The underwater world is alien to most land animals, a place we only visit briefly before our breath runs out. A few terrestrial animals extend their stay by bringing air with them, such as diving beetles that smuggle bubbles under their wings, or human divers with compressed air tanks. The newest member of the terrestrial SCUBA club are several species of semi-aquatic Anolis lizards, as described by Christopher Boccia, a researcher from the University of Toronto, Canada, and more than a dozen colleagues in their recent study.
Spurred on by their own natural history observations of tropical anoles serenely resting at the bottom of streams, the authors rounded up 20 species of anoles (including five considered to be semi-aquatic) at field sites across the Americas and the Caribbean. Using buckets, aquaria or even kiddie pools, they let the anoles dive voluntarily or gently submerged them, paying very close attention to what happened next.
When submerged, the anoles took on a silvery sheen, caused by a thin layer of air (a ‘plastron’) forming between the water and their skin. The quicksilver-like coating also invariably connected to a large air bubble around their head to their nostrils. Most species at least occasionally inhaled and exhaled – effectively re-breathing – the bubble while submerged. And when the team compared how many times the different species resorted to re-breathing, the semi-aquatic anoles re-breathed more often and for longer stretches than their non-aquatic cousins. They also stayed underwater longer, with the champion diver of the group staying under for 18 min. Perhaps, the authors mused, the exaggerated re-breathing response in semi-aquatic anoles was an adaptation to their waterfront lifestyle.
But that's a hypothesis that only holds if re-breathing serves a physiological function, such as providing oxygen for respiration. To test this idea, the researchers started jabbing an oxygen-measuring probe into the bubbles, looking for evidence that the anoles sucked oxygen out of their bubbles during their dives. They found that the oxygen levels decreased over time in the bubbles and larger anoles consumed oxygen more slowly than smaller ones, which is exactly what they expected from respiring lizards on land. It seems that diving anoles are probably re-breathing the air they take down with them from the surface.
There are a few ways that breathing and re-breathing the same bubble of air could help anoles make longer dives. The first is that re-breathing mixed the air inside the anole's body, helping them to extract as much oxygen as possible from the air already in their lungs and throat. The bubble may also help anoles dump carbon dioxide into the water around them, preventing the waste from building up in their bodies. Finally, and most intriguing, the anole might take a page from the book of diving beetles and use bubbles like a physical gill. As the oxygen in the bubble declines, more oxygen is pulled from the water into the bubble, which could allow these air breathers to supplement their breath holding with water breathing!
The key to the anole's bubble breathing strategy is probably their waxy skin and the plastron that forms around it, connecting the bubble with their internal respiratory system via the bubble around their nostrils. All anoles get a silvery coating when dunked in water, regardless of their habitat preferences, and so their peculiar skin likely evolved for something other than SCUBA diving, such as keeping clean or dry, and semi-aquatic anoles then co-opted it to create the plumbing of their bespoke underwater breathing apparatus. Anoles may dive deep to keep away from predators or find food, but the trick they use to do it is only skin deep.