Scientists have found clues in the lungs of marine mammals that hint at why the creatures are able to traverse the depths of the ocean without getting the bends.
Their new study also suggests a potential mechanism underpinning some of the whale-stranding events that have been linked with navy sonar exercises.
Some marine mammal species – including sperm whales and Cuvier’s beaked whales – are capable of diving up to 1,500m on a daily basis to feed.
Despite this, the animals seldom suffer from the affliction that has long plagued human divers.
Also known as decompression sickness, the bends is a condition that arises when divers ascend too quickly from the depths of the ocean. Dissolved nitrogen gas then enters the skin, joints and brain, and the results can be fatal.
Now, researchers have analysed the anatomy of marine mammals in an effort to understand how they avoid the bends, and have come up with a potential explanation.
"How some marine mammals and turtles can repeatedly dive as deep and as long as they do has perplexed scientists for a very long time," said Dr Michael Moore, director of the Marine Mammal Center at Woods Hole Oceanographic Institution and co-author of the study.
"This paper opens a window through which we can take a new perspective on the question."
Historically, researchers assumed the chest structure of marine mammals meant their lungs compressed automatically at great depths, an adaptation that prevented them from taking up excess nitrogen and getting the bends.
However, mass strandings of whales in places like the Canary Islands forced scientists back to the drawing board, as autopsies revealed some of the creatures had gas bubbles in their tissues that suggested decompression sickness.
The discoveries suggested that while rare, it was actually possible for the animals to get the bends. Moreover, scientists suspected that naval activity in the area was somehow involved in this process.
To try and resolve this mystery, Dr Moore and his colleagues analysed the available evidence in a paper published in Proceedings of the Royal Society B.
They also conducted their own experiments, making CT scans of a dead dolphin, seal and pig while they were being pressurised in a hyperbaric chamber.
What they observed was a distinction between the marine and land animals’ lungs.
In the marine mammals, their lung architecture leads to the formation of two distinct regions under pressure – one air-filled and the other collapsed.
The researchers suggested that when they are diving, blood flowed primarily through the collapsed part of the animals’ lungs, minimising the exchange of nitrogen.
According to the study’s lead author, Dr Daniel García-Parraga of the Fundacion Oceanografic, this mechanism could prevent marine animals from taking up excess levels of nitrogen and therefore minimise risk of the bends.
This discovery also hinted at why human activity – specifically noises made by navy sonar exercises – could lead to animals suffering from decompression sickness.
"Excessive stress, as may occur during exposure to human-made sound, may cause the system to fail and increase blood to flow to the air-filled regions,” said Dr García-Parraga.
“This would enhance gas exchange, and nitrogen would increase in the blood and tissues as the pressure decreases during ascent."
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