Natures wonders: Porpoises control heart rate in dive (Introduction)

by David Turell , Monday, November 21, 2016, 18:37 (2922 days ago) @ dhw

In order to handle deep dives, the porpoise can control heart rates to manage oxygen consumption and the 'bends', nitrogen bubbles on ascent:

https://www.newscientist.com/article/2113451-porpoises-plan-their-dives-and-can-set-the...

"Two captive harbour porpoises called Freja and Sif have helped to reveal that porpoises —and probably all cetaceans — consciously adjust their heart rate to suit the length of a planned dive.

"By doing this, the animals optimise the rate at which they consume oxygen beforehand to match the intended depth and length of their dive.

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"Researchers discovered as far back as 1975 that pinnipeds — such as sea lions — had the ability to consciously control their heart rate.

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“'We saw that the porpoises didn’t lower their heart rates as much during the initial 15 seconds of a 20-second dive as they did during an 80-second dive,” says Elmegaard.
On average, Freja’s and Sif’s heart rates were 15 and 26 per cent lower when they performed the longer dives, compared with shorter ones, suggesting that this would help them optimise oxygen use while swimming down.

"Importantly, these differences occurred in response to a mental cue — the sound that distinguished the shorter and longer dives — demonstrating that they were premeditated. “We concluded that the porpoises have cognitive control of their heart rate by adjusting their dive response in anticipation of the dive duration,” says Elmegaard.

"It’s not clear how they do it yet, but it involves lowering the heart rate and, at the same time, constricting blood vessels. Combined, these decrease heart output and perfusion of oxygen into organs, maintain blood pressure and conserve blood oxygen, essentially redistributing oxygen to the brain and heart, which are the organs most sensitive to lack of oxygen.

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"But alongside optimising blood oxygen levels, brain control of heart rate also probably helps cetaceans avoid a build-up of potentially toxic nitrogen in tissues, which causes decompression sickness. As with human divers, if cetaceans surface too quickly from depth, nitrogen bubbles can build up in tissues instead of being safely dissolved in the blood.

“'The bubbles can cause damage similar to a blood clot if blocking capillaries,” says Elmegaard. “If bubbles form in joints or muscles they can cause massive pain and damage, and in nerve tissue can cause damage potentially leading to paralysis and death.”

“'The findings are clear – that the porpoises control heart rate voluntarily,” says Paul Jepson at the Institute of Zoology in London. “I also agree with the authors that this is likely to apply to other cetacean species like dolphins – not just porpoises.”

"The heart rate would indeed also regulate nitrogen levels in body tissues, he says.

“'If abnormal cognitive physiological control of dive profile occurred, for example due to naval sonar exposure, this could result in radically altered nitrogen gas kinetics and – potentially – increased risk of decompression sickness – as has been suggested for some mass strandings of beaked whales linked to human-made mid-frequency sonars,” he says”."

Comment: It is hard to imagine this system of control occurred by chance, since it is so important to survival in deep dives.