April 9, 2014 report
Study suggests global warming causing changes to the pitch of frog calls in Puerto Rico
(Phys.org) —A trio of researchers has published a paper in the journal Proceedings of the Royal Society B: Biological Sciences, describing field studies conducted by lead Peter Narins and others. He and colleagues have found that the pitch of male coqui frog calls in Puerto Rico has changed over the twenty two year period between 1984 and 2006, which appears to correlate with the amount of air temperature increase the frogs have been exposed to due to global warming.
Narins and a group of students visited Puerto Rico in 1983-84 and recorded frog calls (and frog body size) along a path that led up into the mountains. At the time, Narins noted that the frog calls changed in pitch in relation to the elevation at which they lived. The higher up they lived, the bigger they got and the lower the pitch of the calls. Narins and a new group of researchers visited Puerto Rico again in 2006, and repeated the exercise, following the same path and making recordings and measuring frogs. In so doing, they report that the frog calls have changed dramatically as has their average body size.
More specifically, Narins and his team found that it was the second syllable of the call that was most impacted. Prior research has shown that the first syllable is generally aimed at warding off other males, the second is directed at females. Over two decades, the second syllable has grown higher in pitch and lasts for a shorter amount of time. The team noted that some of the calls from higher elevations in 2006 were nearly identical to calls from frogs in lower elevations in 1986, suggesting a definite link between frog calls and changes in air temperature. Further analysis revealed that the amount of change in pitch could be mapped to the changes in temperature, which happened to also correlate to the actual average air temperature increase experienced by Puerto Rico over the same time span. The team also reports that average frog size has decreased as well.
Frogs are cold-blooded of course, which means they are more susceptible to changes in air temperature. The problem with changes in male frog calls is that quite often it means directing them at females that are unable to respond to them, or in some cases, hear them at all. That of course could lead to rapid population decline, or possibly extinction.
Temperature affects nearly all biological processes, including acoustic signal production and reception. Here, we report on advertisement calls of the Puerto Rican coqui frog (Eleutherodactylus coqui) that were recorded along an altitudinal gradient and compared these with similar recordings along the same altitudinal gradient obtained 23 years earlier. We found that over this period, at any given elevation, calls exhibited both significant increases in pitch and shortening of their duration. All of the observed differences are consistent with a shift to higher elevations for the population, a well-known strategy for adapting to a rise in ambient temperature. Using independent temperature data over the same time period, we confirm a significant increase in temperature, the magnitude of which closely predicts the observed changes in the frogs' calls. Physiological responses to long-term temperature rises include reduction in individual body size and concomitantly, population biomass. These can have potentially dire consequences, as coqui frogs form an integral component of the food web in the Puerto Rican rainforest.
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