Author: Lindsey Swierk

Over the past several years, semi-aquatic anoles experienced a bit of viral fame for “scuba diving,” a nickname for their ability to rebreathe a bubble of air over their nostrils while diving underwater. Rebreathing allows anoles to remain underwater for a long time and theoretically escape their terrestrial or aerial predators. My collaborators and I have clocked rebreathing semi-aquatic anole dive times of about 20 minutes, though — who knows – it may even be longer! Chris Boccia and Luke Mahler led a collaborative study a few years ago in which we found that these rebreathed bubbles do decrease in oxygen over a dive, which tipped us off that anoles are actually using bubbles in respiration.

Water anole (Anolis aquaticus) rebreathing a bubble of air. Photo by: L. Swierk

But aside from being just a mind-boggling behavior to watch and a nerdy party factoid, the existence and function of rebreathing immediately hatches dozens of ecological, evolutionary, and physiological questions. One of the most fun and puzzling of these is: how are anoles actually able to stay underwater so long just by using the oxygen in their old, exhaled breath? We were puzzled by this too since, despite the relatively low oxygen demand expected of a lizard in cool stream water, we already knew that there was something funny going on with oxygen availability in these rebreathed bubbles toward the ends of dives. Instead of decreasing linearly like you would expect, the oxygen decrease in bubbles actually slowed over time. Could this mean that – when oxygen was needed most — the rebreathed bubbles were picking up oxygen from the water surrounding them?

That air-breathing animals extract oxygen from water via bubbles is certainly not a new idea. There is solid evidence of so-called “physical gills” in many air-breathing invertebrate species, including beetles, water bugs, spiders, and even scorpions! These species maintain bubbles on or near their bodies, and they get enough oxygen from the diffusion of dissolved oxygen from the water into their air bubbles to respire and remain underwater for long durations (sometimes indefinitely!).

Given the relatively small size of these invertebrates, versus the larger sizes and greater oxygen demands of semi-aquatic anoles, we thought it extremely unlikely anoles would be able to entirely rely on physical gills for indefinite respiration. But… perhaps oxygen diffusing into their bubbles could at least extend their dives? Even only a small increase in dive time could offer a benefit when it comes to predator avoidance.

My then-PhD student, Dr. Alexandra Martin, an NSF REU student, Diane Cordero-De La Cruz, and I decided to design an experiment to begin to test this idea, using our lab’s favorite (don’t tell!) semi-aquatic anole: Anolis aquaticus. In controlled lab conditions, we altered the levels of dissolved oxygen in tanks, predicting that if lizards were able to use their bubbles as physical gills then they would be able to stay submerged longest in the most highly oxygenated tanks. We were surprised and intrigued to find exactly this – A. aquaticus dive durations increased significantly when dissolved oxygen in the water was highest, and dives were shortest when dissolved oxygen was lowest. Anoles also rebreathed fewer bubbles as dissolved oxygen increased. These patterns suggest that rebreathing bubbles may be more than just an oxygen “tank”… bubbles may also be functioning as a physical gill, replenishing the air bubble with oxygen from the surrounding water. Use of a physical gill would be a first for any known air-breathing vertebrate.

Dive duration and numbers of rebreathed bubbles (shown as estimated marginal means; EMM) of water anoles diving in low, medium, and high dissolved oxygen (DO) tanks. Figure from Martin et al. 2025, Journal of Experimental Biology

There are many next steps to confirm the mechanism and adaptive function of our results, one of which is to directly measure oxygen diffusion into the bubble. But we are fascinated by the story that these findings are beginning to tell: that anoles may be pushing the envelope of vertebrate respiration in ways we’re only beginning to appreciate. As always, anoles find a way.

You can read more about our findings in our new paper in the Journal of Experimental Biology: “High dissolved oxygen extends dive duration and suggests physical gill use in a vertebrate.”

