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Harbor / harbour seal - Phoca vitulina - also known as the common seal at the sandy beach of Düne belonging to Helgoland Island - Germany

Wearable device provides new insights into how seals dive and swim

Image credit: Dieniti | Dreamstime.com

A wearable non-invasive device can be used to investigate blood volume and oxygenation patterns in freely diving marine mammals, providing new insights into mammalian diving physiology.

The study, conducted by J. Chris McKnight of the University of St Andrews and his colleagues, consisted of a wearable non-invasive device for the seals based on near-infrared spectroscopy (NIRS).

McKnight and his colleagues adapted NIRS technology for use on freely diving harbour seals to investigate blood volume and oxygenation patterns specifically in the brain and blubber, using a device called the PortaSeal.

Glued to the animals’ fur - either on their heads to measure cerebral blood, or on the shoulder to monitor peripheral circulation - the PortaSeal device was used to obtain detailed continuous NIRS data from four seals swimming freely in a quasi-natural foraging habitat. These devices were later easily, and painlessly, removed and the data was downloaded.

The results have provided new insights into how voluntarily diving seals distribute blood and manage the oxygen supply to their brains and blubber, yielding important information about the basic physiological patterns associated with diving.

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

The study showed that seals routinely constrict their peripheral blood vessels, accompanied by increased cerebral blood volume, around 15 seconds before submersion.

According to the researchers, these anticipatory adjustments suggest that blood redistribution in seals is under some degree of cognitive control and is not just a reflex response to submersion.

Furthermore, they observed that seals routinely increase cerebral oxygenation (flow of oxygen to the brain) at a consistent time during each dive, despite a lack of access to air.

“It gives a new perspective on the capacity to control the body's fundamental physiological responses,” McKnight added. “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 performing exceptional feats, like diving to 2,000m for two hours with heart rates as low as two beats per minute (bpm) and yet somehow avoid brain trauma.”

Fig 1. Visualisation illustrating the underlying concept of a three-optode-receiver-channel spatially resolved continuous wave near-infrared spectroscopy sensor

Image credit: McKnight JC, Bennett KA, Bronkhorst M, Russell DJF, Balfour S, Milne R, et al. (2019)

In response to submersion in water, mammals show a suite of cardiovascular responses such as reduced heart rate and constriction of peripheral blood vessels.

However, investigating dive-by-dive blood distribution and oxygenation in marine mammals has up to now been limited by a lack of non-invasive technology that can be used in freely diving animals.

To overcome this issue, the researchers hypothesised that NIRS could potentially address this gap in knowledge, providing high-resolution relative measures of oxygenated and deoxygenated haemoglobin (red blood cells) within specific tissues. In turn, this could be used to estimate changes in blood volume.

The authors proposed that the ability to track blood volume and oxygenation in different tissues using NIRS will enable a more accurate understanding of physiological plasticity in diving animals in what is an increasingly disturbed and exploited environment.

From one marine mammal to another, researchers from the University of Zurich (UZH) conducted a study on the survival rate of the iconic Indo-Pacific bottlenose dolphin population in Shark Bay, Western Australia.

As they investigated the effects of an unprecedented marine heatwave in the area in 2011 and its impact on the dolphin population, they found that climate change may have more far-reaching consequences for the conservation of marine mammals than was previously thought.

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