Heart rate controllers Solvin Zankl/Alamy Stock Photo

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.

“Until now, we knew that the heart rates of porpoises and cetaceans in general correlate with different dive factors, such as dive duration, depth and exercise,” says Siri Elmegaard of Aarhus University in Denmark, who led the research. “Now we can conclude that harbour porpoises have cognitive control of their heart rate.”


The discovery might also provide another explanation for how exposure to loud noise from shipping, sonar or subsea exploration harms cetaceans and possibly triggers strandings.

If their concentration is disrupted by sudden loud noise, it could prompt animals to panic and resurface too quickly, triggering potentially fatal decompression sickness, pain or confusion, and leading to possible stranding.

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

“What is remarkable is that it has taken four decades since the pioneering work on the cognitive control of heart rate in sea lions for researchers to ask the same question for cetaceans,” says Terrie Williams of the University of California, Santa Cruz. “This latest discovery is indeed an exciting advance in our understanding of the dive response in whales and dolphins.”

Cognitive control

Elmegaard and her colleagues discovered the cognitive control phenomenon after fitting Freja and Sif with equipment to measure their heart rates for the first 15 seconds of dives at the Fjord & Bælt centre in Kerteminde, Denmark. They taught the pair to perform dives that lasted either 20 or 80 seconds, then familiarised them with a sound cue associated only with the 20-second dives.

“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.

“This trait of cognitive heart rate control is likely to be shared by all cetaceans because it makes a great difference to dive ability, survival and fitness,” says Elmegaard.

She says previous research showed that seals diving with an initially low heart rate made oxygen in the blood last twice as long than if heart rate declined gradually during the dive.

Decompression sickness

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.”

Given the newly discovered importance of cognitive control of heart rate — and therefore also of oxygen and nitrogen levels in the body — Elmegaard warns that cetaceans may panic and relinquish that control if they are startled by loud, unexpected noise underwater, something the team plans to investigate further.

“It would be interesting to see how some sounds affect the cardiovascular response of the trained porpoises in a controlled setting,” she says.

“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”.

Journal reference: Current Biology, DOI: 10.1016/j.cub.2016.10.020