How female frogs use acoustics to choose a mate

Loud sounds are important in both the design of the car and the flirtation of the frog.

So New Hampshire biologists brought an acoustic camera used by car designers to spring frog mating pools to study women’s preferences. Researchers now suspect that a man’s chances of becoming a father depend in part on which boy group he belongs to.

We humans can name our own examples of hum-hum people who are attracted to membership in the right group, says evolutionary biologist Ryan Kalsbik of Dartmouth College. “If Ringo Starr wasn’t the Beatles,” he muses.

Acoustic camera gives biologists a new tool for exploring the strength of membershipwrote Kalsbik and his colleagues in June Environmental letters. Calsbeek credited Dartmouth colleague Hana ter Hofstede, who studied insect sounds and was not part of the study, telling him about this industrial camera and its value to biologists.

The high-tech installation “looks a bit like something you can find on a rover,” he said. A hula hoop-like antenna on a pole holds thin microphones that supply 48 independent audio channels to location software. It uses the small differences in when the same frog call reaches different microphones to calculate the frog’s location.

Calsbeek pulled camera equipment and its substantial battery (sometimes “up to 800 vertical feet with 90 pounds on my back”) to the 11th rendezvous of the spring tree frog pool (Rana sylvatica). In the pool, the impatient men “generate this huge, chaotic swallowing sound” like a turkey herd. Vigorous imitations of Calsbeek on the phone – imagine something like half-absorbed sounds of signals – really give the atmosphere of birds.

Tree frogs maintain their appearance in scattered, crowded night crowds for single people in these pools, where males gather and females shop. Harvesting begins at the beginning of the year, as tree frogs have the rare ability to survive the cold, huddled here and there in the bed of leaves, in some latitudes literally freezing with a stopped heart (SN: 21.8.13). Once thawed back to life, they gather with other boys in the pool, devouring their little warm hearts and waiting for the women to find their way to the party.

Male tree frogs have no anatomy for sperm insertion. Daddy struggles to catch a female and press tight against her so that his semen reaches the eggs when she releases them. With a good grip, the male then turns the pairing of a crowdsourcing frog into a doubles event.

Such frantic predatory men can inadvertently drown females. So once a female jumps into a mating pool, she may not have much choice as to who the father of her offspring is. However, researchers wondered, in places with more than one pool, could she at least choose one group of predatory swallowers over another? Maybe some features of the chorus help her decide.

Most of the extensive research on mating preferences and flirting performances – singing mockingbirds, diving hummingbirds, chirping crickets, etc. – considers a single suitor who appears, usually for a woman (SN: 21.05.09; SN: 4/12/18; SN: 15.12.20). Instead, the Calsbeek team asked, “Does she have a favorite band?”

To see how a man’s membership in a group could give him a sex appeal bonus, researchers created their own frog strips for women in the lab. Combining serenades of individual men, extracted from the many recordings by the pool, the researchers made different trios. Some had the overall tone of piercing little boys; some were mostly roaring bass performances, and some were mixed.

The clearest result so far is that laboratory women seem to like the chorus sequence itself, whether it is piercing at a dominant height or deeper and louder. As a sign that this may be true outside the lab, researchers usually find more than egg-shaped masses, signs of success in mating, floating in pools where choirs maintain more consistent tones.

The tree frog paper caught the attention of longtime frog researcher Michael Ryan of the University of Texas at Austin. Now he would like to know about the feminine side of these choirs, such as how far a woman can hear the pools she can approach.

The acoustic camera itself also intrigued Ryan, who was already browsing shop windows online while answering journalistic questions. For decades, he and his colleagues have studied the calls of wild frogs in more difficult and unpleasant ways. He would place at least three fixed microphones to triangulate the sound position before chanting at night. At the time, he hoped the few men she could track down would show up and not change places much. A mobile acoustic camera with 48 sound inputs, he says, sounds “really cool.”