I was out on a run when I heard this intriguing sound, a chorus of high-pitched chirping noises. Curious to find out what was making this sound, I strayed off the trail and followed the chirps through a field of reeds. Here's what I saw (and heard).
Hmmm... There wasn't much I could see, although I could hear a ton of activity.
I tried to sneak up to a lone, strident voice from the choir. Many failed attempts later, my legs were covered in scratches from bumbling around in the reeds, but I finally managed to get a glimpse of the creature making this sound. It was a tiny brown frog, small enough to sit on your fingertip. The pond was transformed into a chorus of these frogs, all trying to outdo each other in attracting females.
I got close enough to record the sound that made by one of these little guys, and asked twitter to help me identify the frog. (Hit play to hear the recording).
Happily, I woke up the next morning to a tweet from my friend, science writer Sarah Keartes (twitter, website), who forwarded my request to her frog enthusiast colleagues at EarthTouch. They were able to identify the frog as the Northern Spring Peeper, a frog whose recurring cheeping sound mark the onset of spring in Northeast USA.
The latin name of the Northern Spring Peeper is Pseudacris crucifer, which sounds more like a comic book villain or a badass rapper than a tiny frog. A quick youtube search convinced me that this was indeed our guy. Here's what the male looks like when it emits this sound.
And here's a closer look.
That huge pouch that you see is a vocal sac, and it inflates up to be nearly as large as the frog. It's this acoustic resonator that allows this tiny frog to emit such a loud and shrill chirp.
Back home on my computer, I isolated a sample of the frog cheeping from my recording. Click below to listen to this sound and see the shape of the audio signal. Essentially, this is a plot of how your speakers have to wiggle in order to play back the frog's chirp.
This frog's call was remarkably consistent. It was almost exactly the same pitch every time, and precisely timed at evenly spaced intervals, about 40 to 50 times a minute. Neat.
The chirp sounded very shrill, but does it have any overtones, like when a singer hits a note? Or is it a pure note, like when you strike a tuning fork? To find out, I viewed the above plot as a frequency plot. (Mathematically, this is known as taking the Fourier transform, and I've written more about that here).
If you haven't seen these spectrograms before, they're a little tricky to read. On the vertical axis are the different frequencies (or pitches) in the sound, and as before, time ticks from left to right. Think of this plot as distilling the frog's chirp into its constituent notes - low notes on the bottom, and higher notes on top. The hotter colors represent a louder sound, going from cool blues to warm reds to ultra-hot white.