Water anole (Anolis aquaticus) on a rock

Water anole perched on a streamside boulder. Photo by L. Swierk

How Do Body Color Changes Affect an Anole’s Conspicuousness to Conspecifics and Predators?

By Lindsey Swierk

On August 4, 2024

In All Posts

Water anole from the Osa Peninsula, Costa Rica (Photo by Lindsey Swierk)

Rojo A, & L Swierk. 2024. How does rapid body color change affect the conspicuity of lizards to their predators and conspecifics? Behavioral Ecology and Sociobiology 78: 78.

Written by Andrés Rojo:

The water anole (Anolis aquaticus) is a Costa Rican anole that changes color and pattern in different microhabitats as a form of camouflage to avoid detection by their predators, which include birds like motmots and kingfishers. Because of its body color complexity and rapid color changes, water anoles are great study organisms to use when examining how animal camouflage, social signaling, and visual perception interact.

My research project was inspired by Dr. Lindsey Swierk and her lab’s work on water anole color change and camouflage. I joined the Swierk lab as an undergraduate in February of 2021, as I was interested in tropical ecology and animal behavior. I am also a photo hobbyist and experiment with color and full spectrum photography. Dr. Swierk thought that I would be a good fit with the research team studying water anole colors and patterns. When she told me about it, I was motivated by my enthusiasm for photography and wildlife research to take on the project, especially the idea that I could modify research-grade photos to model how animals see one another.

An example color cloud map showing the colors of a water anole (gray) and its substrate (red) as seen through an avian visual system. The X axis represents a green-to-red progression of colors, and the Y axis represents yellow-to-blue colors. Water anoles do not have UV body colors in the regions measured (dorsal and lateral surfaces). Darker tones indicate more pixels of colors at that location within chromaticity space.

Dr. Swierk and I decided to test whether the body coloration of A. aquaticus would be perceived differently by the visual systems of water anoles’ predators and their conspecifics. We used ImageJ and the micaToolbox (QCPA) to model the two visual systems and apply them to each photo taken in the field. The anole and substrate photos were converted into color maps, which could be compared to determine how much the color of the anole overlapped with the color of the substrate in the visual systems of predators and conspecifics – a measure of conspicuousness of anoles according to both visual systems.

A comparison of body color – background color overlap of water anoles as seen through bird and anole visual models, including males (blue) and females (gold).

Although we found no significant difference in how camouflaged A. aquaticus were perceived by the visual systems of predators and conspecifics, our results suggested that males more consistently color matched their microhabitats compared to females and that females were less likely to color match their backgrounds in their lighter phase, suggesting a sex difference in preferred conspicuity in more exposed habitats. These could present topics for future research into how anoles use color camouflage to avoid being detected by predators.

Donations of Shed Anole Skins Needed

By Lindsey Swierk

On April 1, 2020

In All Posts

Anolis aquaticus

Recently, my lab has been excited to begin work on some of the morphological, behavioral, and physiological aspects of so-called “underwater breathing” in semi-aquatic anoles. Given the current circumstances, I need to ask for a little help from the anolologist community. My collaborators and I are studying the microstructure of semi-aquatic anole skin – what about the skin allows a bubble to stick and move around the way it does?

We are seeking shed anole skin samples to compare with Anolis aquaticus. Any anole species or sex or any size sample will do. Really, a bit of shed skin that is even 1 cm long from a single individual would be a “big” sample and enormously helpful!

If anyone out there keeps anoles of any species in lab or at home and would be willing to drop a fragment of shed skin into an envelope for me, it would go a long way to helping us keep this project moving despite the moratorium on fieldwork.

If you are able to help, you can message me below. Thank you all in advance!

In the Eye of the Beholder: How Do Anoles Respond to Human Clothing Color?

By Lindsey Swierk

On January 13, 2020

In New Research

Water anole (Anolis aquaticus), Costa Rica

Have you ever wondered how your clothing color affects how many anoles you see and catch on a given day? When we go out herping, a lot of us favor some sort of clothing color scheme – whether it’s conscious or subconscious, scientific or superstitious. Since anoles have such excellent color vision, and since they’re so sensitive to the signaling colors of the individuals they interact with, this makes sense, right?

