These lizards use bubbles to breathe underwater

Semi-aquatic anoles have a nifty trick for extending their underwater escapades.

Published 13 May 2021, 13:47 BST
rebreathing-lizard
An adult male Anolis aquaticus (water anole) underwater with a partially formed bubble on its snout
Photograph by Lindsey Swierk

Aquatic insects and other invertebrates are well known for carrying bubbles underwater for respiration. Now scientists have found tropical lizards that also “breathe” underwater this way—a first-of-its kind discovery.

New research shows that a dozen species of Caribbean and Latin American anoles, a type of lizard, can exhale air to create large, oxygen-filled bubbles that cling to their head. The anoles were seen periodically inflating the bubbles and then drawing them back in through their noses—so to test if they use these bubbles to breathe, scientists observed the reptiles staying underwater for more than 15 minutes, according to a paper published May 12 in the journal Current Biology.

“We think this is operating like a rebreathing device,” says study first author Christopher Boccia, a doctoral student at Queen’s University in Kingston, Ontario. That device is an apparatus that allows divers to extend time underwater by recycling exhaled air and breathing the previously-unused oxygen in it.

Roger Seymour, a biologist at Australia’s University of Adelaide who wasn’t involved in the paper, suspects this behaviour could help the animals rid their lungs of accumulated carbon dioxide, which could escape into the water over the large surface area of the bubble.

It’s also possible that in well-oxygenated streams, the bubbles could function as a kind of “physical gill” that pulls the gas from the water via diffusion, says Boccia, who performed the research while completing a master’s degree at the University of Toronto.

Diving for cover

Well over 400 anoles species, spanning a wide diversity of colours and sizes, live throughout the tropics. Some of the most unique are the semi-aquatic species, which protect themselves from predators by diving into streams.

Over the course of the study Boccia collected, observed, and released multiple species of these lizards in Mexico, Costa Rica, and Colombia. Often these dappled anoles sleep at the end of branches, ready to wake and leap as necessary to avoid snakes. “That makes them easier to find for humans at night,” says Boccia, whose research was partially funded by the National Geographic Society.

Boccia then observed the anoles’ rebreathing behaviour when they were placed in various containers filled with water. In his experiments, he found that six species could expel and re-inhale large bubbles, also called plastrons, which remain where the lizards can breathe them by clinging to the animals’ water-repelling scales. When Boccia and colleagues measured the oxygen levels in the bubbles, the levels went down over time—an indication that the anoles were consuming the oxygen.

Scientists say it’s also possible that some lizards use their rebreathing technique to forage underwater, for example on small fish.

“No one would have predicted that anole lizards would rebreathe a layer of gas [clinging] to the outside of their skins,” Seymour says. But because the study scientists “followed up on the observation with careful measurements... this makes the study both bizarrely interesting and scientifically rigorous.”

Underwater adaptations

By noting which anoles exhibit this behaviour and whether they are taxonomically related, scientists found that the behaviour has evolved five separate times, independently, in five different lineages of these lizards. Anoles in general are well known for exhibiting convergent evolution—that is, evolving similar features in disparate environments independently, yet ending up looking very similar. It seems there are only so many ways to be a successful anole, Boccia says.

Several types of insects can carry air bubbles with them underwater. They include species of beetles and arachnids, such as diving bell spiders. In some cases, these plastrons do act like gills, Seymour says.

Some insects can survive indefinitely underwater by breathing through these bubbles. However, compared with lizards these insects have significantly lower metabolic rates,  need less oxygen, and are smaller, and the bubbles have a larger surface area to body mass ratio.

By potentially helping the anoles clear carbon dioxide from the air in their body, and possibly even absorbing oxygen from the water, this bubble-rebreathing behaviour likely extends their ability to stay underwater, Boccia says—but to what degree remains unclear. He hopes to study the phenomenon further.

Philip Matthews, a biologist who studies animal respiration at the University of British Columbia and wasn’t involved in the paper, says the behaviour “may be increasing dive time on the order of seconds to a minute. But this could be all a submerged lizard needs to gain an edge.” 

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