Traffic noises stress frogs, but some have adapted

Frogs in noisy ponds near highways have changed their stress and immunity profile compared to those of quieter ponds – modifications that reduce the negative effects of traffic noise on amphibians. According to a new study, when calm pond frogs are exposed to traffic noise, this noise is stressful and hampers the production of antimicrobial peptides, an important defense mechanism against pathogens. However, frogs in ponds near highways show a reduced stress response and altered immune profile, both of which can reduce the cost of traffic noise. The study appears online on November 21, 2018, in the Journal of the Royal Society Proceedings B.

"In the United States, traffic noise can be heard almost everywhere," said Jennifer Tennessen, the newspaper's first author. She was a graduate student at Penn State at the time of the research and is currently a research associate at Western Washington University. "Noise can have a number of negative consequences for wildlife, for example by interfering with communication and reducing the ability to find food." Frogs are particularly vulnerable to noise because they depend on sound to find a partner. to breed in the spring to mate and lay their eggs, but many of these ponds are located near noisy roads.We wanted to know if the traffic noise had negative physiological effects on wood frogs and, in the affirmative, if they could adapt. "

The researchers collected eggs in ponds within 300 feet of highways and major highways, as well as in ponds in more isolated locations, less than 3 miles from major roads. Eggs were reared in the laboratory and the resulting frogs were exposed to traffic noise or ambient noise similar to that observed in the calm ponds for 8 days, about the time they would spend in the water. breeding pond.

The researchers found that the frogs in the calm ponds showed an increase in corticosterone levels, a stress hormone, after exposure to traffic noise for 8 days, which indicates that the noise is stressful. Exposure to noise also affected immune function in frogs at quiet sites, increasing the number of white blood cells called monocytes – an indication of an immune response to noise – and altering the production of important compounds on the skin called antimicrobial peptides.

"In the frogs of calm ponds, exposure to traffic noise has hindered the ability to produce antimicrobial peptides in the families of brevinines and temporins," said Louise Rollins-Smith, professor of pathology, microbiology and Immunology at the Vanderbilt University School of Medicine and author. paper. "Antimicrobial peptides are components of the immune defense system that provide important protection against pathogens such as bacteria and fungi." Peptides in the brinainin family in particular strongly inhibit the fungal pathogen responsible for cancer. infectious disease chytridiomycosis, responsible for the widespread mortality of amphibians around the world. "

Researchers also investigated whether frogs had developed ways to deal with these negative consequences of traffic noise.

"A long-term elevation of stress hormones can have negative immune consequences," said Tracy Langkilde, professor and director of biology at Penn State and lead author of the article. "We could therefore expect a suppression of the stress response after long-term exposure to stress in order to reduce the immune costs, such as the modified production of skin peptides that we have documented here." In the frogs of the noisy ponds, we see exactly that. "

Frogs in noisy ponds did not show high levels of corticosterone after exposure to traffic noise for 8 days, suggesting that frogs in ponds in very noisy areas have suppressed stress response.

"We do not know if frogs at noisy sites have a suppressed stress response to noise specifically or if they have a suppressed stress response overall," Langkilde said. "Both offer the advantage of avoiding stress-related noise costs, but an attenuated stress response in general could have other negative effects, for example, not being able to correctly prepare a behavioral response to predators. "

In contrast to quiet pond frogs, which had increased monocyte counts when exposed to traffic noise, frogs in noisy ponds actually had an increase in monocyte counts when exposed to ambient noise in the wild. calm ponds. Researchers believe that this type of immune response can occur in unusual situations and support the idea that frogs at noisy sites have adapted to avoid the physiological costs of noise.

"In the future, we hope to identify the mechanism by which frogs adapt to noise," Tennessen said. "The roads near the ponds that we studied were built between the 1940s and 1960s, so these changes could have occurred between 15 and 35 generations of frogs." People generally think that adaptation and the evolution unfolds on huge time scales, but here we see that animals can react relatively well to new threats quickly, even if the consequences of this response are still unclear. "