In the tropical forests of southern Costa Rica, a small, semi-aquatic lizard has evolved an unusual survival technique. Water anoles create a bubble over their nostrils to breathe underwater and avoid predators, a behavior that researchers at Binghamton University, State University of New York, have recently confirmed has adaptive significance.
Underwater Escape
When threatened, water anoles dive into streams or ponds, producing an air bubble that sticks to their heads. This bubble allows the lizards to remain submerged for extended periods, often outlasting lurking predators. Lindsey Swierk, the lead researcher, and an assistant research professor of biological sciences, explains that these lizards use the bubble to “pull oxygen” while submerged.
“Anoles are kind of like the chicken nuggets of the forest. Birds eat them, snakes eat them,” said Swiek. “So by jumping in the water, they can escape a lot of their predators, and they remain very still underwater. They’re pretty well camouflaged underwater as well, and they just stay underwater until that danger passes.”
“We know that they can stay underwater at least about 20 minutes, but probably longer,” she added.
Swierk’s team had previously observed this behavior but wasn’t sure if it played a direct role in helping the lizards stay underwater. To test the theory, they conducted an experiment by preventing some lizards from forming the bubbles. When they could not produce bubbles, they surfaced sooner than those in a control group, staying underwater 32% less time on average.
This shows that the bubbles are more than just a byproduct of the lizard’s hydrophobic (water-repelling) skin. Rebreathing the air trapped in the bubble is key to their extended dives.
Previously, a 2021 study by researchers at the University of Toronto found that the anoles periodically inflate the oxygen-rich bubbles that cling to their heads, then draw them back into their nostrils. This is akin to a “rebreathing device”, similar to the one used by divers to extend their time underwater by recycling exhaled air, which contains some unused oxygen.
Besides providing oxygen, the bubbles may extend time underwater by helping remove the carbon dioxide from exhaled air. The greenhouse gas is highly soluble in water. And, since the levels of carbon dioxide in the bubbles are higher than the surrounding water, the gas dissolves into the water rather than being re-inhaled.
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The Next Frontier: Bubble as a ‘Physical Gill’?
Swierk is now exploring whether these bubbles function like a “physical gill.” Some insects are known to draw oxygen from water into a bubble, allowing them to breathe underwater like fish. However, lizards have greater oxygen demands than insects. Future experiments will look at whether the water anoles can extend their dive times by slowly absorbing oxygen into the bubble.
While these lizards may seem ordinary to most, their ability to “scuba dive” is a testament to nature’s ingenuity. Swierk believes that the excitement surrounding this research can inspire further exploration into other little-known animal behaviors.
“I’ve had people talk to me about how much they love scuba diving and freediving, and how they’re interested in how animals might do the same thing,” said Swierk.
“So, there’s a great opportunity to get people excited about science by having this relationship between what they love to do and what’s evolved in nature. Even in animals that seem commonplace — you’re always finding new things.”
The findings appeared in the journal Biology Letters.