WEST PALM BEACH, FLORIDA—A century ago, the cast of circus characters often included bearded ladies, women with decidedly manlike facial hair. Eastern fence lizards have their own bearded ladies. Males sport prominent blue patches on their undersides, and many females have smaller, lighter blue splotches on their chests and necks. Female lizards find the blue in potential mates quite sexy, so evolution has favored brighter patches. But male lizards are turned off by blue females, so it’s a mystery why females have any blue at all.
Tracy Langkilde has some clues. A biologist at Pennsylvania State University, University Park, she and her colleagues have evaluated how blueness affects female lizards. Although there are some downsides to being blue, bearded females run faster and have young that survive better than do offspring of nonblue peers, Langkilde reported here this week at the annual meeting of the Society for Integrative and Comparative Biology.
“It’s a very eloquent study, important to our understanding of why males and females [of all species] are different,” says Erica Westerman, an evolutionary biologist at the University of Chicago in Illinois who was not involved with the work.
In every species, males and females are built from the same set of genetic instructions and so, in theory, should look the same. But they don’t. To understand how this so-called sexual dimorphism arises, Langkilde turned to the eastern fence lizard (Sceloporus undulatus), a 15-centimeter-long reptile that lives on rocks and tree stumps in the eastern United States. “Our work suggests that traits can have different costs and benefits for the two sexes,” she explains.
Langkilde’s earlier studies and other work showed that the blue patches on the undersides of males are badges of maleness that appeal to females. Then in 2013, she and her colleagues found that males prefer to mate with nonblue females. When blue females do mate, they are slower to lay their eggs by about 2 weeks, and their clutches of eggs weigh less than clutches from nonblue females. Langkilde wondered if males tended to shun these females as mates because of these deficiencies. “Not all traits are benign for the sex that doesn't need them,” agrees Simon Lailvaux, an integrative evolutionary biologist at the University of New Orleans in Louisiana, who wasn’t involved in the study. “Studies like this tell us what those costs are specifically for females.”
Now, Langkilde and her colleagues have taken a closer look at these females, not only in the lab but also in the field. They suspected that high testosterone levels cause the blue splotches and reproductive problems, so they gave pregnant nonblue females this hormone. And indeed, treated females laid their eggs later than untreated females, and the resulting young were smaller and didn’t survive as well as the young of untreated females, Langkilde’s team reports.
But the researchers also found that blue females sprint faster, reaching speeds of 1.5 meters per second compared with the 1.2 meters per second achieved by nonblue females. And more of the field-caught young with blue moms survived after 2 months in the lab than did young of nonblue moms. “These results suggest previously unreported, possible fitness advantages for bearded ladies,” Langkilde says. In addition, males may mistake blue females for other males, and so those females are less harassed by overly eager suitors, who can court so much that they interfere with the female’s other activities. Given that 70% of the females in some lizard populations have blue on them, “there may be some unexplained trade-offs in the costs (delayed reproduction) and benefits (higher offspring survival) for females bearing male traits that may explain the existence of these traits,” writes Peter Zani, an integrative biologist at the University of Wisconsin, Stevens Point, who was not involved with the research, in an e-mail.
“Attempting to understand evolutionary trade-offs is of interest to a broad swath of biologists,” Zani adds. Indeed, such work can tell us about ourselves, Westerman says. “It’s difficult to study the costs and benefits of females having malelike traits in our own species, but there might be benefits that we don’t understand yet.”