Grade 4

Do Sharks Have a Sixth Sense?

Hammerhead sharks by the hundreds swirl around Peter Klimley. This sounds like a nightmare with teeth, but for the adventurer and scientist, it's just another swim with his favorite animal.

Klimley, a marine biologist at the University of California, Davis, has spent many hours in the water with these sharks—some more than four meters long. Although hammerheads have a “man-eater” reputation, they've never harmed him. “I once wanted to get a hammerhead on the research boat, so I lassoed it by the tail,” says Klimley. “I was afraid it would turn around and bite me, but it just swam faster.”

As a teenager, Klimley raised tropical fish. He became interested in sharks as a college student. “There was something romantic about sharks,” he says. “It was my dream to study sharks in the water. I believe that if you want to understand an animal, you have to enter its environment.”

Scalloped hammerheads are one of the largest of the nine species of hammerheads. As a young scientist, Klimley heard about scalloped hammerheads schooling in huge numbers at remote seamounts (underwater mountains) in the Gulf of California. He traveled to Mexico and slipped into the water with them, not quite sure if he would end up an observer or an appetizer.

Most people think of sharks as mindless eating machines. But by watching the hammerheads, Klimley discovered a shark “society.” Large females use head-slaps and fancy corkscrew swimming to rule the school. The females joust for a place near the center, where males come courting.

Klimley has studied many different sharks, but he thinks that hammerheads are special. “I love hammerheads,” he says. “They're the PhDs of the shark world.”

Every evening, the hammerheads leave the seamounts. Where do they go? Klimley decided to track them with an ultrasonic transmitter. He attached the transmitter by diving down and jabbing a barb into a hammerhead's back with a spear. The transmitter floated behind the barb, sending signals to a computer aboard the research boat. Klimley could now track the hammerheads day and night.

The scientist discovered that the hammerheads traveled to deeper waters every night to feed, sometimes swimming 16 kilometers from the seamount. They returned every morning. “I was amazed,” says Klimley. “We tracked the sharks from our boat, and sometimes it was so dark that we had a hard time finding our way back.”

How do hammerheads navigate through a dark ocean? Klimley considered many possibilities. Were the sharks getting their bearings from the moon or stars, or following features on the sea floor? He looked at the swimming depth recorded by the transmitter. The hammerheads swam too deep to see the moonlit sky, and too shallow to follow the bottom.

Klimley knew that the sea itself might give the hammerheads navigational clues. The open ocean isn't one big bathtub. It has thick layers of different temperatures that can move in different directions. He mapped currents and water temperatures near the seamount, but there was no connection with the sharks' paths.

There was another possibility: the Earth's magnetic field. The Earth is magnetized along a north-south axis. But magnetic minerals in the Earth's crust cause small changes in this main magnetic field—bands or spots of stronger or weaker magnetic force. The bands and spots are faint, but perhaps they could act as navigational markers for a creature with a magnetic “sixth sense.” A creature like a shark.

Sharks have thousands of tiny holes on their bodies that are connected to little fluid-filled sacs. Called the Ampullae of Lorenzini, these sacs are used by sharks to sense the faint electromagnetic fields generated by all living things. A shark can find prey hidden under sand or in murky water. If a shark can find a fish using an electromagnetic sense, could it also find its way through the ocean by using magnetic clues?

Klimley mapped the magnetic field around a seamount. The mount had a strong magnetic field, with positive and negative poles, and the hammerheads always schooled on the northern side. He discovered magnetic “peaks,” “valleys,” and “ridges” flowing outward—a hidden underwater landscape. During their nightly feeding jaunts, the sharks seemed to follow these magnetic features the way a helicopter pilot might follow a river or a ridgeline.

The scientist thinks that the hammerheads use the seamounts as highway markers and rest stops on their longer migrations. Scalloped hammerheads usually spend the summer in the Gulf of California, but no one knows where they go the rest of the year. He hopes to solve the mystery by using special long-distance satellite tags.

Klimley also hopes to find more evidence for his theories about hammerhead migration. He plans to someday run a large wire coil around a seamount and reverse its magnetic poles. If hammerheads orient using a magnetic sense, they should change where they school on the seamount.

Wherever the hammerheads spend the winter, and however they navigate, one thing is certain: They'll find their way back for another swim with a certain scientist friend.

Vocabulary

ultrasonic:
Of or relating to acoustic frequencies above the range audible to the human ear, or above approximately 20,000 hertz.

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Activity

  1. What did Peter Klimley want to know about sharks?
  2. What are Ampullae de Lorenzini? What two things might a shark use electromagnetism to do? What role does electromagnetism play in these activities? Write a few sentences to explain your answer.
  3. How would changing the poles of underwater features, such as seamounts, affect a shark?
  4. Do you think it is important for scientists to study how sharks use electromagnetism? Why or why not? Write a sentence or two explaining your opinion.