Repeated Evolution of Underwater Rebreathing in Diving Anolis Lizards

Semi-aquatic Anolis lizards have some of the most fascinating ecologies, colour patterns, and behavioural strategies in the genus (though I may be biased). Twelve of these neotropical streamside specialists are distributed across much of mainland Latin America and on the two largest islands of the Caribbean. All are rarely found more than a few meters from a stream and some have been observed to consume semi-aquatic prey (or, in the case of A. vermiculatus, even small fish and freshwater crustaceans).

A riparian lifestyle is also responsible for the signature move that unites all species of semi-aquatics—escape dives! As anyone who has encountered one of these lizards in the wild can attest, semi-aquatics will readily dive underwater when approached. They can stay down for awhile too—up to 18 minutes by my count (Mexico’s A. barkeri currently holds the record). Diving anoles have attracted the attention of tropical biologists for more than half a century now (e.g., Robinson 1962; Brandon et al. 1966; Campbell 1973; González Bermúdez and Rodríguez-Schettino 1982; Birt et al. 2001; Leal et al. 2002; Henderson and Powell 2009; Muñoz et al. 2015; Herrmann 2017) and this work has begun to fill out our natural history knowledge of these enigmatic lizards. However, understandably, most work to date has focused on what these lizards are doing when they’re not in the water. And, as it turns out, there’s a lot to learn if we look below the surface…

In 2009, while studying Anolis eugenegrahami, an endangered semi-aquatic anole from Haiti, Luke Mahler and Rich Glor noticed that an individual they had just released into a clear, shallow stream proceeded to repeatedly exhale and re-inhale an air bubble as it clung to the rocky bottom. Luke and Rich had to move to their next site later that day, so weren’t able to learn more. Sadly, a follow-up field season was cancelled in the aftermath of the 2010 Haiti earthquake.

Years later, when I started my MSc thesis on aquatic anoles in at the University of Toronto, Luke shared this observation with me. When an anole does something once, another anole somewhere else usually does it convergently, so we couldn’t help but wonder whether aquatic anole species elsewhere also exhibited this apparent “rebreathing” behavior. So, when I was planning my first field season in Costa Rica, on a hunch, we purchased an oxygen microsensor, and I set out to establish whether this intriguing behaviour occurred in any other semi-aquatic anoles.

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The aquatic anoles did not disappoint! During my Master’s, along with an amazing team of colleagues, I visited stream habitats in Costa Rica, Colombia, and Mexico, studying A. oxylophus, A. aquaticus, A. maculigula, and A. barkeri along with the non-aquatic anoles we were able to find at each site. I found that each of these species routinely performed the same behaviour that Luke and Rich had observed in A. eugenegrahami! We named this phenomenon “rebreathing” after the SCUBA apparatus. All of the semi-aquatics we observed performed rebreathing extensively during experimental submersions and are from five phylogenetically distinct lineages, showing a pattern of remarkable behavioural convergence!

As I was conducting these experiments, “rebreathing” was independently discovered in Anolis aquaticus by Lindsey Swierk (see image below, and Lindsey’s 2018 AA post). Lindsey is the world authority on Costa Rica’s diving anoles, and has reams of firsthand knowledge about their ecology and behavior. So we did the obvious thing when we found out about her observation – we invited her to join our project. We managed to deliver our oxygen sensor to Lindsey in Costa Rica via a colleague with overlapping travel plans, and she helped fill out our oxygen use data set for the Costa Rican diving anole species. In addition, Luke tested Anolis lynchi in Ecuador, and various non-aquatic species during fieldwork there and elsewhere (Dominican Republic, Jamaica) to help round out the data set.

Speaking of non-aquatic anoles, what role do they play in this story? An interesting one, as it turns out. Rebreathing clearly seemed fascinating, but one possibility was that it was relatively ubiquitous and that all anoles would rebreathe if you submerged them. To find out, we did just that, carefully dunking aquatic and non-aquatic anoles alike in aquaria or buckets at our field sites.

