Walking through La Selva’s forested trails, many sounds greet you: peccaries chomping on leaves or tree roots, crested guans trilling from the canopy, lizards scuttling at sonic speeds over leaf litter, and cicadas chirping so loudly and monotonously that they almost fade into the wind. Cutting through these noises you can hear the call of the strawberry poison dart frog, a squeaky, repetitive ch-ch-ch-ch sound that can last anywhere from 10-40 seconds. Males call from their territories (about 2-3m diameter of forest floor) to advertise themselves to females, who have the luxury of choosing the guy with the most attractive salutation – usually, they go for males with fast, long, and loud calls. With all of the noises that surround them in the forest, how do male frogs decide when it is worthwhile to call, and when they might get drowned out? We set out to investigate this question empirically, and as our interference noise of choice we selected one of the most common natural noises that is likely to mask a frog’s call (since it occupies the same sound frequencies)—a cicada call.
Before we could perform any experiments, we had to catch our test subjects. Strawberry poison dart frogs are active throughout the day, but calling activity is highest from about 7-9:30 a.m. Their bright red aposematic (warning) coloration makes them easily visible against the drab shades of brown that compose the leaf litter, but unless you’re up close their small size (less than an inch long) often makes them undetectable by the human eye. Instead of going by sight, we relied on sound to guide us to the frogs. After just two days of practice in the field, we had our method down:
Step 1) Walk slowly down the forest trail with your ears peeled.
Step 2) When you hear a call (ch-ch-ch-ch….) gauge how close you think you are to the frog (walking with quiet feet in the direction of the sound with help with this). If it’s more than about five meters off the trail, forget about it—the frog will stop calling well before it comes into view. But if it’s nearer than that, proceed to step 3.
Step 3) Contrary to intuition, do not try to sneak up on the frog. Most will cease to call when they hear footsteps approaching even from a decent distance, so taking brisk strides in the direction of the call will get you closer to the frog before it stops calling.
Step 4) Get as close as you can before the call stops. Scan the area, taking care to check perching hotspots, such as tree roots, logs, and tops of leaves (exposed places where a female would be likely to see him). Nine times out of ten, a little strawberry-red dot will catch your eye.
Step 5) Lunge. These frogs move in quick, jerky hops that make them hard to follow or corner. So roll up your sleeves, keep your eye on the prize, and grab!
These tactics worked pretty well for catching male frogs, but we also needed some females to provoke the males to call in the lab. Since females don’t provide calls to follow in the forest, we had to rely on sight and a bit of luck to find females hopping along the forest floor. Males and females are easily distinguished by examining the gular (throat) region, which is dark grayish-blue on males and red or asymmetrically patterned on females.
Back in the lab, we set up a cardboard box arena to conduct our experiments. We placed one male and one female on opposite ends of the arena and waited for the male to call, which usually happened after only a minute or two. When the call began, we started audio recording on our fancy recording device, i.e. the voice memo app of an iPhone 5. Depending on whether the trial was a “noise” or a “silence/control” treatment (predetermined) we would wait 10 seconds into the frog’s call and then either play the cicada call playback (obtained from Macaulay Library, Cornell Lab of Ornithology) from a speaker for 10 seconds or do nothing during that time. We continued to record the frog calls for 10 seconds after cicada playback to get an idea of how quickly they can rebound from being interrupted by interference noise.
After testing over 60 frogs, we ended up with a pretty nifty result. Frogs lowered their call rates (speed of the ch-ch’s) during cicada noise compared to pre-playback, indicating a decrease in calling investment when faced with interference noise. Interestingly, males neither increased nor decreased their call volume from pre- playback to during the cicada noise, suggesting that males always call at their maximum volume and maintain it even during masking noise. This could mean that energy expenditure is mediated more by how fast the calls come rather than how loud they are; in other words, calling loudly is not as big an investment as calling rapidly.
What does this mean for frogs? Cicadas and other sources of noise are ubiquitous in the forest, and a frog that calls to attract females only in periods of silence will likely suffer reduced reproductive success. Studying how natural acoustic interference affects animal communication can help us predict the effects of added anthropogenic noise as forested areas become more developed.