Because overheating can be fatal, humans and other animals find ways to cool off
By Ivan Amato,
It may seem like plain-old common sense to seek cooler air on sweltering summer days. But there’s a complex, carefully orchestrated biological imperative behind that hunt for air conditioning and a chilled drink.
First and foremost, we need to stay cool to stay alive. Thankfully, like all members of the animal kingdom, we have a repertoire of behavioral and physiological tactics to keep from fatally overheating. This enables us to pursue a strategy that might, in Washington-speak, be called a Save the Proteins campaign.
Our body’s thousands of specialized proteins, such as the actin and myosin that make up the molecular gadgetry of our muscles, can do their jobs because of their precise three-dimensional shapes. Warm those proteins just a few notches above 98.6 degrees and the chemical fasteners maintaining these shapes begin to loosen and the proteins can’t function properly. Heat those proteins to, say, 106 or higher, and they can’t hold it together at all. If the proteins in the brain cells that help regulate breathing reach such temperatures, for example, you’ll stop breathing.
Luckily, evolution has come up with some basic, commonsense coping mechanisms. For animals known as Washingtonians, this almost always means seeking air conditioning. (This is common sense as long as you ignore the climate-changing consequences.) Small animals with big enough tails, such as the Cape ground squirrel of southern Africa, can sometimes use their rear appendage like a parasol. A standard hippo option is to bask in cool river water.
But in places where it’s boiling hot in the shade for days, months or even longer, some resourceful animals resort to estivation, the hot-climate version of hibernation. One of the most extreme estivators is the lungfish, according to Gregory Florant, a zoologist at Colorado State University in Fort Collins.
Several species of African lungfish, eellike creatures with lungs that enable them to breathe air when there isn’t enough oxygenated water around, live in habitats where heat can be scorching and moisture can be minimal even for years. “Lungfish burrow in the mud, secrete a crusty mucus cocoon around themselves and drastically lower their metabolism until conditions are better for them to move around,” Florant explains. Gross and boring, yes, but effective.
Moving into shade, migrating to more temperate places and basking in the sun are behavioral tactics animals use for maintaining their cool. But they also have an array of physiological tools to manage the heat. The most familiar of these involves taking advantage of the cooling effect of evaporating water — in other words, sweating.
When the body begins to heat up, an ingenious system of temperature sensors informs the thermoregulation centers in the brain that orchestrate the body’s couple of million sweat glands. The sweat that is produced absorbs body heat as it vaporizes and dissipates, thereby keeping your body temperature close to your normal set point.
Horses, humans, donkeys and other beasts of burden are champion sweaters, which is one reason they are so good at exerting themselves for long periods of time. Those basking hippos also are famous for a sweat that is red, and a team of Japanese chemists discovered that the sweat has antibiotic and sunblocking characteristics, which might trump any role it plays in cooling.
Animals with fur generally lack sweat glands or have them in limited distribution, such as on footpads. Still, they can lower their body temperature using a mechanism similar to sweating. When dogs pant, for example, they inhale through their nose into a bony, scroll-like structure that operates, in engineering parlance, as a countercurrent heat exchanger, according to physiologist George S. Bakken of Indiana State University in Terre Haute. There the air flows over moist tissue, causing evaporation. Many animals — among them dogs and elephants — increase the efficiency of the process by panting: A wet tongue and mouth can serve as ideal surfaces for further evaporative cooling.
The brain is particularly sensitive to overheating, which is why some animals such as birds, fish and dogs have a super-efficient type of countercurrent heat exchanger, sometimes called a rete mirabile (miraculous net) in which an enmeshed, double network of small arteries and veins help cool down the brain’s blood supply.
Another physiological tactic for cooling down — as long as the air temperature is below body temperature — is to expand blood vessels near the surface of the body. Known as vasodilation, this works by shunting warm blood from the body’s core to surfaces where it can radiate away. Scientists have long argued that the elephant’s enormous ears serve as heat radiators by way of vasodilation, but some investigators, including Bakken, who expects to make some measurements this summer at the Indianapolis Zoo, remain skeptical and wonder if the ears’ flapping action might cool off the beasts more than any vasodilation does.
When behavioral and physiological cooling measures aren’t enough, the body can turn to biochemical emergency responses. It can, for example, manufacture heat-shock proteins, which are able to rescue other crucial proteins that are overheating and coming undone. It’s a temporary act of desperation, however. Unless a heat-stressed creature can quickly adopt less drastic cooling measures, it will lose its personal Save the Proteins campaign and pay the ultimate price.
Amato is a writer in Silver Spring and host of the DC Science Cafe series at Busboys and Poets.