(a) Water anole dewlap; (b) Orange and blue shirts; and (c) green shirts worn in this study.

Bree Putman (Cal. State San Bernardino), Andrea Fondren (an undergraduate researcher), and I teamed up to determine if there was any truth behind the superstition. In an effort to understand how anthropogenic colors affect the behavior of lizards, we designed a study to test whether researcher shirt color would influence the sighting and capture rates of water anoles (Anolis aquaticus). Male water anoles have large orange dewlaps, which is their most conspicuous sexual signal. Using a modified version of the species confidence hypothesis, we predicted that water anoles would be least fearful of anthropogenic colors that most closely resembled the color of their own sexual signals (orange). To test this, a group of us spent the summer surveying water anole populations wearing three different shirt colors: orange, blue, and green. We carefully allocated shirt colors to research teams and study sites, planning it out in advance so that all shirt colors were worn by all researchers and used at all sites evenly in the study.

Our results, published in Biotropica, may make you rethink your own field shirt color choices. Both our sighting rates and our capture rates of water anoles were significantly higher when wearing orange shirts – matching the color of water anole dewlaps. Lizards likely have sensory biases for colors used in their species-specific displays, and we found that this translates into clear differences in behavioral responses to anthropogenic colors. On some level, I think many of us expected that the green shirts, which camouflaged us beautifully in the forest, would have been the most successful. Not so. Looking more broadly, Bree Putman’s previous study on western fence lizards (Sceloporus occidentalis) helps to solidify the idea that lizards are biased toward anthropogenic colors that they themselves “wear.” Western fence lizards have blue sexual signals and – corresponding nicely to our water anole results – these lizards are less fearful of researchers wearing blue clothing. Taken together, these findings remind us that placing a human perspective on animal perception can sometimes lead to flat out wrong conclusions.

As ecotourism is on the rise, it’s worthwhile to consider how the anthropogenic stimuli that ecotourists bring into a species’ native habitat can affect species behavior and survival. Reducing a species’ exposure to more frightening stimuli (such as unfamiliar colors) can reduce stress and disturbance. Something for the ecotourism community to consider as researchers continue to explore ways to minimize disturbance of natural populations, while promoting sustainable use of natural areas.

As for me, it looks like I’ll be buying quite a few orange shirts this year.

Underwater Breathing by a Tropical Lizard

By Lindsey Swierk

On December 20, 2018

In Anole Videos, Natural History Observations, Notes from the Field

Adult water anole (Anolis aquaticus) from Coto Brus, Costa Rica, with an exhaled – and recycled – air bubble. Photo by Lindsey Swierk.

Over the past few years, I’ve been accumulating evidence that the water anole (Anolis aquaticus) might be a tiny scuba diver in the streams of its home in southern Costa Rica. Anolis aquaticus takes to the water as refuge from predators, swimming and often diving underwater for long periods of time – the record at my study site is currently 16 minutes! To enable these crazy-long dives, it’s possible that anoles may have come up with a scuba-tank method of sorts to “breathe” underwater. I’ve compiled a video of what appears to be underwater respiration of a recycled air bubble that clings to the anole’s head. (A few more details about these observations will be in the upcoming March issue of Herpetological Review.) For now, enjoy the video!

