does the hammerhead have more ampullae than other sharks, given its wider head? Is the head teeming with senses that are superior to those of other sharks? Could the tubercles assist with the shark’s yaw (sideways turning motion)? New questions surface all the time. For instance, bonnethead sharks have been found to feed on seagrass, which sometimes makes up as much as half their stomach contents. Even though they are able to partially digest the grass, do they swallow it intentionally? That preference would make the hammerhead an omnivore—the only known omnivore among shark species. Scientists will be exploring the answers to these questions for many years.
But here is what we do know. Hammerhead sharks migrate extensively, and they face constant risks from humans in oceans around the world. Commercial fishermen have devised all kinds of nets. One of the deadliest is the drift net, which is left to drift in coastal waters where various fish species get entangled. When a hammerhead struggles to free itself, the shark only gets more entangled and, once immobilized, will suffocate to death. Since hammerheads travel mostly in coastal waters, they often fall victim to these nets. Moreover, recreational anglers pursue the hammerhead as a trophy. In the United States, some of the 49 million licensed recreational anglers, armed with equipment worth $45 billion,25 descend on sharks, many of them hammerheads.
While hammerheads are hunted constantly, their lives are quite ordinary; they live in their home territories, find mates, reproduce, and die. In that sense, there is little difference between the life of the hammerhead and the life of humans. Yet humans are different, and we have unique attributes. Darwin made the distinction clear. He said, “The love for all living creatures is the most noble attribute of man.” Humankind needs to widen its circle of love to encompass sharks now more than ever. However, while humankind can be noble, we can also be victims of self-delusion. As Darwin also observed: “Great is the power of steady misrepresentation.” The misrepresentation of sharks has led to the greatest threat to the hammerhead’s existence in 20 million years. And as we’ll see in the next chapter, our misconceptions about sharks as solitary, bloodthirsty species continue to be upended. Scientists continue to discover that sharks have more in common with people than previously believed.