How frogs adapt to life near noisy highways

(Credit: Getty Images)

Researchers have discovered that frogs living in ponds near noisy highways have altered stress and immune profiles compared to frogs from quieter ponds—changes that reduce the negative effects of traffic noise on the amphibians.

When researchers experimentally exposed frogs from quiet ponds to traffic noise, however, they found the noise stressed the frogs and impaired their production of antimicrobial peptides—an important defense mechanism against pathogens.

“In the United States, traffic noise can be heard nearly everywhere,” says first author Jennifer Tennessen, who was a graduate student at Penn State at the time of the research and is currently a research associate at Western Washington University.

wood frog on leaves
Wood frogs rely on sound to find mates and reproduce, but many breeding ponds are near noisy roads. (Credit: Lindsey Swierk)

“Noise can have a number of negative consequences on wildlife, for example by interfering with communication and reducing the ability to find food. Frogs are particularly vulnerable to noise because they rely on sound to find mates and reproduce,” Tennessen says.

“Wood frogs travel to ponds in the spring to mate and lay their eggs, but many of these ponds are located near noisy roads. We wanted to know if traffic noise has negative physiological effects on wood frogs and, if so, whether they can adapt,” she says.

Quiet, please

The researchers collected eggs from ponds located less than 300 feet from interstates and major highways and from ponds in more isolated settings, up to three miles from major roads. The researchers raised the eggs through metamorphosis in the lab, and then exposed the resulting frogs either to traffic noise or to ambient noise similar to that at the quiet ponds for eight days, about the length of time they would spend at breeding ponds.

Frogs from the quiet ponds had increased levels of the stress hormone corticosterone after exposure to traffic noise for eight days, indicating that the noise is stressful. Noise exposure also affected immune function in frogs from quiet sites, increasing counts of a type of white blood cell called monocytes—an indication of an immune response to noise—and impairing the production of important compounds on the skin called antimicrobial peptides.

“In frogs from quiet ponds, exposure to traffic noise impaired the ability to produce antimicrobial peptides in the brevinin and temporin families,” says Louise Rollins-Smith, professor of pathology, microbiology, and immunology at the Vanderbilt University School of Medicine and an author of the paper.

“Antimicrobial peptides are components of the immune defense system that provide important protection against pathogens like bacteria and fungi. Brevinin-family peptides in particular strongly inhibit the fungal pathogen that causes the infectious disease chytridiomycosis, or chytrid, which is responsible for widespread mortality of amphibians around the world,” Rollins-Smith says.

Stress response

The researchers also investigated whether frogs have developed ways to deal with these negative consequences of traffic noise.

“Long term elevation of stress hormones can lead to negative immune consequences,” says senior author Tracy Langkilde, professor and head of biology at Penn State. “So we might expect to see suppression of the stress response after long-term stress exposure to reduce immune costs, such as the altered production of skin peptides that we documented here. In frogs from noisy ponds, we see just that.”

Frogs from noisy ponds did not have elevated levels of corticosterone after exposure to traffic noise for eight days, suggesting that frogs from ponds in high-noise areas have a suppressed stress response.

“People usually think of adaptation and evolution occurring over huge timescales, but here we see that animals can respond relatively quickly to new threats…”

“We’re not sure if the frogs from noisy sites have a suppressed stress response to noise specifically or if they have a suppressed stress response overall,” Langkilde says. “Both offer the benefit of avoiding stress-related costs of noise, but having a dampened stress response in general could have other negative effects, for example not being able to properly mount a behavioral response to predators.”

Unlike frogs from quiet ponds, which had increased monocyte counts when exposed to traffic noise, frogs from noisy ponds actually had increased monocyte counts when exposed to the ambient noise heard at quiet ponds.

The researchers believe that this kind of immune response may happen in unfamiliar situations, and supports the idea that frogs from noisy sites have adapted to avoid the physiological costs of noise.

“In the future, we hope to pinpoint the mechanism of how frogs are adapting to noise. The roads near the ponds we studied were built between the 1940s and 1960s, so these changes may have occurred within 15 to 35 frog generations,” Tennessen says.

“People usually think of adaptation and evolution occurring over huge timescales, but here we see that animals can respond relatively quickly to new threats, though the consequences of that response are still unclear.”

The study appears in the Proceedings of the Royal Society B.

Additional coauthors are from Penn State, Vanderbilt University, Syracuse University, Binghamton University, and the University of Pennsylvania School of Veterinary Medicine. Penn State, the Penn State Applied Research Laboratory, Sigma Xi, the American Society for Ichthyology and Herpetology, and the National Science Foundation funded the work.

Source: Penn State