Frogs get a hop on noise pollution

In an increasingly noisy world, it may seem impossible to find peace amongst the cacophony of busy roads, blaring sirens, and roaring machinery.

That is, unless you are a wood frog.

New research from Pennsylvania State University shows these pocket-sized pollywogs are becoming immune to the stresses of noise. Compared to their quiet country cousins, wood frogs living near roadsides did not show the same kind of stress response to traffic noise, suggesting they may be adapting to noise pollution.

Researchers have long known that noise can alter how wildlife behave and communicate, but this is the first evidence that animals can rewire the way their bodies cope with noisy environments.

As natural sounds are being drowned out by noise pollution from human activities, both people and wildlife are feeling stressed. But while we seek refuge behind white noise machines and meditation apps, wildlife are left with few coping strategies, and their stress hormones skyrocket.

When those hormones are elevated around the clock, they can have a laundry list of downstream effects on animals, said lead author Jennifer Tennessen.

“Chronically elevated levels of stress hormones can have all sorts of negative effects on wildlife,” Tennessen said. “They can suppress growth, reproduction, [and] immune function.”

Noise can also put a real damper on amphibious romance. Back in 2014, Tennessen found that female wood frogs exposed to road noise had sky-high stress hormone levels and were less likely to look for mates.

But during that study, something else caught Tennessen’s attention: Even though wood frogs were stressed by noise, she still found lots of them living next to roadways.

“If noise is stressful, how is it that frogs are able to survive and reproduce in these potentially stressful environments?” she asked. “See[ing] that there were still wood frogs in these noisy sites led us to wonder, well, maybe they’ve adapted.”

To investigate further, Tennessen collected frog eggs from quiet and noisy ponds and hatched them in identical conditions. Once the frogs reached adulthood, she compared their reactions to traffic noise. Frogs from quiet areas were stressed by noise and less healthy as a result; they had elevated stress hormones, stunted growth, and difficulty producing a natural antifungal agent on their skin.

But frogs from noisy sites did not experience stress like their counterparts from quiet areas. When exposed to traffic noise, their stress hormones did not budge from baseline levels.

“We were excited to see that that yes, it does appear that [the frogs] have adapted to the stress from traffic noise, at least in the populations we studied,” Tennessen concludes.

That is good news for these frogs, but Tennessen is hesitant to hail this as an ecological triumph of animal over pollution. She hopes to investigate the noise-tolerant frogs further; if many frogs died off in the process of the populations adapting to noise, those remaining may be inbred and thus less capable of adapting to future environmental changes.

She also wonders if there might be potential negative side effects of being immune to stress.

“The stress response is… [shared] across vertebrates because it’s so useful,” said Tennessen. “It’s a necessary step for initiating key processes of the body to aid in survival, so if all of a sudden the stress response is muted, then perhaps escaping from predators won’t be as effective.”

In other words, a fearless frog is a dead frog.

Overall, though, Tennessen finds her results to be encouraging in the uphill battle of conservation.

“A lot of wildlife populations have some degree of resilient capacity wired into them,” she says. “And so, it’s sort of a reason to be hopeful.”