It’s not just his croak

Even at night, female tree frogs prefer males with redder vocal sacs

Coloring matters. Even to frogs that court in the dark.

FROG COMPUTER DATING FROG COMPUTER DATING. A female European tree frog watches a video, dimmed to simulate a moonlit night, showing a courting male. Offered pairs of videos, females showed a preference for certain male coloration, even though frogs mate at night. Doris Gomez and Marc Théry

CHOOSE ME Portraits of courting male frogs come from paired videos that researchers showed females to test for preferences in male coloration: paler vs. redder vocal sacs (top) and darker body stripe vs. hardly any stripe (below). Doris Gomez and Marc Théry/Proc. B

European tree frogs choose their mates at night, but a matchmaker video test finds that females still care about a guy’s coloring, says Doris Gomez of France’s National Museum of Natural History in Brunoy.

More females hopped over to a video of a male with extra red-orange in the vocal sac of his throat than to a rival video of a male with a paler sac, Gomez and her colleagues report online March 24 in Proceedings of the Royal Society B. Males with a darker stripe down the side of their body proved more popular too.

Many studies have focused on how frog croaking attracts mates. “Relative to that vast literature, this [visual study] is really quite novel,” comments behavioral ecologist Karin Pfennig of the University of North Carolina at Chapel Hill. Human researchers, she says, may have been affected by their own sensory biases, assuming that if they couldn’t see frog colors, a female frog couldn’t either.

European tree frogs (Hyla arborea) are active during the day, lurking in trees to catch small insects. During mating season, though, the action’s at night. Males a few centimeters long gather at ponds and croak rapidly, their vocal sacs ballooning out and rippling the water.

 “They are very noisy,” says coauthor Marc Théry, also at the museum in Brunoy. Researchers working with the frogs in an enclosed lab wear ear protectors.

Studies in other frog species have found remarkably powerful night vision. And only males have the reddish vocal sac, so researchers wondered whether females might use the color in checking out a mate too.

Researchers studied the sacs of 125 male frogs in the wild and created courtship videos that fell within the ranges of the body coloring measured. Some wild males have stripes on their sides, but others don’t, so another set of videos offered striped vs. unstriped males.

No one has yet characterized this tree frog’s vision, so the researchers used information about a related species’ color sensitivity to make sure the video wasn’t dropping wavelengths that tree frogs could see.

In a dark lab, female tree frogs were released within sight of two dimmed video screens. In the wild, a female does the choosing, listening and watching the males on display and then hopping up to one and tapping him. In the lab, the researchers found, females tried to do the same thing. “They went crazy,” Gomez says. They hopped toward one video screen or the other, sometimes climbing a screen as if determined to touch real frog flesh.

Males with a very red-orange vocal sac and darker stripes attracted more females. Female European tree frogs do indeed use these color markings as they cruise among their nighttime callers, the researchers conclude.

Pfennig praises the effort to reflect frog visual sensitivity. Her own work shows that night-mating female spadefoot toads pay attention to the various shades of male yellow-green. But Pfennig had no information on visual systems for her study and made do by applying a human’s estimations of the coloring to clay frogs.

Just what it is about the tree frog’s popular coloring patterns that lures the ladies will take more testing, Théry says. Because the paler sac wasn’t popular, he speculates that it’s not just an issue of brightness. Also he notes that males’ sac coloring comes from food-derived carotenoids, which also power the immune system. A male may need to be healthy and well-fed if he’s going to spend precious carotenoids to keep his vocal sac a sexy red.

All this coloration work is starting to open the way for looking at why females would bother looking at males in the dark in addition to listening to them, Pfennig says. The next question, she says, is “how females put together complex pieces of information.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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