An orange Anolis sagrei used in the study. Image by Beth Reinke.
Readers of Anole Annals know that Florida populations of Anolis sagrei now include red-orange individuals [1, 2, 3]. I learned more about this new color by conducting the first scientific study on orange skin coloration in Anolis sagrei.
Before I go any further, I owe a thank you to those who documented their orange A. sagrei findings on Anole Annals. Previous posts confirmed that what I was seeing in the lab wasn’t an anomaly. As I learned more about the sightings of these orange anoles, it became apparent that the orange phenotype was rather common. The posts also helped me understand when this odd coloration was first noticed (only in the last decade!). I was even able to meet with one contributor in person.
The first thing I noticed was that there is quite a bit of diversity in the distribution of orange coloration on the bodies of the lizards themselves. Most of the posts on Anole Annals showcase full-bodied orange lizards [1, 2, 3]. I found that partial orange coloration was just as common. Take, for example, this male whose orange coloration was limited to his tail and hind legs.
A biologist’s first intuition is to wonder how differences in coloration might influence survival. Most of my research project was focused on identifying fitness differences between brown and orange lizards. I was working under the impression that orange skin suddenly appeared in the population and became common very quickly. I knew that there are cases when new phenotypes become common for no reason (genetic drift). Nonetheless, we don’t normally expect to see a new phenotype become common in a short amount of time. I suspected that orange lizards had an easier time surviving or breeding than the brown ones. But I was surprised that a color as conspicuous as orange could be so successful. I reasoned that it couldn’t have helped them camouflage, so why are orange lizards surviving and reproducing?
Maybe it had something to do with mate choice. Since males use their orange dewlaps to attract females, it might be that a completely orange male would look particularly stunning to a female. Even though orange might have made the males an easier target for predators, the effect on reproductive success may have outweighed the risk of predation. This is the hypothesis that I had in mind for most of the project and the one that made the most sense to me. It’s fitting, then, that when I ran a behavioral experiment in the lab, the females didn’t care at all about color! They were much more interested in males that performed a lot of pushups and head bobs (behaviors that many species of lizards use to communicate). These pushups and head bobs demonstrate a male’s physical fitness to a female.
Maybe orange reflected something in their physiology, then? I ran two different experiments to test endurance and sprint speed. The tests of endurance and sprint speed in particular took up most of the time of the project; it turns out live animals don’t usually do what you need them to do. Despite their penchant for sprinting out of sight in the wild, getting lizards to run in the lab was more difficult than you might guess. The endurance tests involved a custom-built lizard-sized treadmill. More often than not, the lizards would treat it like a moving sidewalk you’d find at the airport. Other times they’d wriggle into the machine itself (at no risk to them) and I’d have to take apart the treadmill, one screw at a time, to fish them out. No images of that, sadly.
To measure sprint speed, I needed the lizards to run up a wooden pole. Here’s a video of me trying to convince lizards to run up that pole.
I became more interested in paleontology after this project. Dead animals behave more predictably.
After all that, the data didn’t point to any difference in orange and brown lizards’ endurance or sprinting ability. I took a step back to get to the bottom of something I knew I could answer. I wanted to identify the pigments that they were using to color their skin. Having read about what gives Anolis sagrei dewlaps their red and orange color, I was expecting to see two classes of pigments in orange lizard skin: carotenoids and pterins. No one had extracted pigments from even brown A. sagrei skin before, but I wasn’t expecting to see much in non-orange skin.
I boiled lizard skin in all sorts of carcinogenic solutions to extract the pigments.
Then I separated the two types of pigments in test tubes – carotenoids at the top and pterins at the bottom.
As expected, the dewlaps had both types of pigments. Unexpectedly, brown lizard skin contained pterins. I thought this was a little odd since we don’t see red or orange on brown lizards. But, no one had done this before, so I didn’t quite know what to expect. Like brown lizard skin, orange lizard skin had pterins, but not carotenoids. This surprised me because it suggested that the orange color in orange lizards might not be due to the addition of a pigment so much as the absence of one. Melanin (another class of pigment that produces brown and black colors) typically masks the effects of other pigments that may be present. So, although I was unable to test this myself, I now suspect that the orange color is caused by a lack of melanin.
It was time to revisit that camouflage idea. I had taken for granted that orange was too conspicuous to conceal a lizard, but I needed the data to back up my claim. I collected quantitative data on brown and orange lizards’ skin color by using a spectrophotometer, which records color as the wavelengths of light reflected off a surface. The result is something that looks like this:
What A. sagrei dewlaps look like to a spectrophotometer.
One of my collaborators, Dr. Beth Reinke, applied these data to a visual model to predict how A. sagrei’s bird predators would see the new color. She identified that orange anoles are less conspicuous to bird predators. Now the strongest lead is what I had ruled out when I first began the project: camouflage!
So what’s up with orange A. sagrei? The color doesn’t make them more attractive to mates nor does it correlate to increased physical fitness. Because orange and brown skin contain the same kind of orange-producing pigment, my best guess for the mechanism is a lack of melanin in the areas that appear orange. And, although the new color looks conspicuous to humans, it may help orange individuals hide from bird predators. The benefits of orange as camouflage may explain why the new color persists in south-Floridian populations of A. sagrei.
There’s a lot left to know about orange anoles. A good next step would be to test the “orange as camouflage” result in the field. Additionally, research into the genetic basis of this phenotype may identify how it arose and the mechanism behind it. Some breeders have suggested that orange coloration is genetically dominant over brown coloration. This is something I wanted to identify in breeding experiments, but time ran out before I graduated from college.
Orange A. sagrei remain enigmatic. I hope to hear more about orange anoles from enthusiasts in the lab and the field!
Paper: Erritouni YR, Reinke BA, Calsbeek R (2018) A novel body coloration phenotype in Anolis sagrei: Implications for physiology, fitness, and predation. PLoS ONE 13(12): e0209261. https://doi.org/10.1371/journal.pone.0209261