Male lizards prefer mates with the most feminine features.
Researchers studied the "mating behavior and blue-color patterns of fence lizards in Arkansas, Alabama, and Mississippi," a Penn State news release reported.
Male fence lizards have bright blue markings on the sides of their throats and bodies. Past research suggests these markings are caused by testosterone, making them a "manly" symbol. Sometimes females have smaller and subtler versions of these testosterone markers.
"Just as some human females have male-pattern facial hair, albeit less pronounced than in males, some female fence lizards display the typically-male blue markings," Tracy Langkilde, an associate professor of biology at Penn State University, said. "However, whereas in human females the masculine characteristics are less common within the population, in fence lizards, we see the opposite pattern: About three quarters of the females are so-called 'bearded ladies,' making masculinized females much more common than their counterparts with little or no blue ornamentation."
The researchers observed whether the male lizards were more attracted to "bearded ladies" or their more feminine unmarked counterparts.
"We found that, although males do not say 'no' to bearded ladies, they clearly discriminate against blue-ornamented females, opting more often to court females without coloring," Lindsey Swierk, a graduate student in Langkilde's lab, said. "The question is 'why'? Is it possible the males mistake the bearded ladies for fellow males? Or are bearded ladies somehow less fit and, therefore, less attractive to males?"
The team discovered the bearded ladies laid eggs that weighed less than the more feminine lizards'; they also laid them 13 days later.
The team believes the eggs are lighter because they have smaller yolks, which means fewer nutrients for the fetus.
"As for the timing, the 13-day difference is significant. It means that the bearded ladies' offspring hatch later, so they have less time to gather food and to prepare for overwinter hibernation, which is a tough period that few babies survive. As a result, females with less blue coloration may have an evolutionary advantage with regard to the fitness of their offspring. This might explain why males tend to prefer them," Langkilde said.
Since unmarked females are less common, the researchers believe the species may be moving towards more sexual dimorphism, which are physical differences between males and females.
"It is possible that, over the course of several generations, we will see the more-feminine lizards winning out over their bearded-lady sisters," Langkilde added. "In time, the percentage of bearded ladies could dwindle and the balance could shift. However, another possibility is that bearded ladies have some other evolutionary advantage that keeps their numbers high within populations."
The extra testosterone in the bearded makes them more aggressive, and thus more likely to fight off predators.
"Bearded ladies also may be more sexually aggressive so, although the males don't prefer them, they may initiate more of the courtship and mating and produce as many or more offspring for this reason," Langkilde said.
The research suggests female fence lizards prefer the males with the brightest blue colors.
