They’ve been called vampire fingers. White and bloodless, they are my fingers post most dives. Try as I may to stay warm using various practices, my body doesn’t respond well to our temperate ocean water. Regardless of my reptilian response, I’m still considered warm blooded, as are all mammals ” those able to internally regulate body temperature to a limited degree despite temperature swings in the external environment.
Cold-blooded creatures like fish don’t have our internal control and, instead, require surrounding temperatures to stay consistent. But just because the warm-blooded are grouped together, it doesn’t mean all mammals employ the same methods to warm up or cool down. For example, we humans shiver (muscle tissues contract and expand), which creates friction to raise body temperature. To chill down, we perspire. As moisture from body tissues travels to the skin’s surface, it evaporates, and skin temperature drops, cooling the blood in the skin’s dilated blood vessels before it cycles back to the body’s core.
Pinnipeds, like our local sea lion (Zalophus californianus richardsi) and harbor seal (Phoca vitulina), boast a sophisticated counter-current mechanism in their flippers to internally regulate temperature. Here, arteries, which carry warm blood from the heart, are closely surrounded by an elaborate network of veins that carry cooled blood from the extremities back to the heart.
Because the blood vessels are positioned so close to each other and close to the skin’s surface, arteries and veins can influence each other’s temperature. For example, for warming, arterial blood transfers some of its heat to the otherwise cool venous blood to warm it before it is returned to the heart. On the flip side, the arteries dilate after strenuous activity, allowing warm blood from the heart to rush to the veins in the extremities where it is cooled by the skin’s surface, before being returned to the body’s core.
Have you ever seen sea lions floating together on the ocean’s surface with their pectoral flippers raised in the air? They aren’t signaling for help. While resting, this so-called “raft” of sea lions may also be warming or cooling themselves. The sun warms or ocean breezes cool the network of blood vessels, setting the counter-current system in motion. Harbor seals don’t raft, maybe because they have more blubber than sea lions so are less apt to get cold. Plus they have a special ability to keep blood near to their vital organs by reducing blood flow to the flippers. Should a chill set in, seals hug themselves in a behavior called “bottling.” By tightly tucking in their flippers to their body while floating upright like a cork with their head out of the water, they rest and stay warm.
Pinnipeds fully exploit their flipper facilities on land, like dipping them into the ocean if overheating, not unlike us submerging our feet to cool off.
Alternatively, we both may fan ourselves but pinnipeds use their flippers since they typically don’t have access to electric fans. Tossing sand over themselves is another flipper application: wet sand to cool and sun-baked sand for insulating warmth. And if you’ve ever sat on your hands to warm up, you’re in good company because seals do the same with their flippers after positioning themselves on heated sand.
Although pinnipeds are adapted to hold onto warmth and enjoy a body temperature three degrees above ours, they still must dry-dock themselves almost daily to rest and warm up. To see lolling pinnipeds, find a small cadre of sea lions hauled out on rocks in front of the caves by Goldfish Point in the La Jolla Ecological Reserve. Observe harbor seals south of La Jolla Cove at Seal Rock Reserve or on the sandy beach at the Children’s Pool.
Pinnipeds and humans are both successful at managing hot and cold but the system can go haywire. As a human example, I suffer from Raynaud’s phenomenon, a poorly understood malady in which blood vessels constrict in the fingers and toes. Despite my extreme protective gear, which suits other divers just fine, my body incorrectly registers hypothermia ” blood retreats from my extremities and makes haste to my vital organs. I try to compensate by channeling sea lion behavior when back on the surface. After raising my hands to face the sun, the warmth seeps in to thaw my digits. When in Rome “¦
” Judith Lea Garfield, biologist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. www.judith.garfield.org. Questions, comments or suggestions? Email [email protected].