The Lizard That Breathes Underwater with a Recycled Bubble

A brown lizard perches on a wet boulder above a fast-flowing stream in southern Costa Rica. When a predator approaches—a bird, a snake, or an approaching researcher—the animal does not dart into the forest canopy. It drops directly into the water, sinks to the rocky stream bed, and stays there.

Lindsey Swierk, an assistant research professor of biological sciences at Binghamton University, recorded Anolis aquaticus remaining submerged for more than sixteen minutes in field observations in 2019. Land-dwelling reptiles rely on atmospheric air, yet this creature manages to hide beneath running currents until terrestrial threats pass.

The mechanism is visible on the lizard’s snout. Upon diving, an air bubble expands over its nostrils, collapses as the lizard inhales, and expands again upon exhalation. It functions like an external recycling system, pulsing against the stream flow in rhythmic cycles.

The Hydrophobic Layer

Air-breathing land vertebrates do not typically hold air on their skin. Water anoles manage it because their skin microarchitecture is highly hydrophobic. Microscopic, water-repellent scales trap a thin sheet of air—a plastron—across the head and body when the lizard submerges.

When the anole exhales underwater, air leaves the nostrils and merges directly with this surface layer. Instead of floating free and rising to the surface, the exhaled breath expands into a single, cohesive bubble held in place by the hydrophobic skin around the snout. When the lizard inhales, it draws the pocket back into its lungs.

In a 2021 study led by Christopher Boccia and published in Current Biology, researchers measured the oxygen partial pressure inside these rebreathed bubbles over time. They confirmed that oxygen levels steadily dropped throughout the duration of a dive. The lizards were actively harvesting oxygen from their exhaled air pockets.

Rebreathing allows the animal to clear dead-space air—the residual volume left in the trachea and mouth cavity after exhaling. By cycling this air through the external bubble, the lizard can repeatedly draw usable oxygen back into its respiratory tract.

Testing the Mechanism

To confirm whether this rebreathing bubble actually extends survival time underwater, Swierk designed an experiment published in Biology Letters in 2024.

She applied a commercial emollient to the snouts of one group of water anoles. The emollient disrupted the skin's hydrophobic properties without harming the animals, preventing the air from clinging tightly to the scales and stopping the formation of a stable bubble. A control group of anoles was left untreated to dive normally.

The control group remained submerged an average of 32 percent longer than the lizards with impaired bubble formation. Without the ability to re-inhale their external air pocket, their dive durations dropped sharply. The bubble was not a passive side effect of hydrophobic skin; it actively prolonged their underwater refuge.

The Physical Gill

The bubble does more than just store exhaled air. It acts as a physical gill.

Insects like aquatic beetles and diving spiders have long been known to use physical gills. They carry air bubbles underwater, where gas exchange occurs passively across the boundary layer between the bubble and the surrounding water. If dissolved oxygen in the water exceeds the oxygen level inside the bubble, oxygen diffuses into the air pocket. Carbon dioxide, being highly soluble in water, diffuses outward.

In a 2025 study in the Journal of Experimental Biology, Swierk and her co-authors tested whether Anolis aquaticus derives the same physiological benefit. They submerged lizards in water tanks with varying concentrations of dissolved oxygen.

When dissolved oxygen levels in the water were elevated, the anoles stayed submerged significantly longer. The oxygen gradient between the cold, oxygenated stream water and the depleted air inside the bubble forced oxygen to cross the bubble's outer membrane, replenishing the air before the lizard drew it back into its lungs.

It is the first demonstrated case of a vertebrate utilizing a physical gill.

Measured.

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