New research by a Carnegie Museum of Natural History curator and her student shows that deforestation and increased road infrastructure may cause tree frogs in Borneo to increase the volume of their calls. The call of Kurixalus chaseni, a small brown and green tree frog, grew louder in response to playback of traffic noise. Such a change in call volume may be harmful to tree frog populations in the years to come. The study, entitled Effects of traffic noise on vocalisations of the rhacophorid tree frog Kurixalus chaseni (Anura: Rhacophoridae) in Borneo, was published in Raffles Bulletin of Zoology.
In September 2017, Yeo Zhi Yi, a student at Yale-NUS College (YNC) in Singapore, designed an experiment under the mentorship of Jennifer Sheridan, Rea Curator of Amphibians and Reptiles at Carnegie Museum of Natural History, who was an assistant professor at YNC and instructor of the Conservation Biology class. Yeo played recorded traffic noise from portable speakers to tree frogs at Danum Valley in Sabah, Malaysia during a class field trip. He observed that, unlike other frogs that shifted the frequency of their calls in particularly noisy settings, the tree frogs shifted the amplitude of their calls, the first time such a response has been seen in frogs.
“Calling louder is actually much more energetically expensive than calling at a higher frequency, and research shows that an increase of 3 decibels, like we saw in our frogs, doubles the energetic cost of calling,” says Sheridan.
Given the continuing expansion of road networks across the region, the increased biological cost of calling could make this species vulnerable to reduced growth and survival in areas that are within earshot of roads, even if those areas are technically protected. While legislation can protect specific areas from development, those same areas cannot be protected from noise pollution. This study helps demonstrate the indirect impacts that deforestation and road networks can have on wildlife.
“I was brainstorming research ideas for the class and read up on how different anthropogenic noises affected wildlife. This sparked my curiosity to find out whether traffic noise could affect calling frogs in more pristine habitats such as those in Danum Valley, but I wasn’t sure if my experiment would work,” says Yeo Zhi Yi.
“Zhi Yi came up with a simple and elegant experiment, but with acoustic research, you never know whether you’re going to see a response in a previously-unstudied species. I figured he had maybe a 50-50 chance of seeing any response at all, and I don’t think either of us expected to see a shift in amplitude but not frequency,” says Sheridan.
Research completed prior to this study suggests that anthropogenic noise can decrease foraging efficiency, increase anti-predator behavior, and alter mate attraction and territorial defense in many species, including frogs. For frogs, most data on traffic noise impacts comes from temperate regions or the neotropics, with little insight on the impacts of traffic noise within tropical Southeast Asia. Southeast Asia faces the highest rate of deforestation in the world, along with a rapid expansion of road networks, increasing the urgency of understanding the indirect impacts of this expanding infrastructure.
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Effects of traffic noise on vocalisations of the rhacophorid tree frog Kurixalus chaseni (Anura: Rhacophoridae) in Borneo
Abstract: Transportation networks are currently growing at rapid rates, and the increase in roads can have detrimental effects on biodiversity. Increases in anthropogenic noise have been demonstrated to negatively impact several types of behaviour across taxa, as well as to mask key elements of vocalisations required for communication. While much information has been collected for species in temperate regions, fewer data are available for Southeast (SE) Asian species. Given that SE Asia has the highest rate of deforestation, which is the largest driver of road expansion, more data are needed on the impacts of traffic noise on SE Asian species. To that end, we exposed calling tree frogs (Kurixalus chaseni) to traffic noise to determine the impacts of two levels (low and high) of anthropogenic noise on dominant frequency, mean amplitude, and signal rates. While there was no observed impact of exposure to low traffic noise, we observed an increase in mean call amplitude when frogs were exposed to high traffic noise.
Increased amplitude is energetically expensive compared to changes in frequency or signal rate, which indicates that increased traffic noise may have negative impacts on long-term fitness in this species. We encourage further studies on the relationship between traffic noise and reproduction in this and other species across the region.