A male brown anole extends its bright orange dewlap to signal to another anole. (Photo courtesy of John David Curlis, University of Michigan; illustration by Emily Faith Morgan, University Communications)
Wait, you didn’t know that Monday is World Lizard Day? What rock have you been living under?
To celebrate, two University of Virginia lizard-ologists want you to see these tiny reptiles’ true colors shining through.
There almost aren’t enough crayons in the box to capture the scale of their scaly vibrance. Add to that their amazing ability to sometimes change colors, and you can see how it’s easy to become a fan of the fan-throated lizard, or geek-out over the crested gecko.
Christopher Robinson, a doctoral candidate in the lab of biology professor Robert Cox, is one such devotee. He’s currently a Doffermyre Family Jefferson Dissertation-Year Fellow.
“Lizards are such wonderful organisms to study,” Robinson said. “They can be stunningly beautiful, exhibiting vibrant colors and fascinating patterns that are useful for investigating principles in evolutionary biology.”
Robinson and Cox decoded for UVA Today some of the many colorful mysteries behind lizards’ pigmentation.
Q. Why do lizards around the world vary so much in their colors?
Robinson: The evolution of color is driven by the selective pressures an animal encounters in their environment, and the way that they can be seen in it. For example, the coloration of a lizard in a deeply shaded forest and a species in a bright desert are usually quite different.
Similarly, lizards that are only active at night use color differently than lizards that are active during the day.
At left, Robert Cox is an evolutionary biologist and ecological researcher who runs the lizard lab at UVA. At right, Christopher Robinson is a graduate student in his lab. (Photos by Dan Addison, University Communications)
Two of the biggest drivers of color evolution are predation and social interactions, such as finding a mate or protecting a resource.
All of these selective pressures interact to produce the stunning variation we see in lizard colors around the world.
Q. Is predation the primary driver of why the colors have evolved?
Robinson: For some species, yes, but for others, probably not. While predation is a strong selective pressure, the ability to find a mate is just as important as not being eaten.
Many species of lizards have sexually dimorphic coloration, meaning males and females have different colors. Often, females are less colorful than males. The female coloration allows them to be cryptic and avoid predators, while the vibrant male colors act as signals to mates and rivals.
Q. Does more intense color indicate they’ll taste bad to a predator?
Robinson: In Virginia, we don’t have any lizards that are unpalatable to predators, although we do have amphibians, such as the red-spotted newt, which are bright red as juveniles to warn predators that they contain lethal toxins, which are potent enough to kill humans if they are consumed.
Cox holds a brown anole on his finger. These lizards can rapidly change from dark to light, but their scales essentially still stay brown. (Photo by Dan Addison, University Communications)
Cox: Bright warning colors that signal toxic or unpalatable prey are common in insects and amphibians, but not in lizards or other reptiles. However, bright color can be a warning that a species is venomous.
Gila monsters and beaded lizards of the southwestern United States and Mexico are the only truly venomous lizards, and they are strikingly colored orange and black as a warning.
Q. How does color help with mating?
Robinson: Male lizards often use coloration and colorful ornaments to signal to mates, and in some species, variation in coloration is associated with individual quality. This might mean that the individual is healthy and free from parasites, able to secure food easily, or strong.
In theory, a female can then use coloration to assess whether the offspring she has with a male are likely to be successful themselves.
Q. What are the most colorful lizards we can see in Virginia?
Robinson: The lizard most people contact me about is the common five-lined skink. Juvenile skinks have dark backs with bright yellow stripes and a vibrant blue tail. But as they mature, the blue color in their tail fades, and they tend to lose their yellow stripes.
Male, left, and female, right, desert spiny lizards demonstrate the variety of colors that occur with sex differences. (Photo courtesy of Christian Cox, Florida International University)
However, during the breeding season, adult males develop bright orange heads and enlarged jaw muscles, which they use in fights with other males.
Cox: Eastern fence lizards, which we study, are also very colorful, but you might not know it at first glance.
Juveniles and adult females have subtle brown and black patterns on their backs that help them blend in with tree bark and avoid predators. Even adult males are not particularly colorful from above.
But, if you look at the underside of a male, you will see vibrant blue patches that they display to rival males or potential mates – by doing push-ups.
Q. Why do some lizards’ bodies seem to be segmented into two or more distinct colors?
Robinson: There are several reasons why this can occur, but if we revisit the example of the common five-lined skink, with a striped body and a blue tail, it is primarily an anti-predator tactic.
Robinson, a Doffermyre Family Jefferson Dissertation Year Fellow, discusses aspects of his research. (Photo by Dan Addison, University Communications)
When the lizard moves quickly, the stripes running down its back are disorienting and make it hard for a predator to focus on the body. The blue tail serves as a visual attractant, so that predators are encouraged to attack an expendable body part.
The tail, as you may know can break off, allowing the lizard to escape and live another day while the tail regenerates.
Cox: In other species, the distinct colors may be a way of generating a contrasting pattern that stands out during visual communication with other individuals. Lizards generally do not vocalize, so most of their social communication occurs through visual displays or chemical signals.
Many lizards also have ornaments that are brightly colored and stand out from their background color. For example, we study anole lizards that use an extendable throat fan called a dewlap to communicate with one another. The color and pattern of the dewlap is highly specific to the particular species of anole.
Q. What is color-changing used for?
Robinson: Color change is often used as a social signal to indicate something about an individual’s current state. If a male lizard has just won an aggressive interaction with a rival, it may signal this by changing its color to help attract a mate, or to advertise its victory.
Because lizards do not produce their own body heat like birds or mammals, many species also use color change to thermoregulate and control their body temperature.
A lizard that is cold can darken its skin to help absorb more solar radiation and quickly warm up. Once it has reached its optimal body temperature, it can lighten its skin to slow the rate of solar absorption and maintain its preferred temperature.
Contrary to popular belief, there is much less evidence that rapid color changes are used for background matching and camouflage.
Q. What are the biological mechanisms that create the color changes?
Robinson: Different cell types contain different pigments and structures that contribute to color. For example, melanin pigment absorbs light and appears brown or black. Lizards can darken their color by dispersing melanin throughout the cells that contain it, or lighten their color by concentrating the same melanin into a small area of each cell.
Color change often involves several cell types. In the green anole, cells that reflect blue light interact with other cells that contain yellow pigment to produce their green color.
Cells containing melanin sit beneath these blue and yellow cells, but they have arms that extend above the other cells to the surface of the skin. When the lizard darkens, melanin disperses to the tips of these arms, covering the other pigment cells and producing a brown color.
Q. How will you be celebrating World Lizard Day?
Robinson: I will be working on my next lizard-based paper and likely watching “Rango” in the evening.
Cox: I’ll use it as an opportunity to read a few recent papers on lizard evolution to find some new material to incorporate into my teaching.