Walking around at night in Puerto Rico, you’re likely to hear the dulcet tones of the male coqui frog (Eleutherodactylus coqui). Beloved in Puerto Rican culture, the small brown tree frogs emit loud calls to attract females and fend off other males and are an integral part of the local ecosystem. But one thing may be putting a frog in these frog’s throats: climate change.
According to a study published today in the Proceedings of the Royal Society B, rise in temperature over the last 20 years may correlate with shorter and higher pitched coqui calls in a given area. Compared to other impacts of warming, altering frog calls in a region might seem less frightening. However, the researchers argue that the consequences could be dire—temperature’s potential impact on the frog vocalizations could come with body size reductions and ramifications across the local food web.
Coqui frog calls include two notes—a “co” note and a “qui” note. The “co” note is meant as a warning to other males, while the “qui” advertises to local females.
In a 1986 study, UCLA biologist Peter Narins found that those “qui” notes vary with altitude. Along a mountain road in the Puerto Rican rainforest, he and his student, Sheila Smith, sampled frog calls from elevations close to sea level (18 meters) to higher elevations (1000 meters) near the top of El Yunque Peak. Males at low elevations called rapidly at high frequencies or pitches, while males living near the top of the mountain produced slower, lower pitched calls.
In fact, the calls varied so much that high-altitude frogs were less responsive to low altitude calls and vice versa. The frogs also varied in body size: “It’s the same species, they just get bigger and bigger as you go up this mountain,” says Narins.
Frogs are ectotherms or “cold blooded” organisms, meaning their environment drives their body temperature. Temperature influences a coqui frog’s body size, and researchers think their size is key to the frequency of their calls and the pitch sensitivity of their inner ear. With global changes in temperature brought on by climate change, Narins wanted to go back and recreate their original study to see if changes in local coqui calls corresponded to changes in temperature.
In 2006, Narins and his colleague Sebastiaan Meenderink traversed the same road up El Yunque Peak and recorded the vocalizations of 116 male coqui frogs at 28 locations ranging from 10 meters to 1020 meters in elevation. From the recordings, they were able to determine the pitch and call duration of each frog. They also measured the body size of as many animals recorded as they could.
The researchers found that the frog calls heard in any particular region had changed significantly: both notes were higher and shorter compared to the original study.
However, the frogs themselves may not be undergoing physiological changes. Shrinking body size in response to climate change has been predicted in other studies, and the researchers were able to see a trend towards smaller frogs. But, it wasn’t clear if the observed shift in pitch and call length was a result of smaller frogs migrating further up the mountain to adapt to temperature shifts, or the result of new-found stunted growth in frogs living at specific elevations.
The frog call data could even be used to estimate how much the temperature had shifted over the last 23 years: Based on the change observed in call length and pitch in the two data sets and the rate of temperature change with altitude, the researchers predicted an increase of 0.26 to 0.86°C, while local weather stations detect an average uptick of 0.37°C.
If temperature continues to change at this rate, coqui frogs as a whole may sound and look quite different by the next century. “We think that at the turn of the next century for example, that these animals will be smaller. Their calls will be higher pitched than they are now, and they will be shorter in duration,” says Narins.
Let’s say the frogs are changing in physiology. Whether or not these changes impact the survival of coqui frogs depends on how the female coqui’s inner ear adapts to these changes. If females can’t pick up higher pitched calls, mating issues could cut coqui populations. Having fewer coqui or even smaller coqui to eat or to eat other organisms could destabilize the local food pyramid.
Long term studies of these physiological changes in frogs are rare, but they are becoming more and more useful in predicting how indirect impacts of climate change might pan out. Because they are so sensitive to temperature, frogs and other ectotherms may face higher risks with climate change in general, and their communication systems would be more indirectly at risk.
Just because an organism is warm blooded doesn’t mean they’re off the hook either: Rising air temperature alters the basic sound transmission in bat echolocation. And some scientists argue that ocean acidification will cause low-pitched sounds to travel farther under water, creating a cacophony beneath the sea surface that could disorient marine mammals and other animals sensitive to sound.
So for better or worse, thanks to climate change, the bioacoustic soundtrack of the next century could sound totally different from the one we listen to today.