This may be the only lizard with an oxygen tank

Produced by: Science Popularization China

Author: Su Chengyu

Producer: China Science Expo

Editor's note: In order to decode the latest mysteries of life science, the China Science Popularization Frontier Science Project has launched a series of articles called "New Knowledge of Life" to interpret life phenomena and reveal biological mysteries from a unique perspective. Let us delve into the world of life and explore infinite possibilities.

The reason why Svirk carefully applied moisturizer to the top of a lizard's head dates back to 15 years ago... In 2009, during a field survey in Haiti, evolutionary biologist Luke Mahler and his colleagues accidentally discovered a tree-dwelling lizard moving underwater.

Arboreal Lizard

(Image source: Document 1)

This is a species of Anolis, which lives mainly in tropical and subtropical forests, grasslands, coastlines and even urban areas. This genus has a high species diversity, with more than 400 known species, making it one of the richest genera among vertebrates.

Anolis lyra

(Image source: mahlerlab)

Mahler thought the lizard would surface within a few seconds, but the lizard stayed submerged for much longer than they expected. When it finally surfaced, Mahler and his team were puzzled: How did this tiny reptile stay underwater for so long? He always wanted to figure out: Did this lizard really find a way to "breathe underwater"?

lizard

(Image source: Document 1)

Solving the Mystery

A few years later, as his interest in evolutionary biology grew, Mahler finally met another like-minded scientist, Lindsey Swierk, a professor at the State University of New York. Swierk had also discovered similar lizard behavior in Costa Rica, so the two hit it off and decided to study this mystery together.

In 2017, Mahler's student Chris Boccia traveled to various parts of Central America, including Panama, Costa Rica, Mexico and Colombia, to closely observe lizards in different habitats.

Chris Boccia with his anole lizard

(Image source: University of Toronto official website)

During an observation by a stream, he saw a tree-dwelling lizard lurking in the water. At first it just lay quietly at the bottom of the water, but then Boccia noticed that there was a small bubble of air at the tip of the lizard's nose, tightly attached there. Then, the lizard seemed to suck the air from the bubble back into its nasal cavity, and then continued to stay calmly underwater.

Figure (a) shows the rebreathing bubble located on the dorsal side of the lizard's snout.

Figure (b) shows the bubble located laterally between the eye and nostril.

These diagrams show the location of air bubbles and how lizards exhale and hold them underwater.

(Image source: Document 2)

"I couldn't believe my eyes!" Borgia recalled the scene, still full of excitement, "Could this bubble be its 'oxygen tank'?"

After returning to the laboratory, Borgia carried out detailed research with his mentor Mahler.

They embarked on a massive expedition from the mountains of Colombia to the streams of Costa Rica to the wetlands of Jamaica, using fishing rods and their hands to pluck the tiny lizards from the grass, 32 species of anoles and four other lizards in total, and brought them back to the lab for a special underwater test.

(Image credit: Lindsey Swierk)

The lizards were gently placed in the clear water of the laboratory, and the researchers observed how they dived and floated freely in the water. For each experiment, the researchers waited patiently until the lizards voluntarily surfaced or showed signs of fatigue. In order to protect these little explorers, each lizard was only given a maximum of five trials, with at least fifteen minutes of rest between each trial.

To capture every tiny movement, the entire experiment was videotaped for later analysis. The scientists used a very sensitive oxygen probe, gently placed above the lizard's nostrils, to record the changes in oxygen in the underwater bubbles.

The process of expanding bubbles

(Image source: Document 1)

The video shows that when anole lizards sink into the water, a thin film of air forms on their skin, which acts like a small oxygen tank. The lizards repeatedly exhale and inhale these bubbles to keep their lungs filled with oxygen.

A thin film of air forms on their skin.

(Image credit: Lindsey Swierk)

Scientists define this behavior as "rebreathing," the ability to re-inhale air that has already been exhaled. Data show that 18 of the 32 species of anoles tested exhibited rebreathing behavior in at least one individual. Among them, the rebreathing behavior of semi-aquatic lizards is particularly prominent. In semi-aquatic lizards, the maximum number of rebreathing in a single trial reached more than 5 times, and their diving time was also significantly longer than that of non-aquatic lizards.

Non-anole lizards (such as Basiliscus galerius) do not form an air layer underwater. In these species, exhaled air is lost to the surface as small bubbles that cannot be re-breathed. The red arrow in the image indicates a small bubble of air exhaled from the nostrils, indicating that these lizards have lost the ability to re-breath air.

(Image source: Document 1)

Measurements from the oxygen probe showed that the oxygen partial pressure inside the bubbles gradually decreased during the experiment, demonstrating oxygen consumption by these bubbles during the rebreathing process.

