Noise pollution from nearby cars on highways is known to bother many species. But researchers recently found some wood frogs (Lithobates sylvaticus), which rely on sound to find mates, have adapted to the noise, showing lower stress hormone levels and white blood cell counts when exposed to traffic noise.
A study in 2014 conducted by lead author Jennifer Tennessen showed that frogs exposed to traffic noise experienced higher levels of the stress hormone corticosterone, indicating that traffic noise stresses frogs. But then, Tennessen and her colleagues started noticing something strange.
“There were populations of frogs breeding alongside highways,” said Tennessen, a research associate at the department of biology at Western Washington University. “There was this mismatch going on. It didn’t fit what we know. We know frogs are stressed from noise, and prolonged stress has all sorts of detrimental consequences on body functioning that ought to make roadside habitats unlikely areas for populations to persist, so we wondered, why are we seeing frogs breeding in these areas?”
In a study published in the Proceedings of the Royal Society B: Biological Sciences, Tennessen and her colleagues raised wood frogs from eggs in the lab to find out why. They collected eggs from both noisy areas near highways and from quiet areas far from highways in Connecticut, Pennsylvania and New York. Then, after the frogs metamorphosed and grew, they randomly assigned the frogs to two groups in the lab. One group was exposed to eight days of continuous traffic noise. Another was exposed to a quieter control sound.
Then, the team collected blood plasma. The researchers found that frogs from noisy ponds that were exposed to traffic noise in the lab had lower levels of the stress hormone, corticosterone, than those from quiet ponds exposed to the same traffic noise signal. These frogs also had lower white blood cell counts in response to traffic noise, suggesting modified physiological stress and immune responses. On the other hand, frogs taken from quiet areas and exposed to traffic noise had higher levels of stress hormones and white blood cell counts.
“The picture emerging was, something is happening to frogs in noisy sites,” she said. “They’re not being stressed out by traffic noise.”
On the surface, this may seem like the frogs are resilient and adapting well to a changing environment, Tennessen, but, “from a management perspective, we don’t know what is lost in the process,” she said. She suggests populations may be smaller now, as only the individuals adapted to traffic noise may have been able to survive. She also said this may mean that frogs adapted to traffic noise may also have a lowered stress response to predators or other real stressors, which can be a problem.
“That’s where we have to be cautious,” she said. It could also indicate the individuals couldn’t mount an appropriate response at the appropriate time.”
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