What we discovered is that most non-aquatic anole species are indeed capable of basic rebreathing, but for the most part, they don’t rebreathe anything like the semi-aquatics do. If they rebreathed at all, non-aquatic species tended to do so only occasionally and irregularly (usually only one or a few re-inhalations). Since semi-aquatic anoles performed rebreathing extensively and consistently, while non-aquatics were capable of the basic components of rebreathing, but did not rebreathe regularly, we think consistent rebreathing may have evolved when natural selection found a new utility for a trait that all anoles possess—hydrophobic skin. The hydrophobicity of anoles’ scales is likely what enables the air bubble to adhere to the diving anoles’ heads (and thereby also enables re-inhalation).  All anoles therefore appear to be capable of forming a thin layer (or ‘plastron’) of air along their scales during submersion, but only semi-aquatics appear to make regular use of this ability (see plot below). Hydrophobic skin evolved in anoles long before it was co-opted for rebreathing in stream-dwelling species, and likely had nothing to do with the use of aquatic habitats. In this way, the innovation of underwater rebreathing apparently owes its origins to a fortuitous ‘evolutionary accident.’

Although we observed regular rebreathing in all aquatic anole species we studied, we discovered some interesting differences in the way they go about it. There were three main locations along the head to which diving anoles would exhale bubbles (see image below). We noted some variation in the bubble positions used by semi-aquatics, perhaps indicating that are multiple ways to achieve the same rebreathing function.

To determine if ‘rebreathing’ was truly involved in respiration, we used our oxygen sensor to measure the oxygen concentration of the bubbles produced by diving semi-aquatics. This is not as easy as it sounds; bubbles were frequently re-inhaled quickly and diving anoles do not take kindly to being accidentally poked in the nose with a probe. But we persevered, and found that bubble oxygen levels decreased through time, consistent with the respiration hypothesis!

We found some evidence that oxygen decrease followed an exponential decline curve, suggesting either that anoles extract some additional oxygen from the surrounding water by rebreathing (thus slowing the rate of oxygen loss from the bubble), or that metabolic rate (and thus oxygen demand) drops over time during submersion (see figure below). We compared our results to diving insects that use a similar rebreathing apparatus while submerged and found that anole oxygen use matches up well with our expectations for their sizes, and that the metabolic rate of anoles is probably too high for them to remain underwater indefinitely using oxygen captured from the water by the rebreathing bubble (the same is true for the largest diving insects).

The consistency with which unrelated semi-aquatic anoles rebreathed suggests that rebreathing is adaptive for semi-aquatic living; however, our data currently do not allow us to favour a particular physiological functionality for this behaviour. Our top three (not mutually exclusive) hypotheses are: 1) rebreathing allows anoles to access air trapped in their head cavities or within the plastron, which might otherwise not be incorporated into their air supply; 2) the rebreathing bubble functions as a physical gill (as has been observed in diving insects), allowing diving semi-aquatics to extract some oxygen from the surrounding water; and 3) bubble exhalation and re-inhalation allows anoles to remove excess carbon dioxide which builds up during dives. We hope to investigate these possibilities during future work!

We published this work in Current Biology (Boccia et al., Repeated evolution of underwater rebreathing in diving Anolis lizards, Current Biology (2021), https://doi.org/10.1016/j.cub.2021.04.040)

See also coverage from National Geographic, the University of Toronto, and Binghamton University. Special thanks to Day’s Edge Productions who created the amazing video summary!

References

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Brandon RA, Altig RG, Albert EH. 1966. Anolis barkeri in Chiapas, Mexico. Herpetologica. 22(2):156–157.

Campbell HW. 1973. Ecological observations on Anolis lionotus and Anolis poecilopus (Reptilia, Sauria) in Panama. Am Mus Novit. 2516:1–29.

González Bermúdez F, Rodríguez-Schettino L. 1982. Datos etoecologicos sobre Anolis vermiculatus (Sauria: Iguanidae). Poeyana. 245:1–18.

Henderson RW, Powell R. 2009. Natural history of West Indian reptiles and amphibians. Gainesville: University Press of Florida.

Herrmann NC. 2017. Substrate availability and selectivity contribute to microhabitat specialization In two Central American semiaquatic anoles. Breviora. 555(1):1–13. doi:10.3099/MCZ33.1.

Leal M, Knox AK, Losos JB. 2002. Lack of convergence in semi-aquatic Anolis lizards. Evolution. 56(4):785–791. doi:10.1111/j.0014-3820.2002.tb01389.x.

Muñoz MM, Crandell KE, Campbell-Staton SC, Fenstermacher K, Frank HK, Van Middlesworth P, Sasa M, Losos JB, Herrel A. 2015. Multiple paths to aquatic specialisation in four species of Central American Anolis lizards. Journal of Natural History. 49(27–28):1717–1730. doi:10.1080/00222933.2015.1005714.

Robinson DC. 1962. Notes on the Lizard Anolis barkeri Schmidt. Copeia. 3:640–642.