In 2021, another member of the research team, Lindsey Swierk, designed a new experiment to further verify the function of semi-aquatic lizards using rebreathing bubbles. Similarly, they first caught 30 semi-aquatic anole lizards. These lizards were randomly divided into two groups: a normal bubble group and a damaged bubble group.

They simply applied a little water to the heads of the lizards with normal bubbles, while for the lizards with damaged bubbles, the researchers applied a thin layer of moisturizer, which is mainly composed of water-based moisturizers. Its function is to make the lizards' skin lose its original hydrophobicity, so that the lizards can no longer form the air bubble on their heads for rebreathing.

The blue part is the area on the lizard's head where moisturizing lotion was applied

(Image source: Document 2)

The scientists found that the lizards that did not apply moisturizer were able to form a stable air bubble underwater and repeatedly use this air bubble to breathe again and again throughout the dive. In contrast, the lizards in the damaged bubble group could only occasionally form small bubbles near their nostrils, but these bubbles were not effective in helping them to extend their dive time.

The formation of a single back bubble: the time interval between images is 0.2 s (Figs. i to iv) and 0.07 s (Figs. v to vii). The bubble is briefly held after formation and slightly expands (Figs. vii to viii, time interval 2.0 s) before being reabsorbed.

(Image source: Document 2)

Those with normal bubbles were able to stay underwater for an average of 67.5 seconds, while those with damaged bubbles stayed underwater for an average of 32 percent less time. At the same time, the scientists found that males stayed underwater for 20 seconds less than females. This may not sound like much, but in nature, 20 seconds can mean the difference between life and death. A hungry bird might decide that searching for 20 extra seconds isn't worth the effort and would rather go downstream to find better luck.

Do anoles have "physical gills"?

The research does not end here, because scientists have only proved that they use bubbles to consume oxygen underwater, but they do not know the principle. They speculate that the bubble anole lizard may use the same routine as the diving beetle, that is, "physical gills."

So-called "physical gills" are the mechanism by which some aquatic insects breathe using bubbles . Diving beetles and water flies, for example, store air bubbles between hairs on their abdomens or elsewhere on their bodies. When they dive into the water, the air bubbles form a stable film on these hairs.

Physical gill diagram

(Image source: UQ eSpace)

These bubbles not only provide them with oxygen, but also act like gills by exchanging oxygen with the water, extending their stay underwater. Oxygen diffuses from the water into the bubbles, while carbon dioxide diffuses out of the bubbles. These insects have low metabolic demands, so this mechanism can effectively meet their oxygen needs without quickly depleting the oxygen in the bubbles.

Schematic diagram

(Image source: Document 3)

Insects can have air bubbles because their bodies are covered with tiny hairs that hold air in place, forming a stable bubble membrane.

Hair can hold air and form a stable bubble membrane

(Image source: Document 3)

As for anole lizards, they have no hair, but their skin is covered with fine hydrophobic scales and tiny protrusions. These irregular and dense hydrophobic surfaces can prevent water from spreading completely, and bubbles will remain tight due to surface tension, reducing the contact area with water, thus maintaining stability in the water.

In addition, microscopic structures such as tiny protrusions and scales can reduce the contact angle of water, making water droplets on the surface more spherical and enhancing hydrophobicity. This allows the lizard's skin to effectively capture air and prevent bubbles from bursting due to water pressure.

The skin of the anole lizard is somewhat waterproof

(Image credit: Lindsey Swierk)

Why do anole lizards go into the water?

The question is, why do these anole lizards stay underwater for so long for no reason?

Svirk described the anole lizard as "chicken nuggets in the forest." Whether it's birds or snakes, there are guys in the sky and on the ground who like to eat them. Although escaping on the ground is a good way, if there is water nearby, they will dive in without hesitation to reduce the risk of being eaten.

However, diving also has a price: the body temperature can drop by up to 6°C. Reptiles are ectotherms, relying on the external environment to maintain their body temperature. Staying in cold water may cool the body faster, which in turn affects a range of body functions, and muscles may not work well.

In the future, we may be able to learn from the adaptation mechanism of the anole lizard and develop human equipment with the ability to "re-breathe" to help us survive more efficiently in underwater environments. Every in-depth exploration of these biological structures may reveal more unsolved mysteries in nature and help us understand how life survives in various extreme environments in amazing ways.

References:

1.Boccia CK, Swierk L, Ayala-Varela FP, et al. Repeated evolution of underwater rebreathing in diving Anolis lizards[J]. Current Biology, 2021, 31(13): 2947-2954. e4.2.Swierk L. Novel rebreathing adaptation extends dive time in a semi-aquatic lizard[J]. Biology Letters, 2024, 20(9): 20240371.3.Ditsche-Kuru P, Schneider ES,

2.Melskotte JE, et al. Superhydrophobic surfaces of the water bug Notonecta glauca: a model for friction reduction and air retention[J]. Beilstein journal of nanotechnology, 2011, 2(1): 137-144.