Student-Produced Short Film on a Day in the Life of an Anologist

By Lindsey Swierk

On July 11, 2018

In Anole Videos, Education and Anoles

Water anole (Anolis aquaticus), photo by Lindsey Swierk

For three years now, my students and I have studied the fantastic water anole (Anolis aquaticus) at Las Cruces Biological Station in Costa Rica. Each summer, I work with aspiring undergraduate scientists of minority backgrounds on their independent research projects on this quirky anole species. This year, my students had the opportunity to participate in a science filmmaking workshop while in the field. Specifically, they wanted to show everyone what it’s like to be an anole field researcher! In their own words:

“In the last two days, we had the amazing opportunity to be part of a science communication workshop led by Nate and Kori from Day’s Edge Productions to learn more about making science films and all the behind-the-scenes action that happens behind the camera. In groups of four, we were given the challenge to plan, shoot, and edit a short film in less than 24 hours (more like 12).” – Diana Lopera (University of Hawaii)

“We decided to try to capture a day in a life of a field biologist to showcase the hard work that happens behind the scenes of research. I am super happy with what our group was able to come up with and hope to really show our appreciation for those film artists and scientists alike working hard to understand these difficult questions.” – Maegan Delfin (University of Guam)

Special thanks to my colleague, Bree Putman, and her students Austin Carriere and Andrea Fondren for being brilliant water anole collaborators and inspirations in the field. Aside from providing an interview, Bree and I had no involvement with the filming or storyboarding, so the video represents our students’ perspective on field research – and is all the better for it!

“At the end of each day, we come out of the forest with more than just data. We come out with a greater appreciation for the hard work scientists do to find the answers to better understanding the natural world.” – Diana

Also special thanks to Day’s Edge Productions for a great workshop that inspired all of our students in this REU program to become enthusiastic science communicators.

Enjoy the peek into the daily grind of an anologist!

Request for Anolis aquaticus Photos and Sightings

By Lindsey Swierk

On September 20, 2017

In All Posts

Photo by Anolis aquaticus from wildherps.com.

Hello anole enthusiasts!

A quick note and a request from your Anolis aquaticus correspondent. Our new paper on stress-related body color brightening in Anolis aquaticus was recently selected as Editor’s Choice in the Canadian Journal of Zoology. In it, we document a genus-atypical direction of color change following exposure to a stressor, possibly related to optimizing camouflage in the water anole’s unusual habitat. Enjoy!

We are embarking on a new research direction with these wonderful watery critters. In our early stages of surveying, I’d like to ask for your help.

We are exploring morphological and behavioral variation across the water anole’s range to explore several hypotheses related to coloration, habitat lighting, temperature, and stress.
For example, dewlap coloration seems to be fairly variable: water anole dewlaps from our sites at Las Cruces Biological Station are red-orange (left), but at Osa the dewlaps are much yellower (e.g., screen shot taken from Brave Wilderness’s video* on the water anole, right).
jennet Osa

In addition, we’re also interested in knowing a little more about water anoles in the riparian zones that are found in otherwise deforested tropical pasture lands. We’ve put together a map of all known collection sites of museum specimens and published studies (sites shown without exact coordinates in the interest of species’ protection; grey sites are approximate).

You can help by sharing with us your photographs of water anoles (dewlaps are of special interest, but any photographs would be appreciated) and/or locality data** of Anolis aquaticus sightings or collection. Locations of sightings in pasture/agricultural areas are especially needed!

Lindseyns @ gmail.com
Lindsey.swierk @ yale.edu

Thank you for your help!

*A very enthusiastic group called Brave Wilderness posted a video about their search for the “mysterious” water anole. I have mixed feelings about it and its less-than-perfectly-accurate information, but it certainly captures kids’ imaginations!

**To keep this charismatic species safe and help prevent poaching, please send any GPS coordinates to me directly rather than posting them publicly.

Super-Honest Dewlaps and Trait Scaling Relationships in Semi-Aquatic Anoles

By Lindsey Swierk

On October 2, 2016

In New Research

Anolis aquaticus, the semi-aqautic anole. Photo by Lindsey Swierk

From backyard anole enthusiasts to researchers with decades of experience, dewlaps are a favorite topic of discussion here on Anole Annals. We love documenting the diversity of dewlap colors and patterns (1, 2, 3, 4), judging “best/biggest dewlap” contests (1, 2, 3), and noting dewlap oddities across the genus (1, 2, 3, 4). We’re slowly piecing together an answer to the question of what role dewlaps actually serve in signaling and, in particular, what kind of information they might convey. As you might expect, it’s a pretty complex problem, made even more interesting by the fact that dewlap information content probably reflects the unique pressures placed on individual species.

