How female frogs use acoustics to pick a mate

Loud sounds matter in both car design and frog flirting.

So New Hampshire biologists lugged an acoustic camera used by car designers to springtime frog-mating pools to explore female preferences. Now researchers suspect that a male’s chances of becoming a dad depend in part on which pool’s boy band he belongs to.

We humans can name our own examples of ho-hum guys getting an allure boost from membership in the right group, says evolutionary biologist Ryan Calsbeek of Dartmouth College. “If Ringo Starr hadn’t been a Beatle…,” he muses.  

An acoustic camera gives biologists a new tool to explore the power of membership, Calsbeek and colleagues wrote in the June Ecology Letters. Calsbeek credits Dartmouth colleague Hannah ter Hofstede, who has studied insect sounds and was not part of this study, with telling him about this industrial camera and its value to biologists.

The high-tech setup “looks a little like something you might find on a Mars rover,” he says. A hula hoop–like antenna on a pole holds stubby microphones feeding 48 independent channels of sound to location-calculating software. It uses the slight differences in when the same frog call reaches different microphones to calculate the frog’s location.

Calsbeek hauled camera gear and its substantial battery (sometimes “up 800 vertical feet with 90 pounds on my back”) to 11 early-spring rendezvous pools for wood frogs (Rana sylvatica). In a pool, eager males “generate this huge, chaotic gobbling sound” like a turkey flock. Calsbeek’s spirited imitations over the phone — imagine sort of half-swallowed honking sounds — indeed give a poultry vibe.

Wood frogs keep their species going in splashy, thrashing singles night–style crowds at these pools where males gather and females shop. The gatherings start early in the year, as wood frogs have the rare ability to survive cold nestled here and there in leaf litter, in some latitudes literally freezing solid with a stopped heart (SN: 8/21/13). Once thawed back to life, they gather with other guys at a pool gobbling their little warmed-up hearts out waiting for females to find their way to the party.

Male wood frogs lack the anatomy to insert sperm. A dad-wannabe fights to grab a female and position himself tight against her so his sperm will reach eggs as she releases them. With a good grip then, a male turns frog mating from crowdsourcing into a couples’ event.

Such frantic grabby males can inadvertently drown females. So once a female hops into a mating pool, she may not have much choice about who fathers her offspring. However, the researchers wondered, in places with more than one pool, might she at least choose one bunch of grabby gobblers over another? Perhaps some features of the chorus help her decide.

Most of the vast research on mating preferences and flirtatious performances — mockingbirds singing, hummingbirds swoop-diving, crickets chirping and so on — looks at a single suitor showing off, usually for a female (SN: 5/21/09; SN: 4/12/18; SN: 12/15/20). Instead, Calsbeek’s team asked, “Does she have a favorite band?”

To see how a male’s membership in a group might give him a sex appeal bonus, the researchers created their own frog bands for females in the lab. Combining individual male’s serenades pulled from the trove of poolside recordings, the researchers made a variety of trios. Some had the overall pitch of shrill little guys; some were mostly rumbly bass performances, and some were mixed.

In video from what’s called an “acoustic camera,” each blip of a rainbow blob in this New Hampshire pool pinpoints the location of a male wood frog who just gave his come-hither mating call. The colors correspond to the decibel scale at right. The male frogs gather in early spring to call (sounding to human ears like something between a human burp and a turkey gobble). To female wood frogs, though, it’s apparently a signal that males are available and eager for company.

The clearest outcome so far is that lab females seem to like chorus consistency itself, whether shrill in dominant pitch or deeper and rumbly. As a sign that this might be true outside the lab also, researchers typically found more of the jelly-gob egg masses, signs of mating success, floating in pools where choruses kept more consistent pitches.

The wood frog paper caught the attention of long-time frog researcher Michael Ryan of the University of Texas at Austin. Now he would like to know about the female side of these choruses, such as how far away a female can hear the pools she might approach.

The acoustic camera itself also intrigues Ryan, who was already window-shopping online as he answered journalist questions. For decades, he and colleagues have studied wild frog calls in harder and iffier ways. He would set out at least three fixed microphones to triangulate sound position ahead of a night’s chorusing. Then he’d hope the few males he could track showed up and didn’t switch places much. A movable acoustic camera with 48 sound inputs, he says, sounds “really cool.”

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