Researchers are still learning about how marine mammals can return from a long deep dive without suffering the “bends.”
Nitrogen, which is a gas at the surface, will dissolve in the blood with increasing pressure. So, as a dolphin or whale makes a deep dive, more and more nitrogen will dissolve in the blood as it passes through the lungs.
A rapid ascent from a prolonged deep dive should bring on what is called the bends, as the nitrogen returns to a gas, forming bubbles in the blood which can cause massive damage to tissues and small blood vessels.
But on a regular basis, some dolphins dive to 1,000 ft or more for many minutes, and other marine mammals dive much deeper and longer. And neither dolphins nor other deep divers seem to get the bends. How come?
Scientists have been struggling to answer this question for decades, and a new study may shed more light on the subject.
A recently published paper (see below) compared bubbles in dolphins who died after stranding, with dolphins who survived after stranding and were subsequently released, and with wild Sarasota Bay dolphins captured and released for a health assessment.
While somewhat complex, the paper offers a nice review of theories of the physiological mechanisms that allow marine mammals to make repeated long dives without physiological consequences.
Authors on the study include SDRP Director Randy Wells.
Dennison, S., M. J. Moore, A. Fahlman, K. Moore, S. Sharp, C. T. Harry, J. Hoppe, M. Niemeyer, B. Lentell, and R. S. Wells. 2011. Bubbles in live-stranded dolphins. Proc. R. Soc. B published online before print October 12, 2011, doi:10.1098/rspb.2011.1754
Bubbles in live-stranded dolphins
S. Dennison, M. J. Moore, A. Fahlman, K. Moore, S. Sharp, C. T. Harry, J. Hoppe, M. Niemeyer, B. Lentell and R. S. Wells
Bubbles in supersaturated tissues and blood occur in beaked whales stranded near sonar exercises, and post-mortem in dolphins bycaught at depth and then hauled to the surface. To evaluate live dolphins for bubbles, liver, kidneys, eyes and blubber–muscle interface of live-stranded and capture-release dolphins were scanned with B-mode ultrasound. Gas was identified in kidneys of 21 of 22 live-stranded dolphins and in the hepatic portal vasculature of 2 of 22. Nine then died or were euthanized and bubble presence corroborated by computer tomography and necropsy, 13 were released of which all but two did not re-strand. Bubbles were not detected in 20 live wild dolphins examined during health assessments in shallow water. Off-gassing of supersaturated blood and tissues was the most probable origin for the gas bubbles. In contrast to marine mammals repeatedly diving in the wild, stranded animals are unable to recompress by diving, and thus may retain bubbles. Since the majority of beached dolphins released did not re-strand it also suggests that minor bubble formation is tolerated and will not lead to clinically significant decompression sickness.
Keywords: stranding; decompression sickness; gas bubbles; diving physiology; marine mammals.