When the time is right, a good love song can make all the difference.
A study from the University of California, Davis, found that temperature affects the sound and quality of male frogs' mating calls. In the colder, early weeks of spring, their songs start off sluggishly. In warmer weather, their songs pick up the pace, and female frogs take note.
Better songs not only make the males more attractive mates, but they also suggest to females that environmental conditions are suitable for reproduction.
"The song of frogs really depends on the temperature of the environment," said lead author Julianne Pekny , a UC Davis graduate student in the Department of Wildlife, Fish and Conservation Biology when the study was conducted and currently director of conservation science with the Amphibian and Reptile Conservancy in North Carolina. "As ponds warm, male frogs go from sounding slow and sluggish to faster and almost desperate. I can hear it with my human ears, and female frogs are also paying attention."
For the study, published today in Frontiers in Ecology and the Environment, Pekny took to the water's edges of Quail Ridge Ecological Reserve and Lassen Field Station, which are part of UC Natural Reserves. Microphone in hand, she recorded the hot and cold love songs of the Sierran treefrog to gauge the relationship between water temperature and their breeding calls. Songs from the same male were faster when water was warmer, both of which females typically prefer.
"What's interesting to me is this could be a process by which females are tracking how seasonality is changing over time," Pekny said. "As the pond warms, the sexier male calls come earlier, too."
Climate shifts and conservation
The results carry implications for conservation amid climate change. About 41% of amphibian species are threatened with extinction, making them the most endangered vertebrate class. Understanding when they breed, how that may shift as the climate warms, and what is driving those shifts is critical to their conservation.
As breeding time approaches, male frogs concentrate in large numbers around ponds and other aquatic areas. They arrive much earlier than the female frogs and begin to warm up their "voices." But the females do not necessarily come to the pond just because the males are calling. The time has to be right for her eggs to survive. That clue, the study found, lies in the quality of the male's song, which is more attractive once it's warmer.
"It's in the best interest for males to get to the pond as early as possible, before other males," said coauthor and herpetologist Brian Todd , a professor in the UC Davis Wildlife, Fish and Conservation Biology department. "But it's in the best interest of females to get there when it's actually time to go and lay their eggs."
Timing is everything
UC Davis Professor Eric Post , senior author on the study, studies phenology, or the timing of cyclical events in nature, such as flowers blooming, insects emerging and frogs breeding.
"This could potentially revolutionize the study of phenological responses to climate change," Post said. "We're emphasizing a new understanding of the role of springtime vocalizations by frogs and toads. Males may be unwittingly signaling nuances about the appropriateness of environmental conditions for breeding, and females interpret these signals beyond the intentions of males."
Post noted that the study may also apply to insects that produce mating calls: "The implications are fascinating and potentially quite broad."
Listening for spring
The sound of frogs singing at a pond is among the first signs of spring for humans, too. Pekny encourages people to listen for themselves.
"If you can listen over the course of weeks or longer, see if you can notice the difference," Pekny said. "Imagine how the female frog might perceive those male calls at the beginning of the season versus toward the end."
(Listen to the difference between a hot frog song and a cold frog song .)
The study was funded by the Mildred E. Mathias Graduate Student Research Grant, Fran Maurer/Lenora Timm Quail Ridge Student Endowment, and Horodas Family Foundation.
