Author: Lindsey Swierk

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.

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.