Frogs don’t like living near noisy highways any better than people do, but research from Vanderbilt suggests that frogs, like hardened city-dwellers, can learn to adapt to the constant din of rumbling trucks, rolling tires and honking horns. And, just like those urbanites who can’t get a good night’s sleep without the sporadic sounds of sirens, some frogs have grown accustomed to the rattle and hum of the highway.
“The broad interpretation is that frogs adapted to noise are better able to cope with noise,” said Vanderbilt professor Louise Rollins-Smith, PhD, who conducted the research in collaboration with Penn State and three other institutions. “It suggests that these populations that are exposed to noise from the time of road building, which is 1940s, 1960s onward, have actually kind of evolved to accept these kinds of noise conditions.”
Published in Proceedings of the Royal Society B, the study suggests that traffic noise is harmful to frogs, yet frogs can adapt.
To accomplish the study, researchers collected eggs of the wood frog Rana sylvatica from ponds in quiet locations and noisy locations, such as near major highways. Back in the laboratory, the eggs were allowed to hatch and undergo metamorphosis, and then the frogs were split into groups and exposed to a recording of either ambient noise or traffic noise for eight days.
“The main thrust of this,” said Rollins-Smith, “is that the ones from quiet places actually were stressed by the [traffic] noise and the ones that came from noisy places were not so much bothered.”
One of the findings was that traffic noise reduced the ability of frogs originally from quiet ponds to produce the antimicrobial peptide (AMP) brevinin-1SY. AMPs are short proteins that confer protection against a wide range of threats, including viruses, bacteria, fungi and parasites. In frogs, AMPs are secreted from specialized glands in the skin.
“Most species [of Rana] make quite a number of them, and this [species] makes only one well defined and tested antimicrobial peptide,” said Rollins-Smith, a professor of Pathology, Microbiology and Immunology.
AMPs, like brevinin-1SY, inhibit Batrachochytrium dendrobatidis, an aquatic fungal pathogen that is associated with global amphibian declines.
The fungal pathogen “enters through the [frog] skin. And so, this layer of antimicrobial peptides in the mucus of the skin is one of the protective defenses,” Rollins-Smith said.
This finding suggests that traffic noise may contribute to global amphibian declines by reducing the ability of wood frogs to defend against infection.
Traffic noise also impacted frogs’ immune and stress responses. When frogs originally from quiet places were exposed to traffic noise, researchers saw an increase in the number of monocytes, a particular type of white blood cell. But for frogs from noisy places, it was ambient noise that caused an increase in the number of monocytes.
A similar trend was observed for a hormone that becomes elevated in response to stress. Rollins-Smith said, “They were more accustomed to noise, so when it was too quiet, they responded differently.”
The work was supported by Pennsylvania State University, Sigma Xi, American Society for Ichthyology and Herpetology, and by National Science Foundation.