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LatestPhoto
A large male sperm whale in the Maldives. The following photographs show the whale preparing for a deep dive. He begins by taking one last breathe of air.
Photo: Chris Johnson

January 28, 2003
Deep Divers and the Bends
  Real Audio Report
  28k


Log Transcript

This is Genevieve Johnson speaking to you from the Maldives in the Indian Ocean.

Great News! After only 2 days of traveling south from Male, the capitol of the Maldives, we have found sperm whales. Whenever we enter a new ocean area, we never know quite what to expect on our first research leg. The first whale we found here was a single mature male, traveling slowly south and undertaking regular dive cycles lasting over an hour. Because of the calm conditions, we were able to make some excellent acoustic recordings.

The Odyssey crossed the equator on a windless, glassy sea, and so far we have mainly experienced fine weather. Our first encounter with cetaceans in the Indian Ocean's southern hemisphere was a half hour episode with bow-riding, pan-tropical, spotted dolphins. As they glided beneath the calm, mirror-like surface their fluid movements were clearly visible from the bow. Mesmerizing!

A few hours later, we were with sperm whales again, a group of 4 or 5 animals widely dispersed over perhaps a five mile radius and traveling south at 3.5 knots. Most of the time they are foraging at depth, sperm whales emit regular trains of clicks about twice each second. The echolocation clicks of these whales were loud and distinctive.

LatestPhoto
The whale arches its back.
Photo: Chris Johnson

Since finding sperm whales in Maldivian waters, we have noticed that the majority of dive cycles of these whales lasts for over an hour, sometimes up to seventy-five minutes. The duration of the dives, together with the structure of the groups suggests to us that most of the whales we have encountered so far have been mature males, or sub-adult males-not yet quite old enough or brave enough to venture out on their own, but too old to remain with the matriarchal groups that are generally comprised of adult females, juveniles and calves. According to biologists, Jonathan Gordon and Hal Whitehead, - "mature and maturing males usually seem to forage independently, with a coordination of movement over a distance of a few miles." (Gordon & Whitehead, 1992.)

Because they spend almost 80% of their lives at depth, the amount of time we see sperm whales at the surface is comparatively low. As a result, we spend the majority of our time tracking sperm whales by homing in on their sounds. While the dives of adult females tend to last around 45 minutes mature males are capable of staying beneath the surface for more than 90 minutes. Male sperm whales can also dive to astonishing depths. They have been found entangled in submarine telephone cables at depths of 1,000 meters, while a study in the Caribbean Sea using acoustic transponders and sonar revealed whales diving to depths of 1,185 meters and another was possibly to 2,000 meters. (The Smithsonian Answer Book. James Mead & Joy Gold)

People often ask us how these air breathing mammals can dive to such great depths for extended periods of time without suffering from decompression sickness, also known as the 'bends.'

LatestPhoto
The whale raises it flukes signalling the beginning of a long and deep dive.
Photo: Chris Johnson

The 'bends' is a condition suffered by people when they breathe air under pressure and ascend to the surface too fast (for example, when scuba diving). As humans dive deeper, they continue to breathe air at increasing pressure where more nitrogen gets driven by that pressure into the blood stream. Upon a sudden return to the normal atmosphere - i.e., upon surfacing too fast, the nitrogen that has saturated the blood, comes rapidly out of solution and forms bubbles. These bubbles accumulate in joints, in the blood, the brain and other parts of the nervous system, resulting in a blockage which reduces the oxygenated blood supply-something that is usually severely painful and can cause death. To avoid the 'bends', a human diver must return to the surface slowly, stopping at specific depths to breathe for specific times in order to allow the body time to readjust, and for any nitrogen bubbles to dissipate.

Whales are not subject to the 'bends' because they don't breathe air under pressure the way a human SCUBA diver does when diving. Whales breathe at the surface only, then hold their breath and dive. It seems likely that they don't carry enough air down to have the 79% of it that is nitrogen cause them problems. And because they are able to avoid too much excess nitrogen in the blood, they don't get the 'bends'.

Because atmospheric pressure doubles every 10 meters, a whale at a depth of 500 meters experiences a pressure about fifty times greater than atmospheric pressure at the surface. Once it has dived to about 200 meters - the whale has far exceeded the depth at which humans can breathe air and still function properly. The air the whale dove with has been compressed to 25% of its original volume and the lungs and jointed rib cage have collapsed completely. The heart rate has dropped dramatically and non-essential processes have been much diminished because freshly oxygenated blood supply to peripheral regions has been cut off. Because of the compression and collapse of the cavities where gas-exchange takes place, air is excluded from most of the respiratory surface of the lungs, meaning that no gas exchange can occur between the blood and air in the lungs. This prevents absorption of the nitrogen that might otherwise pass into the blood and lead to the bends when the whale surfaces for its next breath.

As our day drew to an end, we once again detected the faint but unmistakable clicks of a distant sperm whale. And as everyone prepared for the onset of darkness, switching on navigation lights, cooking dinner, turning on the radar and beginning the schedule of night watches, the clicks grew in number and intensity. One gets the impression that unlike humans, for a sperm whale there is little recognition of the differences between night and day; their foraging activities continue, unperturbed by the falling sun and the rising moon. But as for uswe will have to wait until sunrise before we will see our companions once again.

LatestPhoto
Pan Tropical Spotted Dolphin.
Photo: Chris Johnson

References:

  • Whales and Dolphins in Question - The Smithsonian Answer Book.
    James G. Mead and Joy P. Gold. Smithsonian Institution Press, 2002. (p.47)
  • Cetacean Societies. Field Studies of Dolphins and Whales.
    Mann, Conner, Tyack and Whitehead. The University of Chicago Press, 2000.
    (p.154-172)

Links:

Written by Genevieve Johnson

 
 
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