Mating call of the Pacific chorus frog can’t compete with traffic noise

CORVALLIS, Ore. – It’s a little frog with a big voice – and a big problem.

The male Pacific chorus frog, known for its classic “rib-bit” mating call heard across long distances, doesn’t adjust its call to compensate for nearby traffic noise that occurs at the same frequency, according to a study by Oregon State University researchers.

Frogs can change several aspects of their call structure, including duration, calls per minute, and frequency, and they can do this in response to high levels of noise, said Danielle Nelson, a doctoral student who led the study.

“The frogs we studied kept calling at the same frequency and at the same time,” Nelson said. “We don’t think they can change. This is a serious problem for maintaining this frog’s populations in noisy traffic areas. The females orient to individual males through their calls. If they can’t find each other to mate, how is their population going to persist?”

The study spotlights the intrusion of human-related noise on wildlife habitat, said Tiffany Garcia, an aquatic ecologist in the OSU College of Agricultural Sciences. Because frogs are less mobile than birds or bats, they may be more impacted by high levels of noise at breeding sites.

“This is a ubiquitous little frog that you wouldn’t think would have problems calling. But it does,” Garcia said. “We’re seeing urban sprawl occurring around our big cities that is impacting wetland habitats. Human activity is impacting this frog’s ability to communicate.”

The finding was published in the journal Ecology and Evolution: Calling at the highway: The spatiotemporal constraint of road noise on Pacific chorus frog communication.

The researchers recommend adding noise barriers along highways to mitigate the traffic noise, ranging from concrete walls to hedges.

The researchers set up recorders at 11 sites in Oregon’s Willamette Valley. The sites were chosen for their range of noise exposure, from relatively quiet wildlife refuges to some that were just off Interstate 5. They recorded the frogs from February to May at the peak time of the frog’s call – from 4 p.m. to midnight. The team also recorded about 100 individual frogs at all 11 sites.

“Pacific chorus frogs call in the upper frequencies that overlap with road noise,” Nelson said. “Rush hour falls just about when chorus frogs are calling, so they are being interfered with there, as well.”

At about two inches in length, the Pacific chorus frog, also known as the Pacific tree frog, is the smallest and most commonly heard frog in Oregon. The males often repeat their call many times in an effort to attract females for breeding. This calling stimulates other males to join in, and large concentrations of these frogs can be heard from far away.

Pacific chorus frogs live in wetlands, meadows, woodlands and brushy areas and breed up to only a few months a year in shallow ponds, slow moving streams, seasonal pools, watering tanks and roadside ditches.

Many of the 6,000-plus species of amphibians around the world are disappearing. About 30 percent of amphibian species worldwide are considered threatened, according to the International Union for Conservation of Nature.

“There are a lot of reasons why amphibians are declining and humans have a lot to do with them,” Garcia said. “We are changing the climate. We are forcing them out of their habitats, and we are changing their habitats at an unprecedented rate. Oregon is a hot spot for amphibians. We have a rich biodiversity of frogs and salamanders. We need to protect the ones that we have.”