Why do animals love to bask in the sun? (Hint: It's not just to stay warm.)

Every summer, beaches around the world fill up with sprawled-out sunbathers, but humans aren't the only ones who make it a point to soak in the sun. Across the animal kingdom, creatures big and small meet biological needs by basking in the rays of Earth's closest star.

This includes familiar sun-lovers like lizards and other reptiles, as well as countless species less well known for their sunbathing habits, from frogs to monarch butterflies to hippos. Even birds like a good sun session: Avians from at least 50 families periodically perch or fly to the ground, stretch out their wings, and bake themselves like beach bums working on a tan.

(These delightful pictures show how animals enjoy sunbathing.)

Scientists continue to unravel clues about why animals actively lounge in the sun, a time-consuming practice that can leave some vulnerable to predation. Here's what's known about this intriguing behavior.

Maintaining temperature

Many creatures bask in the sun to control their body temperature, a process known as thermoregulation.

That's the motive for many cold-blooded ectotherms, such as reptiles and amphibians, many insects, and at least two fish: carp and ocean sunfish, an enormous silvery swimmer that researchers found spends nearly half its time near the water's surface.

Unlike endothermic mammals and birds, ectothermic critters can't maintain a stable body temperature through the heat created by their metabolism. So their temperature fluctuates with their surroundings.

(Meet the animals that survive extreme desert conditions.)

When an ectotherm's environment cools down, so does its temperature. This slows the body's chemical reactions controlling everything from immune function to muscle performance—which might work for a sleeping or resting animal but not for one that needs to do things like hunt prey and scuttle away from predators.

To jumpstart their bodies, ectotherms seek out heat. Some might crawl to a hot rock or swim to toastier water, for example, while others bask in the sun for periods of time that depend on their needs, size, and how well their surface color absorbs sunlight, says Tracy Langkilde, an evolutionary biologist at Penn State University who studies reptiles and amphibians. "As temperature increases, the rate of all these processes will usually speed up," she says. "It's critical for their survival."

Saving energy

The same might be said for some endothermic sunbathers. Although these animals can generate heat internally, thanks to their speedy metabolism, some sunbathe so that their metabolism doesn't have to do all the work.

That's what scientists concluded triggers the sun-seeking habits of ring-tailed lemurs and roadrunners, for example, as well as the Alpine ibex, a type of wild goat that catches rays to conserve energy on frigid mountain mornings in winter, when there's little grass to fuel their bodies.

Also using the strategy: the fat-tailed dunnart, a tiny marsupial found in Australian deserts. They're one of few mammals known to bathe in the sun's warm glow when arousing from torpor, a temporary state in which their metabolism and temperature drops to conserve energy when food runs low. In one study, researchers found that dunnarts that enter torpor and then soak in sunshine afterward can survive on about a quarter of the daily food and water they'd normally need.

A virus buster and vitamin booster

While animals often roast themselves for basic day-to-day functioning, research suggests they also seek sunshine to treat specific health issues.

For instance, there's growing evidence that birds sunbathe to fry parasites hiding amid their feathers. This theory first gained traction in 1993, when researchers observed a group of violet-green swallows that spent more time sunning than other swallows that had been treated for mite and louse infestations. More recent studies have shown that lice on feathers can die when exposed to short blasts of sunlight, further bolstering the pest control hunch.

Similarly, ectotherms may seek sunshine to destroy intruders such as viruses and bacteria. Although this is difficult to test in the field—"as soon as you go up to an animal, they run away," Langkilde says—lab studies examining pathogen-infected green tree frogs, house flies, and other critters suggest that they voluntarily heat themselves beyond their preferred level, inducing a so-called "behavioral fever." This remarkable adaptation has been shown to heal some study subjects, presumably in the same way a natural fever kickstarts the immune system. 

One species—the Western box elder bug—appears to stretch out in the sun to avoid an infection altogether. When this winged insect sunbathes, it triggers a chemical compound that encases fungal spores on its body, providing protection from germs.

Meanwhile, panther chameleons seek sunshine for something it can add to their bodies: vitamin D. When these animals haven't absorbed enough of the vitamin from their diets, research shows they compensate by exposing their vibrant scales to the sun's UV rays, enabling vitamin production. Scientists don't fully understand how the chameleons know to do this—or whether other animals do the same—but they suspect a vitamin D receptor in the animal's brain tips it off to the deficiency.

Burning questions about basking

Despite the progress scientists have made to understand sunbathing animals, there's still a lot to untangle.

The motives of freshwater turtles have been of particular interest, given that nocturnal ones assume basking-like positions at night and one study found that some of them don't raise their body temperature while sunning, ruling out thermoregulation as a rationale. In 2021, another paper nixed leech removal as a likely possibility for their behavior, leaving the question unanswered.

Also unknown: exactly how sunbathing helps birds kill pests, and the full scope of benefits that sunshine provides our furry counterparts.

"Basking has not been studied well [in mammals], particularly in large mammals," Thomas Ruf, a professor of animal physiology at the University of Veterinary Medicine in Vienna, Austria, writes in an email. He suspects it has to do with the challenge of having to analyze them in the field, something he and his colleagues did to study Alpine ibex.

Yet researchers note that understanding this practice—in mammals, as well as in sun-lovers in general—could do everything from improving the care of creatures in captivity to aiding in conservation efforts for wild animals. Future studies will therefore have important implications aside from just unraveling a quirky behavior.

Fish and Amphibians