When it comes to choosing a mate, even lizards have certain standards.
Tracy Langkilde, an associate professor of biology at Penn State University, and Lindsey Swierk, a graduate student in Langkilde’s lab, recently looked at the relationship between body-color patterning and mating behavior in the fence lizard Sceloporus undulatus, and found that the sex lives of these reptiles are more complicated than you might think.A blue-throated male fence lizard. Photograph by Langkilde lab, Penn State University
Males of the species have ornamental blue “badges” on their throats and abdomens that are used in courtship displays and in aggressive encounters with rival males. No surprise there: Studies have linked the blue coloring to testosterone.
The thing is, a majority of the females—so-called “bearded ladies”—sport similar blue-neck coloring to their male counterparts. (Also see “Why Do Mysterious Lizards Have Green Blood?“)
And according to Langkilde and Swierk’s research, published November 6 in the online edition of the journal Biology Letters, male lizards are significantly less amorous when a lady lizard is sporting a brilliant blue neck beard than when she has less coloring and, presumably, less testosterone.
Plentiful Bearded Ladies
Which isn’t to say that they’re all that choosy. The researchers found that male lizards weren’t turning away the bearded ladies as mates, just that they preferred the more feminine look when given the option.
“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,” said Swierk in a press statement.
And they may have good evolutionary reasons to do so. In their research Langkilde and Swierk found that more blandly colored lizards laid more viable eggs—clutches that were heavier and had more nutrients—and were laying earlier in the season, giving their brood more time to develop and therefore an important survival advantage. Males choosing females that are more fit (in reproductive terms) would be classic sexual selection.
But here’s the rub: If bearded ladies aren’t producing offspring that are as viable as their more feminine counterparts, why are they so predominant?
Langkilde and her team found that, on average, 76 percent of female fence lizards were bearded ladies, and that in some populations the number was as high as 95 percent. The scientists believe there may be several factors at play.
Evolving Love Lives
One possibility is that the love lives of fence lizards are in a period of evolutionary flux.
“It is possible that we’re catching a snapshot of the evolutionary process—[that] bearded ladies are very slowly being ‘selected out’ of the population,” said Swierk in an emailed response to questions, adding that changes in social or environmental conditions may be making the bearded-lady phenotype less effective.
It may also be a question of attitude, in which the brassy bearded ladies are simply more likely to get their “man.” (Read more about animal attraction.)
“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,” said Langkilde, who also responded to questions by email.
The team’s previous research has found that unlike males, female lizards find the blue coloring highly attractive, leading to another potential explanation for how bearded ladies keep their genes in the population: They pass on their great “beards” to their male offspring, who reap the mating benefits.
In Langkilde’s words, “bearded ladies may benefit by having especially sexy sons.”
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