_____
Title: Rapture of the Deep
Authors: Michael Tennesen
Source: WILDLIFE CONSERVATION; May/June 2005
Database: D Drive
RAPTURE OF THE DEEP
By Michael Tennesen
During the austral summer, the human population at McMurdo Station on Ross
Island just off the Antarctic shore swells from around 250 to more than 1,000
as the scientists take advantage of 24-hour sunlight and "toasty,"
near-freezing temperatures. When Gerald Kooyman, a research professor at
Scripps Institution of Oceanography in San Diego, California, first visited
the station in the 1960s, he looked out across McMurdo Sound, a sheet of thick
ice wedged between Ross Island and the continent, and wondered what he could
do with this great outdoor laboratory. With no natural predators, the animals
were unafraid of people, so he decided to elicit their help in a diving
experiment. Kooyman transplanted a Weddell seal out to a hole he had drilled
with a large augur into the six-foot thick ice. The hole was placed far away
from other holes or open ice so that the seal would use it to both dive and
to return to the surface. Thus the scientist wouldn't lose the expensive
diving recorder he'd attached to the seal, and no one could claim the seals
went somewhere else during the experiment.
A pioneer in the study of deep diving, Kooyman began his work at a time when
scientists generally thought that animals might be able to descend 325 to 650
feet below the surface. "I remember I was eating a dry cracker when the depth
recorder came back from one seal that showed it dove to 1,970 feet," he
recalls. "I nearly choked on the cracker."
A lot of scientists have since "gagged on their crackers," and called
manufacturers to double-check the accuracy of their depth recorders as, one
after another, the emperor penguin, leatherback sea turtle, northern elephant
seal, bottlenose whale, and sperm whale has met and surpassed that record.
Emperor penguins can dive to 1,853 feet, and sperm whales to 6,560 feet
Compare those depths to that of humans. The world record for a human descent
without diving gear but assisted by ropes and pulleys is 531 feet. And human
divers attempting these depths and greater, with or without gear, are subject
to a host of maladies including shallow-water blackouts (the sudden loss of
consciousness caused by oxygen starvation following a breath-holding dive)
and the bends, or decompression sickness-both of which can be fatal. But
deep-diving animals glide on through water levels dangerous to people without
even elevated heart rates.
Kooyman learned more about the ease with which deep-diving animals accomplish
these phenomenal dives when he returned to McMurdo more recently to test
emperor penguins. For a number of years, Kooyman and his associate Paul
Ponganis, a research physiologist at Scripps and a practicing
anesthesiologist, have been trying to understand some of the remarkable
physiological adaptations that enable these animals to accomplish their
remarkable diving feats.
For better than two weeks, they worked with ten adult penguins, each of which
weighed from 45 to 60 pounds. The scientists divided the birds into two
groups: one group they fed by hand, and the other was allowed to hunt its own
prey through a hole drilled in the ice. By taking blood samples from the two
groups and comparing the results, the scientists hoped to measure the
energetic costs of diving for prey.
But when the results come back they were shocked. Emperor penguins chasing
prey in frigid Antarctic waters are such efficient predators that they don't
use any more energy than if they were simply standing around, chowing down on
handouts. "For penguins and seals, hunting is more like meditation," says
Ponganis,
To get into that state, Weddell seals and emperor penguins exhibit an
exquisite ability to control their own heart rates during dives. Normal
heart rates for emperor penguins are 60 to 70 beats per minute; but when the
birds dive, the heart rate can go as low as 20 to 30. They essentially save
most of their bodily functions-purging carbon dioxide, eliminating waste,
and even digesting food-for the surface, when their heart rates may go up 180
to 200 beats per minute. During deep dives, the emperor's body may even shut
down organs such as the liver, kidneys, and stomach.
In elephant seals, the heart rate of at the surface is 120 beats per minute;
at depths, it falls to 30 to 35 beats, and has been recorded as low as 2
beats per minute. That's 30 seconds between pulses-approaching cardiac
arrest in a human. Unlike in humans, most of the oxygen to sustain the
seals' long, deep dives is stored in blood and muscle, not in the lungs.
Oxygen is bound to the protein myoglobin in the muscles and hemoglobin in
the blood. Deep divers have a high concentration of hemoglobin in their
blood and have a large blood volume. With all this oxygen already aboard,
they don't need to work themselves into a heavy breathing frenzy, like land
animals do, when they are chasing prey.
What emperor penguins, Weddell seals, and elephant seals share with a number
of other deep divers are streamlined bodies and enough mass that they glide
through the water with little effort. During a week on the Island of San
Miguel, off the coast of California, we witnessed how incredibly awkward
elephant seals are on land-lurching across the sand as one scientist put it
like "enormous fat caterpillars."
But when one male elephant seal hauled himself from the shore into the water,
he metamorphosed into a ballet-like swimmer, instantly attaining speed while
displaying little effort. Says Michael Castelini, professor of marine science
at the University of Alaska at Fairbanks, "We [humans] thrash and flail our
way through the water, while the elephant seal glides through it as if it
were on a layer of ball bearings."
Diving may be effortless for elephant seals. Biologists have recorded one
seal diving to 5,479 feet-a record for the species. To put that in
perspective, imagine more than three Empire State buildings stacked on top of
one another. The seal swam from the top of the spire of the uppermost
building to the basement of the bottom building and back up again, a distance
of over two miles, in less than an hour.
