Out and Down

Harbor Seal (Phoca vitulina)

A park visitor stopped me to ask whether I’d seen the Harbor Seal. I hadn’t, and it had vanished again. I turned my attention back to checking for birds along the north side. Then out of the corner of my eye I saw the seal’s head, not twenty feet from the rocks. It looked like the seal was looking at me. Quickly I set up my camera and clicked a few shots. As I did so, the seal turned its head sideways and slid quietly under the surface. Not until I got my images home did I see what the seal was doing. Its nostrils were wide open and it was expelling its breath.

Unlike humans, seals exhale deeply before diving. They can empty their lungs of 90 percent of capacity in one breath. Lungs full of air would affect their buoyancy and hinder their diving. They hold much more oxygen in their blood than humans can. They also slow down their heartbeat from about 100 bpm to only 10 bpm or less while underwater. They usually stay under for only a few minutes, but if they want to they can stay under for half an hour and dive to 1500 feet. Once under the surface they shut their nostrils tight. This one has closed its eyes as it submerges, but they can keep their eyes open and enjoy excellent vision underwater.

Exactly how seals and similar air-breathing deep-diving mammals manage the underwater pressures and depth transitions that challenge human divers remains a subject of scientific exploration. A recent review of the research admits that these abilities “defy our current understanding of respiratory physiology and lung mechanics. These animals cope daily with lung compression, alveolar collapse, transient hyperoxia and extreme hypoxia.” Source.

Here’s the last shot, a fraction of a second after the one above, just before it disappeared. Note the flared nostrils.

Harbor Seal (Phoca vitulina)

More about them: AnimalSpot SeaDoc Wikipedia

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