Seals use thought to prepare body for diving (A juvenile harbour seal © Monica Arso Civil, Sea Mammal Research Unit)

Seals use thought to prepare body for diving

Discovery that seals can "seemingly actively exert control over their circulatory systems is really exciting.”

When crews in submarine films hear the ‘Prepare to dive’ command, they jump into action to get the vessel ready to submerge. A new study, led by Dr J Chris McKnight of the University of St Andrews, suggests seals use an equivalent thought to prompt the physiological changes needed to help them stay underwater for so long.

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Dr McKnight’s team came to this conclusion after studying a group of harbour seals (Phoca vitulina) wearing non-invasive near-infrared spectroscopy devices, which monitor blood circulation patterns. The devices, dubbed ‘PortaSeal’, were attached to the seals’ heads and shoulders to track the changes occurring in their bodies while swimming and diving in a quasi-natural foraging habitat.

“Discovering that seals, which are physiologically fascinating animals, can seemingly actively exert control over their circulatory systems is really exciting,” says Dr McKnight.

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It’s already well known that mammals have a range of automatic cardiovascular responses to being submerged in water, including a reduced heart rate and constriction of the peripheral blood vessels in their limbs and extremities – a process referred to as the mammalian diving reflex. But we’ve been unable to see the full extent of changes that marine mammals, such as seals, undergo before and during a dive because we couldn’t see what’s happening in their bodies while they’re alive and swimming.

Dr McKnight and his colleagues wondered if near-infrared spectroscopy (NIRS), which is used to monitor blood flow and oxygenation levels in brains, might provide a solution to this problem.

NIRS involves shining light that’s almost infrared (with wavelengths between 800-2,500 nanometers) into a patient’s head and then examining the spectrum of light that’s reflected back. Since different molecular bonds absorb different wavelengths of light, the range of reflected wavelengths provides clues as to the amount of blood passing through vessels in the patient’s brain and surrounding tissues, and its chemical composition.

Dr McKnight’s team used an adapted version of NIRS technology, which also has applications in astronomical and agricultural imaging, to provide insights into how a group of four harbour seals distribute blood and manage their oxygen supply while diving.

Intriguingly, the results showed that seals routinely constrict their peripheral blood vessels and increased their cerebral blood volume approximately 15 seconds before submersion. These anticipatory adjustments suggest that blood redistribution in seals is under some degree of cognitive control and is not simply a reflex response to submersion. Seals also routinely increase their cerebral oxygenation at a consistent time during each dive, despite a lack of access to air.

“[This] gives a new perspective on the capacity to control the body’s fundamental physiological responses,” says Dr McKnight. “Getting this insight with non-invasive wearable technology from the bio-medical field offers many exciting future research avenues. We can start to study organs, like the brain, of seals in the open ocean where they can perform exceptional feats such as diving to 2,000m for two hours, with heart rates as low as two beats per minute, and yet somehow manage to avoid brain trauma.”


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