An oppressive heat wave continues to punish the Northeast this week. Temperatures in New York City, Boston, Baltimore, and Washington, D.C., all topped 100 degrees on Tuesday, as residents sagged and perspired. What about our urban animal friends—are pigeons and squirrels getting sweaty, too?
No. Very few animals sweat as prolifically as humans. Pigeons don't perspire at all. Squirrels and rats have a few sweat glands on their feet, but they secrete moisture more to aid in gripping than as a form of thermoregulation. (Horses sweat quite a bit. The equine cooling system is the biggest reason horses are among the few animals that can compete with us in a marathon.) But despite their feathers and fur, the animals of the Northeastern megalopolis aren't likely to drop dead from heat stroke this summer. Pigeons and squirrels have plenty of tricks for keeping cool, and the current heat wave shouldn't threaten their lives.
In the first place, small animals don't have to work very hard at keeping cool. They have a higher surface-to-mass ratio, so they radiate heat more efficiently. Pigeons and squirrels can also tolerate a greater internal temperature range than people. Squirrels maintain about the same core temperature as we do during the day—around 99 degrees—but they can cool off to 95 degrees at night. (Healthy humans generally stay above 97.) A pigeon's core temperature hovers around 108 degrees under normal circumstances—sparrows top 110—so an ordinary heat wave doesn't bother them.
While most animals don't sweat very much, many species rely on the principle behind sweating to keep cool: Moisture on the skin absorbs heat and evaporates, carrying away unwanted warmth. Animals that lack sweat glands just have to find a different way to bring air into contact with moist skin. Many creatures pant, drawing air across their damp tongue. Pigeons employ a variation called gular fluttering, vibrating their throats. Birds and rodents spend a lot of time licking themselves. Squirrels tend to focus the saliva bath on the forearms, where the fur is thin and the blood flow is higher. Some birds even urinate on their legs to evaporate the heat away. When wild animals do die of heat stroke, it's often because a catastrophe separated them from a water source, so they're not hydrated enough for evaporative cooling.
Squirrels also use their tails for thermoregulation. Ambient air chills the blood in the thin tail quickly. In the summer, the squirrel pumps the cooled blood directly back into the torso to lower its internal temperature. In the winter, the circulatory route changes slightly, so that the chilled blood coming back from the tail is heated by the warmer outbound blood before contacting the squirrel's organs. (The tail also serves as a pretty good blanket on a frigid night.) You may see squirrels running around with their tails flipped up over their bodies in the summer. This is because the lighter-colored underside of the tail absorbs less heat from the sun than the darker dorsal side, and it shades their bodies like a parasol.
Small animals engage in a panoply of behaviors to stay cool. Squirrels often abandon their tree cavities in the summer, spending more time in shady, open-air leaf nests. They slow down their activity level—laboratory studies have shown that ground squirrels spend about 10 percent less time running for every degree above 82. Birds turn their bodies to face the sun, which exposes less surface area to the radiant heat.
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Explainer thanks Alan Muchlinski of California State University, Los Angeles; and Michael Steele of Wilkes University.