A chorus of túngara frogs sounds like a Star Wars movie in the rainforest. The male frogs produce two calls: a sharp zing like laser fire and a monotone, robotic staccato. Sabrina Burmeister believes that the frogs, despite their futuristic sounds, will help shed light on the evolution of communication.

Social communication between humans is complex. Simply talking on the phone requires coordination of intricate pathways in the brain. How these pathways came to exist is unknown. “There are a lot of mysteries underlying how complex nervous systems have arisen,” Burmeister says.

It’s hard to study the evolution of the brain because brain tissue rarely fossilizes. The best method for recreating the brain of an ancient animal is to make a mold of the inside of its skull. This mold, called an endocast, can be used for making broad comparisons of brain structure, but not much else. To study the more elaborate features of the brain, scientists have to rely on living animals.

Frogs are good models for communication because their simple brains make it easier to study complex communication circuits. And frogs have predictable communication behavior: they only produce calls when they’re breeding.

To study the túngara frog’s breeding behavior in its natural habitat, Burmeister and her students travel to field stations in Central and South America. When Burmeister bought a map for a trip to Guyana, she was surprised to find only one major highway in the entire country. Guyana is connected by a network of small rivers, so the best way to get around is by boat.

A mating pair of tungara frogs. Photo by Sabrina Burmeister. ©2009 Endeavors magazine.

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Burmeister travels so far because the túngara frog has a special breeding strategy: female choice. In colder climates, frogs have shorter breeding periods, so strategies such as scramble breeding — where many males attempt to breed with a single female — are more common. Because it lives in a tropical climate, the túngara frog is able to breed almost year-round. The male frogs gather along the edge of a mud puddle and call out to the females in the middle. A female selects her mate by swimming over and touching him.

When working with wild animals, Burmeister admits, there’s a lot of variability. The team created standardized conditions by testing all of the frogs in acoustic chambers at the field station laboratory. They put female frogs into the chambers and then played recorded mating calls from the male túngara frog and other species. The female frogs were more active when they heard the túngara calls.

Burmeister and her students were interested in the brain activity stimulated by the calls. They analyzed the females’ brain tissue for genes that indicate neural activity. Lisa Mangmiele, a grad student in Burmeister’s lab, examined a collection of genes called immediate early genes. When part of the brain is stimulated by a sound, sight, smell, or some other sensation, neural activity is triggered like a row of falling dominoes. This activity turns on the immediate early genes.

By tracing the activation pattern of the genes, Mukta Chakraborty, another grad student, was able to see where the dominoes fell. She found that the genes were activated in auditory regions of the brain. Also, the genes were more active when the túngara call was played than when the call of another species was played.

Chakraborty suspected that hormones play a role in response to mating calls. She injected nonmating female frogs with different hormones to see whether they would cause the females to approach a speaker playing the male mating call. She found that estrogen, or estradiol, was all that was needed to get them to approach. Burmeister thinks that the estradiol could be interacting with receptors in the brain that affect how the females interpret the mating call.

While hormones explain why females approach the mating call, they do not explain why females choose one male over another. Burmeister says females overwhelmingly prefer males that produce the staccato tone, also known as the complex call. Eighty percent of the time, females will choose the complex call over the laser fire tone (the simple call). But why?

Túngara frogs’ reproduction depends on the calls. So the auditory circuits of their brains are connected to regions associated with mating. Burmeister and her students examined the gene activity of each region wired to the auditory circuit. But so far their data don’t show an activity pattern that reflects a preference for the complex call. One possibility is that their tests are not sensitive enough to detect the pattern. Another possibility is that the pattern exists in a part of the brain, such as the olfactory region, that’s separate from the auditory circuit. If that turned out to be true, Burmeister says, “it would be a major shift in our thinking of how the frog brain accomplishes these tasks.”

If the complex call is better at attracting females, why have the simple call at all? Because the complex call can be very dangerous. “It attracts frog-eating bats,” Burmeister says. But she also suspects that testosterone, or other hormones or brain chemicals, may affect the frogs’ ability to produce the calls.

For now, Burmeister plans to keep her main focus on the females. Her next step is to pinpoint the brain region that selects the complex call over the simple call. Her students are examining regions outside of the auditory circuit that are active during sexual behavior. Burmeister also wants to make real-time recordings of the frogs’ brain activity with a technique called electrophysiology.

She’s planning to go back to South America to study communication in more frog species, especially relatives of the túngara frog. By comparing species, Burmeister hopes to learn more about how their common ancestors communicated. “If we can get a handle on what those ancestral brain structures might have been like, then we can hypothesize about how the brain has changed through evolution,” she says.