A sub-zero, wind-chilled evening has now enveloped a near-frozen body of freshwater. You, the endothermic (warm-blooded) mammal, find yourself about to take the “polar bear plunge” into that ice littered body of water, egged-on by the louder-than-life chants of your fellow thrill-seekers. You begin breathing deeply, you’re exhalations condense in clouds of courage―you jump. Instantly, every muscle-laden sinew contracts from thermal shock. But your conscious, homeostatic mind’s screaming: “Find dry land, get warm.”
Stoke by numbing stroke, you locate that outlet of escape at the waterhole’s edge, pulling yourself up-and-out of the temperate condition. Still, in a delusional state of mind, your fellow “jumpers” cloak you in a thermal blanket and parade about your courageous endeavor; it’s a paradoxical situation, somewhere between bravado and idiocy. You’re grinning―and warming-up. At this point in time, you’re more aware than ever before that you’re human, and endothermic being; it’s a sublime anatomical quirk. And that, ladies and gentlemen, is homeothermy—thermoregulation that allows an organism to stabilize its own body temperature in lieu of external thermal assistances. And, if you’re an aquatic organism—let’s say a fish, for example—this is a biological blessing.
Never Frozen in Time
Generally speaking, most Artic fish can solely rely on the ice structuring proteins (ISPS) found in their circulatory system. These peptides, quite literally, act as a naturally occurring antifreeze, allowing these aquatic “polar bear club” enthusiasts to live-and-thrive in those said conditions. However, there’s a “catch”—fish that rely primarily on this protein are lethargic, plankton-feeding organisms.
Waste Not, Need Not
But for the active, quick-witted salmon shark (Lamna ditropis) that inhabits those same arctic waters, those protein structures just won’t cut it for their predatory habits—they’re noted as the “miniature Jaws” of the northern Pacific. This vivacious mackerel shark has a trick up its dorsal fin—vascular co-current heat exchange. I know, it’s a mouthful of scientific jargon but bear with me. Allow me to make that more approachable: in essence, they’re able to retain any metabolically created heat—for example, the heat generated by kinetic muscle movements—that is later recycled from the animal’s dorsal aorta and on into the surrounding vascular tissue. Metaphorically speaking, this method of homeothermy is akin to an electrical blanket. The electrical heat source is generated from the electrical supplied outlet where it’s then radiated throughout the cotton-blanketed sheath. Nifty, isn’t it?
Salmon sharks aren’t the only animal to practice this method of internal heat exchange. Bluefin tuna, another highly predatory fish, have been documented to utilize vascular co-current heat exchange. See the food-webbed pattern here—predation requires higher-levels of athleticism, and therefore homeostatic warmer bodies.
Now fully clothed and muscularly functional, you begin to understand the gravity of the feet of elemental bravery you just achieved. Your fingertips regain sensitivity, cementing your endothermic being. What a gift, an anatomical advantage. Then, out of the corner of your slightly blurred vision, you witness an ice fisherman reel-in an artic cod; a metaphorical image that, for some reason, lingers. And just as you’re about to load-into a heat-seated automobile, an epiphinal inner dialogue finds its voice. “Life will always find a way to thrive, warm-blooded or otherwise. Adaptation is the key to thriving and surviving.”