Tuesday, 23 June 2009

It's a small male sperm whale, only a year or two old—maybe 16 feet long if it were all here. Conveniently, only the head remains, at most a ton of blubber, skin, and bones resting on a wooden pallet. "You can lift it with a forklift! I love it," says Ketten, a biologist who studies the hearing of whales. Because this whale has been kept frozen, it is the freshest sperm whale she has cut into in years, and she can't help contrasting it with her last sperm whale dissection in 1999, on New Year's. She was at a party, "in velvet minidress and three-inch heels," when she got the call—a whale had beached and died on Nantucket Island. She dropped everything and got on an airplane. The whale stank horribly. She and her team dissected the ears and returned to the airport. On the way home everyone smelled so bad they were put on another plane by themselves, Ketten recalls.

Her subject this day had stranded on a beach in the Gulf of Mexico months earlier. Bathers poured water on him and covered him with their towels, but he was too sick to return to the sea, too big for any wildlife rehabilitation center. To end his misery, he had to be killed. Because his peripheral veins had collapsed, it proved impossible to inject a mortal dose of sedative. Finally, a veterinarian administered a local anesthetic, cut an artery, and let the whale quietly bleed out into the shallow water. Then the vet and a team from the National Park Service cut off his head, packed it in 150 bags of ice bought at a minimart, and trucked it to a walk-in freezer.

Ketten cuts through layers of blubber and muscle, searching for the tympano-periotic bulla, a bone complex that houses the middle and inner ear. She shows a dry specimen, smaller than her fist, taken from a whale that stranded in 1964. It may be hollow, but it is extraordinarily heavy. Ear bones of cetaceans—whales, dolphins, and porpoises—are the densest bones in the world, protecting delicate inner-ear tissues from damage and the tremendous pressure of dives. Sperm whales are thought to dive as much as a mile below the surface in search of squid and other prey.

Blubber is surprisingly attractive: a spotless milk-white layer, inches thick, beneath the whale's deep, rich black skin. Beneath the blubber, Ketten finds jaw fats, creamy in color, far softer. When she tentatively identifies the shape of the casing that holds the fats, she says: "It's so cool! It's a sort of ovoid lobe of fat—if my theory is correct—that runs along the jaw, conducting sound waves." She describes the lobes as shaped like a pair of rabbit's ears, one on each side of the jaw.

As dusk falls, she reaches the ligaments behind the bulla and calls for a flashlight. Cutting it loose, she holds it up for all to admire before injecting it with formalin to preserve the cochlear structures inside. "It's a rock that has really delicate membranes in it," she says.

On the second day, the biologists tip the head over with a forklift so they can work on the other side. Ketten injects methylene blue dye into the outer ear, a slit about a third of an inch wide and shaped like a sound hole on a violin. The dye travels less than two inches before it hits an obstruction, possibly a lump of wax and dead tissue similar to those Ketten has seen in other whales. The canal may be a blind pouch, a useless relic of the whale's ancestry as a land animal. Ketten says she will examine it "slowly, tediously, carefully" in the laboratory to figure out whether it has any function.

Next, she saws out a block of tissue that contains the middle and inner ear so it can be put through a CT scanner, membranes intact. When the block finally comes loose, she peers into the space behind the ear and points out the enormous auditory nerve that passes through a hole in the skull from the brain to the ear. The nerve is big not only because whales are big; it is big because hearing is a whale's most important sense.

Because cetaceans have evolved so that their outer ears do little if any work, researchers had suggested that jaw fats receive sounds. Ketten was the one who put forth convincing evidence that the soft fat shaped like a rabbit's ear in a whale's head will pick up sound waves as the mammal moves through water and carry the waves to the middle and inner ear. "This particular type of fat has an acoustic impedance that's similar to seawater," she says, referring to cetaceans affectionately as "acoustic fatheads."

While the structure of cetaceans' middle and inner ear is similar to that of land mammals, including humans, Ketten has found differences that allow whales and dolphins to hear higher frequencies than they otherwise might, improving their ability to echolocate. She has determined that cetaceans can hear much higher and lower frequencies than humans because they have evolved a bigger range of widths and stiffnesses in the basilar membrane in the cochlea of their inner ears.

Ketten also discovered that cetacean ears fall into three anatomic groups based on their lives in the water: "The frequencies they hear tell you something about what's important to them in their environment."

For example, odontocetes—toothed whales and dolphins—come in two flavors. Type I odontocetes hear upper-range ultrasonics, peaking above 100 kilohertz, about 80 kilohertz higher than human ears can hear. These animals include species such as the Amazon dolphin, which navigates in narrow spaces and clouded waters. Type II, the lower-range ultrasonic odontocetes, peak below 80 kilohertz. They are creatures of the coast and the open sea, needing lower frequencies to echolocate over longer distances in the search for, say, herring. There's something of a trade-off involved: Higher frequencies give precise images in echolocation; lower frequencies travel much farther but miss very small objects.

So little is known about so many species of whales and dolphins that Ketten becomes frustrated when she is pressured by environmentalists and government agencies to give definite answers. She grumbles that "marine mammalogy is a field in which the plural of anecdote is data." And although she is eager to study the blocks of tissue she cut from the Fort Walton Beach sperm whale, she has had to put much of that work on hold to focus on the most demanding, high-profile case she has ever undertaken: 16whales that beached in the Bahamas two years ago.

Most were beaked whales, and they stranded in the Providence Channels in the northern Bahamas. At the time, the U.S. Navy was testing tactical mid-frequency sonar in the area. Six of the whales died. Ten were pushed back to sea and may have survived. The deaths—and the possibility that sonar was responsible for them—triggered a controversy that is still unsettled.

In 1986, when Darlene Ketten was working at Harvard, she happened to overhear a conversation about cochlear implants in a hallway. "They were saying, 'Well, we can't get good scans because of the metal implant,' and I said, 'Yes, you can.'"

Much of a cochlear implant is made of platinum, a dense element that plays havoc with scans. As dense as platinum is, it's not much denser than cetacean ear bones, which had been a problem Ketten had to overcome in her scans. Soon she began consulting with implant teams at Harvard's Massachusetts Eye and Ear Infirmary and at the Washington University School of Medicine in St. Louis.