When Froggy goes a’ courting,
MU biologist Carl Gerhardt usually isn’t far behind. For nearly 40 years, Gerhardt and his fellow researchers have eavesdropped on the amorous amphibians with the goal of understanding the surprisingly complicated science of frog communication. In the process they’ve gained important new insights into how frogs — and by implication many other species — may be evolving much more rapidly than scientists had ever imagined.
Gerhardt’s most recent work involves the love life of tree frogs: specifically, how female tree frogs respond to males’ mating calls. Female frogs, he’s found, are genetically hard-wired to target mating calls from males of their own species.
That helps prevent females from mating with incompatible males and producing less viable or even sterile offspring. Male calls also give females subtle clues about the quality of prospective mates, and how likely they are to help produce a healthy new generation of frogs.
When love-struck male frogs gather around ponds on spring and summer evenings, they create a deafening chorus announcing their availability to lovelorn females.
To most people, the sound is just an unbroken wall of noise, but Gerhardt can hear all the complicated nuances of this amphibian orchestra. Like a concert maestro, he can tease apart the separate voices: the piping of spring peepers, the trilling choruses of tree frogs, and the basso profundo blasts from bullfrogs.
The musical score for this chorus of calls isn’t inscribed in sharps and flats and arpeggios. In their lab, Gerhardt, his colleagues and students chart the male calls with precise sonograms that analyze their energy, and with graphs that detail their tonal frequency, pulse rates and the duration.
Gerhardt got hooked on frogs early. Growing up in Savannah, Ga., he was fascinated by the strange nighttime sounds that boomed from nearby swamps and pine flatlands. “I started going into those swamps at night and tracking down the noises with a flashlight,” says Gerhardt, a Curators’ Professor of Biological Sciences.
Learning all those calls wasn’t easy. In the southeastern United States, frog choruses can include as many as a dozen separate species, from spring peepers and southern cricket frogs to ornate chorus frogs and bird-voiced treefrogs, just to name a few.
Sometimes, these frog choruses get so loud that it’s even a challenge for a female to find her way to the right mate. “When male frogs call, they usually sit more or less in one place,” Gerhardt says. “The female frog moves near the male or sometimes nudges him to say, ‘Hey, I’m here.’ The male grabs whatever comes around that is about the same size. It doesn’t matter if it’s his species or not. So the choice of the correct mate is all based on the female’s ability to locate and recognize that male.”
To a nonscientist, the tree frog mating ritual might not sound too different from a scene on ladies’ night at a singles bar.
Male frogs gather by a suitable pond and, like a band warming up, lay down a backbeat of noise that grows as more males join the chorus. Some species call from the bank, while others hop into shallow water to let loose.
Meanwhile, female frogs attracted by the noise start slowly hopping toward all the commotion. When a female finds Mr. Right, the male hops onto her back. She then carries him to the spot she’s chosen to lay her eggs, and the male fertilizes those eggs with his sperm.
Gerhardt and his crew can’t precisely reproduce that pond scenario in their laboratory, but they come close. The female frogs are placed in a dark, soundproof chamber so that no outside factors can skew the test results. The researchers play computer-generated male calls, then record females’ reactions on infrared video, watching to see if they hop toward the male calls.
By varying the synthetic sounds he produces in his lab, Gerhardt can identify the acoustic differences that female frogs use in mate selection. That helps him isolate examples of acoustic signal change as succeeding generations of frogs evolve. The research also provides clues to study frogs’ neurobiological mechanisms, such as identifying auditory neurons or networks in their brains.
Back in the real world, however, Gerhardt and other frog researchers have found that love’s course doesn’t always run smoothly. After a long winter’s hibernation, for example, male frogs’ libidos can exceed their judgment and they sometimes grab another male that happens by. The aggrieved male gives a call that says, “Wrong. Release me,” Gerhardt says.
“Sometimes females will make the same sound when they don’t have eggs to lay or it isn’t time for them to mate. This is their ‘I have a headache call,’ and it means get lost.”
Tree frogs don’t always croak sweet nothings. When males get too close to each other, they give out aggressive calls to warn nearby interlopers to take a hike. But even that warning call helps give females some romantic clues. When males are spaced farther apart at the mating pond, it’s easier for females to home in on individual calls and locate a preferred mate.
