Why Do Whales Get the Bends?
The Cuvier's beaked whale is a master of the ocean's crushing depths. It can dive as
deep as 2 kilometers in search of prey, the deepest known for any mammal. So
scientists have been at a loss to explain why, in response to naval sonar
testing, this champion cetacean sometimes succumbs to the same decompression
sickness that afflicts scuba divers. A new mathematical model suggests that, by
replicating the sounds of a predator, sonar forces the whale to adopt a risky
diving pattern.
Researchers have suspected a link between sonar testing and whale deaths for
nearly 20 years. In 2000, the U.S. Navy said its sonar exercises led six beaked
whales to fatally beach themselves in the Bahamas, and stranded whales have died
near sonar-testing sites in at least five other cases since then. It hasn't been
clear how the sonar disorients the animals and causes such strandings, but some
marine biologists suspect that the intense sound waves force whales to shoot to
the surface, and they've found evidence that tiny nitrogen bubbles expand in the
whales' tissues and damage vital organs (ScienceNOW,
9 October 2003). The same thing happens when scuba divers surface too quickly--a
condition known as the bends. But a whale holds its breath when diving,
preventing nitrogen buildup, so the theory didn't seem to hold water. A group
led by marine biologist Peter Tyack of Woods Hole Oceanographic Institution in
Massachusetts suspected that whales alter their diving behavior in some other
way.
Whales make repeated shallow dives when trying to evade predators. The team
wondered whether such behavior could be risky, especially because naval
sonar--which is similar in frequency to the calls of the beaked whale's most
feared adversary, the killer whale--could be forcing the whales to adopt a
similar diving pattern. So the researchers mathematically analyzed dive behavior
in Cuvier's beaked whales and in dolphins to test whether nitrogen bubbles could
expand in whale tissue during repeated shallow dives. The team incorporated
known physiological data into a model that charts how the bubble size might
increase in the circulatory system, brain, muscles, and fat tissues when a whale
dives repeatedly to between 30 and 80 meters for as long as 3 hours.
During normal diving behavior, scientists believe, the lungs of marine
mammals collapse when they plunge past 72 meters in depth. That "clever
mechanism," Tyack says, prevents nitrogen from infiltrating the bloodstream. The
team's model predicts that if the whales' lungs do not collapse during a long
series of shallow dives, the increased pressure can cause nitrogen bubbles to
diffuse into tissues, increasing the risk of bubble formation on ascent.
Limiting the duration of sonar testing may prevent the animals from diving in
these harmful patterns, the team concludes in the current issue of Marine
Mammal Science.
Noting that diving behavior is extraordinarily difficult to study in live
animals, marine biologist Terrie Williams of the University of California, Santa
Cruz, calls the model "extremely useful." As new research shores up gaps in the
model's assumptions--with actual observations to corroborate the avoidance
behavior, for example--scientists can try to home in on a safe length and level
of sonar exercises, clarifying the murky waters surrounding this debate. "Now
it's a question of how quickly [decompression sickness] happens," she says.
