Water anoles, semiaquatic lizards that are shorter than a pencil, are highly sought-after snacks among predators in Costa Rican and Panamanian rainforests. Preyed upon by snakes, birds, small mammals and larger lizards, the anoles hide in vegetation along the rocky banks of streams. Faced with danger, a lizard dives into the water and produces a bubble behind its nostrils that allows it to remain submerged for up to 20 minutes, and perhaps much longer.
Lindsey Swierk, a behavioral ecologist at Binghamton University in New York, worked with Luke Mahler and Chris Boccia at the University of Toronto and other researchers to document the scuba-diving prowess of water anoles. But the purpose of their underwater antics was unclear.
Dr. Swierk wasn’t sure whether the adaptation was a function of survival or just a strange side effect of their hydrophobic skin. While water can’t stick to a lizard’s skin, its composition allows air bubbles to form around parts of its body.
But in findings published Wednesday in the journal Biology Letters, Dr. Swierk found that the bubble allowed this little lizard to stay underwater significantly longer than it otherwise could, supporting the hypothesis that it evolved this ability to evade hungry predators.
Using an underwater camera, Dr. Swierk first viewed the bubble appearing and disappearing atop the anole’s snout. The bubble, which forms mostly as a result of air stored in the lizard’s lungs but also from air pockets on its water-repellent skin, centers over the snout so the animal can rebreathe air underwater. “There are these anchor points on the head where the bubbles seem to be attaching,” Dr. Swierk said.
Researchers followed 28 lizards collected from a rainforest in southern Costa Rica. Half were smeared with body lotion (a daily moisturizer from Honest Company, according to the study) so the respiration bubbles wouldn’t stick to their skin. The other half got a splash of water to keep their hydrophobic skin intact and allow for normal bubble formation.
Lizards with intact bubbles dove underwater 32 percent longer than lizards whose bubble production was impaired. When the lizards weren’t able to rebreathe air within the bubble, they couldn’t dive for nearly as long.
Still, lizard scuba diving is a behavior of last resort. It’s not something that the species uses often, said Kurt Schwenk, an evolutionary biologist at the University of Connecticut who was not involved in the study. Water anoles are coldblooded and regulate their temperature based on their environment. When they dive into a cool stream, it takes them time to warm back up.
And all that basking in the sun takes away from other important lizard duties like hunting or territorial protection. Previous research has even shown that male anoles, which are responsible for territory protection, emerge from the water sooner because they can’t risk an internal temperature drop. “Getting cold causes them to slow down and be metabolically handicapped,” Dr. Schwenk said.
While this is the first vertebrate species to produce bubbles for underwater respiration, it’s a behavior that’s found in small aquatic insects like alkali flies and diving beetles, as well as in some arachnids.
The diving bell spider uses a similarly odd mechanism to breathe underwater. It forms an air bubble that becomes trapped in water-repellent hairs around its abdomen, which allows it to live almost entirely underwater, coming up for air only every 24 hours. The spider’s bubble naturally brings in oxygen and expels carbon dioxide. In future research, Dr. Swierk is interested in whether water anoles can do the same thing.
It’s a behavior with clear evolutionary importance, offering an unlikely escape for a species that nearly every animal in the rainforest likes to eat. “The longer you can stay underwater and not surface, the more likely you are to survive until the predator leaves,” Dr. Schwenk said.
This little lizard has shown that in a world where it’s constantly on the menu, a tiny self-formed bubble has become its saving grace. “This is a wonderful example of how evolution works to help animals adapt to all sorts of funky environments and come up with really interesting solutions to problems,” Dr. Swierk said.
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