After all, it only takes two to tango, and three is definitely a crowd. Gerhardt and former student Sarah Humfeld, now a postdoctoral fellow in his lab, studied a phenomenon in the mating ritual involving what are known as “satellite males.” These are males, usually in poor physical condition, whose vocal performance doesn’t match up to more acoustically desirable competitors. The satellites hang out near calling males and try to intercept females hopping toward their would-be beloveds.
‘The males are both singing the same love songs — just one frog is singing it slower. It’s kind of like the difference between Eric Clapton’s original and his unplugged versions of Layla.’
Through his experiments and other studies, Gerhardt’s lab established the importance of brain and behavior in contributing to species divergence and biological diversity.
Sarah Humfeld, a post-doctoral fellow working in Carl Gerhardt’s lab, introduces a female frog into an anechoic, or “echo-free” chamber located in MU’s Stewart Hall. Researchers turn out the lights, seal the chamber, and turn up the volume on computer-generated male calls. The female frog’s reactions are recorded on infrared video.
“Carl’s research on animal communication and reproductive behavior transcends traditional disciplinary boundaries,” says Mark Bee, one of Gerhardt’s former students who now runs the Animal Communications Lab at the University of Minnesota. “As a result, he has made seminal contributions in fields ranging from evolutionary biology, to behavioral ecology, to auditory neuroscience.”
Gerhardt has published more than 125 articles in scientific journals. His books range from scholarly works, such as Acoustic Communication in Insects and Frogs: Common Problems and Diverse Solutions, to the popular natural history guidebook, Frogs and Toads of North America: A Comprehensive Guide to Their Identification, Behavior and Calls. He has had continuous funding from the National Science Foundation and three Career Development Awards from the National Institutes of Health.
Gerhardt’s fellow researchers credit him with an unquenchable curiosity about the natural world and a passion for science, says John Walker, professor and director of biological sciences.
“As one of his colleagues noted, ‘The true mark of a naturalist is one who not only appreciates our natural world but infects others with that enthusiasm,’ ” Walker says. “That’s Carl to a T.”
In some of his most recent research, Gerhardt has been working with doctoral student Mitch Tucker to study two closely related species of tree frogs native to Missouri: Cope’s gray treefrog and the Eastern gray treefrog.
“To the naked eye — human and frog — the two frogs look exactly alike,” Gerhardt says. “The frogs differ only in the number of chromosomes. The Eastern gray treefrog has double the number of chromosomes.”
Although the two species are visually identical, their mating calls differ, says Tucker. “The males are both singing the same love songs — just one frog is singing it slower. It’s kind of like the difference between Eric Clapton’s original and his unplugged versions of ‘Layla.’”
That doubling of chromosomes happens through a process called “polyploidy,” which scientists think occurs because of abnormal cell division. Gerhardt can induce polyploidy in his lab by chilling fertilized frog eggs through a procedure called “cold-shocking.”
From their earlier studies, the scientists found that tree frogs with more sets of chromosomes have larger cells, which slows down the trill rate in their songs. What researchers didn’t know was whether the females’ call preferences were also linked to chromosome number.
The recent studies found that they were, in fact, linked. “That shows that chromosome number alone can control the behavior that keeps the species separate,” Gerhardt says. “In turn, as chromosome number increases, so does the size of cells, which is probably the immediate cause of the changes in calls and (mating) preferences.”
The discovery also provides insight into how new frog species have evolved. In animals, species origin is often linked to geographic barriers, such as mountain ranges or large bodies of water. These barriers can split an animal population and prevent mating between members of the same species, Gerhardt says.
The Eastern gray treefrog, he says, may represent a rare case of rapid evolution that occurs because of chromosome duplication, changes in behavior and reproductive isolation.
Evolutionary import aside, Gerhardt is not averse to embracing the more fanciful side of his chosen specialization. Frogs, after all, long ago established themselves as stars of folklore whimsy and wonder. And after decades listening to calls in the field and lab, has he learned to speak frog? “I certainly can,” Gerhardt says, only partly kidding. “I can get gray treefrogs to come to me. I can get green treefrogs to come to me.”
Still, he adds, “frogs are not cognitive giants. They’re not thinking about whether they’re going to the right species [to mate with] or get the best snail to eat. It’s just the way their nervous systems work.”