There are animals that live at such depths, such as the coelecanth, but they
often die when pulled to the surface, or live for a only very short time out
of their element. Elephant seals can survive at depth and at the surface,
though these environments are as different as those on Mars and Venus.
What's more, when elephant seals come to the surface, they spend only a few
minutes there, exhaling carbon dioxide and replenishing oxygen, before they
dive again. According to Robert DeLong, research biologist for the National
Marine Mammal Laboratory in Seattle, "You want to think of them not as a
diving animal but as an animal that occasionally surfaces. They spend 90
percent of their time at sea under water."
And they can stay under for long periods without breathing. One female
tagged by DeLong, dove for two hours.
For part of that time, the animal may be sleeping. Diving may be almost an
auto pilot effort, as the seals take short naps on the way down and up.
Castellini believes that elephant seals may sleep as dolphins do: closing
one eye while half of the brain dozes and the other half keeps vigilance,
then switching back and forth. If elephant seals can go deep enough, they
don't have to worry about predation by great white sharks or killer
whales, so swimming at reduced vigilance may not be a problem.
But how do elephant seals avoid rapture of the deep-a common name for
decompression sickness? Researchers believe that an elephant seal's lungs
collapse at a depths between 165 and 230 feet. With no exchange of gases
(particularly nitrogen) between lungs and blood, deep-diving animals avoid
the blood-chemistry imbalances that create human diving problems.
Still they must survive enormous pressures. Every 10 meters a deep diver
descends, the pressure increases by approximately one atmosphere. That means
for a sperm whale to dive 6,560 feet, it must endure pressures equal to about
194 atmospheres or 2845 pounds on every square inch of its skin.
Says DeLong, "Can you imagine what they must look like? Clearly the whole
chest cavity collapses and the animal's face must look like a prune."
Students of Le Boeuf like to paint up Styrofoam mannequins, put lipstick on
them, color their eyes, and then send them down 300 feet, just for fun.
According to Le Boeuf, "They come back looking like shrunken heads."
The first recording devices that were placed on elephant seals were metal
boxes, and they imploded from the pressure.
The speeds at which deep-diving animals make their descents are another
marvel. Says Ponganis, "I talk to physicians who manage experimental human
divers, and they are absolutely floored at how fast seals and whales are
diving. Safely taking a man down to 1,900 feet can take four days, whereas
an emperor penguin can get there in five minutes."
But why go there? What is the advantage? Many of the deep divers are going
to an area called the deep scattering layer, a horizontal zone of living
organisms, usually schools of fish, occurring below the surface in many ocean
areas. The name comes from the fact that this layer scatters or reflects
sound waves. This rich zone contains hundreds of species of fish, which
migrate closer to the surface at night but retreat to the depths by day.
Says Le Boeuf, "The deep scattering layer is where most of the biomass in
the ocean is concentrated. These seals are diving to the center of the
richest part."
By venturing to a zone where few large competitors go, the seals have a
virtual monopoly on some of the richest waters off the continental shelf.
And spending very little time at the surface, they avoid two deadly
predators, the great white shark and the killer whale, which patrol
shallower waters.
The deepest diver may not be a seal or a penguin, but a whale. The sperm
whale, the reigning dive champion, has been recorded at depths of 6,650
FEET. Even leatherback sea turtles have been recorded at depths below
3,280 FEET. However, Sascha Hooker, a biologist with the University of St.
Andrews in the United Kingdom, thinks that if you are interested in what
animals actually do for a living, then looking at their deepest dives is
not as important as looking at their routine dives. Sperm whales routinely
dive to 1,300 to 1,950 feet , elephant seals normally dive to 1,150 to
1,950 feet, and bottlenose whales, which Hooker studied in a submarine
canyon off Nova Scotia, regularly make dives to 3,300 to 4,900 feet. At
these levels there is little or no light. Cameras attached to animals that
go to these depths return with images of a black screen 90 percent of the
time.
Hal Whitehead, a biologist at Dalhousie University in Halifax, Nova Scotia,
says that some fish at these depths are bioluminescent, and whales and
seals may key on this bodily light. Or they may swim beneath their prey in
daylight hours and look upward for silhouettes; hence, the elephant seals'
enormous eyes help them see in the darkness at depths. Many of the whales
use natural sonar systems to locate prey. The nose of the sperm whale,
which Whitehead studies, constitutes a quarter to a third of its total
weight. He thinks that nose contains the most powerful sonar system in
the natural world.
He also thinks that the greatest advantage of diving to depth for a living is
the speed, or lack thereof, of the prey down there. "Animals at the surface
tend to be fast or fierce. This is possible because surface waters have a lot
of oxygen. But when you get down below 1,300 feet, there is very little
oxygen, and the escape responses of prey are very limited."
What that means is if you can handle the crushing pressure, don't get the
bends, hold your breath for ten minutes to an hour, have huge eyes or a sonar
system built into your nose, then the deep sea could just be one of the
biggest all-you-can eat buffets the Earth has to offer.
-- End --
Freelance writer Michael Tennesen last wrote about thieving crows in our
April 2005 issue.