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.
See how there’s a sharp white bar cutting across the chirp? That tells us that the frog chirp is mostly made up of a very loud note with a pitch of about 3000 cycles/second, or G7. And there are some softer overtones as well - those parallel red and pink bars.
To confirm this, let’s make a plot of all the different frequencies that add up to make this sound. This plot is like a recipe for the sound, that tells us which ingredient notes make it up (on the horizontal axis), and at what volume (on the vertical axis).
The peak frequency from the plot above is 3144 cycles/second (or G7), which agrees with what we saw before. How does this compare with the scientific data on this frog?
A classic paper from 1985, Sexual Selection in the Spring Peeper, measured the pitch of the call of 72 Northern Spring Peepers in a lab, and found that the average peak frequency was 3061 cycles/second. That’s pretty close to my field results. Sweet! Science working as it should.
The same paper goes on to show that the female Northern Spring Peepers preferred the males with the loudest calls, and also preferred the males that repeated their calls the fastest. So for example, if there were two males, one that chirps every 1.2 seconds, and the other every 0.9 seconds, then nine out of ten times, the females would pick the faster chirper. For the male frogs, chirping loud and fast is a winning strategy.
The benefits of being loud are apparent. If you’re a frog and you can call out louder then your fellow peeps, you’re likelier to get the female’s attention. But why do the female frogs prefer the fastest chirpers?
It’s because the chirping advertises the male frog’s fitness. The frogs that chirp the fastest tend to be heavier and in better physical condition. That’s because it takes energy to chirp. To chirp faster, a frog has to take in more oxygen, and consume more energy. The frogs that chirp the fastest are the ones with the greatest stamina. Like the fastest long distance runners, they’re able to sustain a high consumption of energy over a long duration.
Which leads us to another puzzle. The very thing that makes the male Spring Peepers attractive to the females - their loud, repetitive calls - would also make them far more conspicuous to any predators. So how do they manage to not get eaten?
One way that these spring peepers avoid predators is by emerging from hibernation very early in spring. But there’s a problem with this strategy. Early spring comes with bouts of cold temperatures, often dropping below freezing (currently the case as I write these words). So the question really boils down to this. How does the frog prevent itself from freezing? The answer to this question totally and utterly blows my mind - the frog doesn’t prevent itself from freezing. Instead, evolution has devised a way for this frog to stay frozen alive.
You see, when the frog emerges from its hibernation in early spring, the temperature can often drop below freezing. If the temperature is -2 or -3 C (27 F), the frog can survive because the water inside it remains in a supercooled state - below its freezing point, but not yet frozen. But once the temperature dips any lower, the water inside the frog can’t stay supercooled, and so it starts to freeze.
For most animals, this would mean a quick death. But not the Northern Spring Peeper. Studies have shown that this frog can survive frozen up to a week . The frog enters a state of suspended animation. Its breathing, blood flow and heartbeat shut down, and its limbs become frozen stiff. The water under its skin freezes, and the contents of its stomach become a solid ball of ice. More than half of the water in its body turns to ice. Yet it can survive in this frozen state for days, and when the temperature goes back up, the frog thaws and eventually goes back to hopping around.
So how does it pull off this incredible trick?
Here’s a clip by David Attenborough that explains this trick in a different species of freeze-resistant frog, the Wood Frog, that uses a similar strategy.
Usually when cells freeze, the ice expands and causes cells to rupture, killing the organism. But this frog, along with a handful of other North American frogs, have evolved an incredible strategy to protect it from frost damage. Within 5 minutes of ice crystals starting to form inside the frog, the frog’s liver goes into emergency rescue mode, and starts to dump glucose into the blood, which then spreads throughout the frog’s body. This sugar serves as a ‘cryoprotectant’ - a biological antifreeze that prevents ice crystals from forming inside the frog’s cells.
The moment that ice crystals form inside the frog, the clock is ticking. It’s a race between the freezing front that’s moving in from the outer parts of the frog, and the glucose being pumped to the rescue, from the inside out. The sugar levels are therefore highest in the frog’s core organs like the liver, heart, kidneys and brain, where glucose levels shoot up as high as 50-fold. Meanwhile, the exterior tissues like skin and skeletal muscle get less glucose because they’ve already started freezing. The really surprising fact here is that the sugar wasn’t in the frog’s bloodstream before the freezing started. It was pumped out by the liver immediately after the frog started to freeze. Once the frog thaws, the glucose is stored back into the liver, ready to be reused.
And this incredible adaptation helps keep these frogs safe. These freeze-resistant Nothern Spring Peepers can emerge earlier in spring than its predators can, ensuring many days of singing and breeding to their hearts content.
Storey, Kenneth B., and Janet M. Storey. “Persistence of freeze tolerance in terrestrially hibernating frogs after spring emergence.“ Copeia (1987): 720-726.
Storey, Kenneth B., and Janet M. Storey. “Natural freeze tolerance in ectothermic vertebrates.“ Annual Review of Physiology 54.1 (1992): 619-637.
Layne Jr, Jack R., and Joseph Kefauver. “Freeze tolerance and postfreeze recovery in the frog Pseudacris crucifer.“ Copeia (1997): 260-264.
Churchill, Thomas A., and Kenneth B. Storey. “Organ metabolism and cryoprotectant synthesis during freezing in spring peepers Pseudacris crucifer.“Copeia (1996): 517-525.
Forester, Don C., and Richard Czarnowsky. “Sexual selection in the spring peeper, Hyla crucifer (Amphibia, Anura): role of the advertisement call.“Behaviour (1985): 112-128.
Zimmitti, Salvatore J. “Individual variation in morphological, physiological, and biochemical features associated with calling in spring peepers (Pseudacris crucifer).“ Physiological and Biochemical Zoology 72.6 (1999): 666-676.
Wells, Kentwood D., Theodore L. Taigen, and Jennifer A. O’Brien. “The effect of temperature on calling energetics of the spring peeper (Pseudacris crucifer).“Amphibia-Reptilia 17.2 (1996): 149-158.
Here’s a incredible video by Robert Krulwich about a “springtime froggy miracle” - the resurrection of the freeze-tolerant Wood Frog.
Want to learn more about supercooling stuff, from horses to the universe? Check out this Radiolab podcast.
And here’s a nice explainer by Malcolm Campbell, that breaks down the science of how Wood Frogs and Birch Trees have evolved similar strategies to survive being frozen alive. Mind blowing stuff!