Frog Mating Calls as Climate Change Indicators

When ice and snow begin to melt in California's high-elevation waters, male frogs hop into action. Emerging from hibernation, they head to lakes and ponds scattered throughout the area and begin calling to females. They have limited time to mate before the water freezes again.

“There's just these really short windows of time that animals have to really get quite a lot done,” says climate change ecologist Julianne Pekny, director of conservation science for the Amphibian and Reptile Conservancy. Some bodies of water are ice- and snow-free for only a couple months.

When male Sierran tree frogs first arrive on the scene, for example, their croaks are distinctly slower and less frequent than later in the season when it’s warmer, Pekny and colleagues report in a recent study published in Frontiers in Ecology and the Environment. Based on data collected when Pekny was a graduate student at the University of California, Davis, the scientists argue that female frogs pick up on the change in the mating calls’ cadence, too—and use those cues to help them decide which waters are warm enough to lay eggs in.

That information is particularly important in places where there is a short window for depositing eggs. And as seasons shift in response to climate change, this amphibious weather app of sorts could help females adjust their mating decisions accordingly.

Pekny, who is also the director of ecological data analysis for Tangled Bank Conservation, says ecologists have assumed that frog choruses are just relaying information related to mate desirability and competition. Male frogs use their croaks to advertise their location and availability to eligible singles in the area, with deeper, more robust ribbits generally emanating from larger frogs. In turn, those croaks—and the frogs they come from—are more attractive to females.

She-frogs are also more drawn to faster, more frequent calls. Pekny thinks that’s partially because females are tracking environmental change through those calls, not just mating signals. While male frogs book it to the water once weather warms, females hang back. That means that although they and the males experience similar air temperatures, it’s the males that know how cold the breeding habitats are.

The study outlines "a new way to think about frog calls as a dual-information system," says Adam Leaché, herpetologist at the Burke Museum of Natural History & Culture and the University of Washington, who wasn’t involved in this study. “It is interesting to think of mating calls as sexual signals and ecological signals at the same time.”

The inkling for Pekny’s theory first emerged while she was doing field work in Northern California, and could hear the difference in frog choruses throughout the season. She decided to collect 35 frogs from Quail Ridge Ecological Reserve in Napa County, and test how their ribbits reacted to temperature change. She put the frogs in aquariums with varying water temperatures and, with a tiny microphone in hand, recorded the call rate and duration of their croaks. Then, she shuffled the frogs to new aquariums to see how they would respond to different temperatures.

It turned out the colder water did make frog songs more sluggish, while the warmer water livened them up. The finding has been suggested for other frog species as well.

Pekny also found that existing literature on frog choruses tended to attribute females’ attraction to faster-paced songs to mate selection. But using male frogs as tiny, amphibious weathermen makes sense too, especially in a place with snow-melt lakes and ponds, she says. It’s in a female’s best interest to avoid arriving at the water too soon because males approach to mate shortly after her arrival.

“There's a lot of aggression from males,” Pekny says. “There's a risk of mortality for the female even, because it's that aggressive.” Water temperature can also vary across elevations, so it would be helpful for females to know which areas are ideal for egg-laying.

Luke Larter, a behavioral ecologist at Brown University who wasn’t involved with this study, isn’t so sure females are gleaning that information from calls—only further experiments will reveal if it’s the case. But he says the study still presents an important idea.

“We don't have a full picture of all the dynamics going on...that's why ideas like this are valuable,” he says. Pekny agrees further research is needed, in particular teasing apart a female’s surroundings and the acoustic landscape she’s exposed to. If her theory is true for the Sierran tree frogs, it could also be applicable to other temperate frog species that have narrow mating windows, Leaché says.

Given that amphibians are the most at-risk animal group globally—around 41 percent are threatened with extinction—scientists are particularly invested in understanding their interactions and relationships to their habitats. Overall, it seems that many amphibians, including frogs, are adapting their behaviors to match the rate of climate change. One grave consequence of that could be a timing mismatch between frogs and their prey. If frogs end up mating earlier as a result of earlier springs, their tadpoles could emerge before there’s enough food available. And later, adult frogs could be hopping around prior to their prey.

“Climate is really affecting all wildlife across the board,” Pekny says. “The actual impacts of it are more difficult to understand because [frogs] are so connected to other animals.”