I’ve recently been working on untangling the mystery of dewlaps in a quirky species of anole, Anolis aquaticus. This water-loving anole is found along streams in pockets of southern Costa Rica and northern Panama, and it has the delightful habit of diving into water when startled. Even among the aquatic-specialized anoles, A. aquaticus is different: it tends to live in ultra-close proximity to water, preferring boulders and crevices directly in the “splash zone” instead of streamside vegetation such as other aquatic species like A. oxylophus. There’s also good reason to think that A. aquaticus has a pretty rich social life – male-female, male-male, and female-female pairs can be found within a few centimeters of each other, and often in dense groups on small rocky islands.

In light of their unusual habitat and living arrangements, we decided to explore how dewlaps correlated with multiple morphological parameters in A. aquaticus. In particular, we decided to use this species to explore a long-standing, but recently debated, paradigm that most sexually selected traits (like dewlaps) scale to body size with positive allometry – or, in other words, that they’re disproportionately large in larger individuals. Last year, we captured male and female (who lack the characteristic reddish-orange dewlap) A. aquaticus and measured multiple sexual and non-sexual traits to test this idea. Our results, available in an accepted article in Integrative Zoology, allowed us to contribute our perspective to the greater understanding of the relationship of sexual selection and allometric scaling patterns. Spoiling part of the punchline, our findings do not support the traditional idea that positively allometry is a hallmark of sexual selection.

The dewlap of Anolis aquaticus. Bar represents 1 cm.

But, equally as notable, our results also suggest some interesting features of this species, including the information content of its dewlap and how allometric patterns interact to produce sexual dimorphism. We found that dewlaps are “super-honest” signals in A. aquaticus; they could serve to amplify size differences between males signaling at a distance because of their positive allometric scaling with body size. Consequently, our study and a recent study by Driessens et al. 2015 (on A. sagrei), oppose previous ideas that dewlaps approach an asymptote of optimum size to balance the pressures of signaling with predation. Our findings are also novel in that they suggest that dewlap color (redness) may serve to convey information about male weaponry: anoles with redder dewlaps were found to have head shapes that correspond to producing greater bite force.

By comparing allometric relationships between males and females, we can also begin to identify how sexual differences in proportionality link to sexual dimorphism and ecology. For instance, male hind limb length in A. aquaticus is on average larger than that of females, but becomes disproportionately smaller as male body size increases. This opens the door to the idea that, because males are larger than females, limb length sexual dimorphism might be the result of an optimal limb-body size relationship regardless of sex; A. aquaticus of either sex with overlong limbs would probably be at a disadvantage if they needed to flee over narrower surfaces such as branches or vines.

Scaling relationships of snout-vent length and a) mass, b) limb length, and c) head length for male (closed dots, solid line) and female (open dots, dashed line) Anolis aquaticus. Axes are log scaled.

Finally, our results hint at the existence of two life-stage male morphs in A. aquaticus, as already identified in other anole species. Body scaling relationships show that small males have disproportionately small dewlaps, small heads, and large limbs, whereas large males have bigger dewlaps, bigger heads, and smaller limbs than should be expected for their body size. Taken together, these results provide a foundation for future research into “heavyweight” and “lightweight” male morphs and associated behaviors. With their small home ranges and apparently high tolerance for same-sex home range overlap, this could be an especially exciting avenue of exploration in A. aquaticus. In any case, it’s certain that there will be much to learn from this watery, elusive, semi-aquatic anole.

You can read more about this project in our accepted manuscript published online in Integrative Zoology. My co-author on this study, Maria Petelo, is an undergraduate at the University of Hawaiʻi who was supported by OTS/NSF’s Native American and Pacific Islander Research Experience, a program designed to increase the representation of underrepresented groups in the natural sciences.