EPOW - Ecology Picture of the Week

So What's the Temperature?

Sound Spectrogram of Cricket Chirps, prob. Field Cricket (Gryllus sp.), Family Gryllidae
Portland, Oregon

Explanation:  What are we seeing in this week's image, and how does it relate to the temperature?

Think of a warm summer evening out in the back yard.  Half moon, gentle breeze, and ... crickets chirping.  

The image is a sound spectrogram of a chirping cricket in my own back yard.  I'm unsure of the species, but the genus is likely Gryllus, a common orthopteran found in many suburban areas.  

Once upon a time, actually in 1897, a professor named Dr. Amos Dolbear of Tufts University flashed on the idea that crickets sing faster when it is warmer out.  Well, he probably wasn't the first to realize this, but he did invent what came to be known as Dolbear's Law that relates cricket chirp rates to ambient temperature.  His formula looks like this:   T  =  50 + (N-40)/4   where T is the temperature in Fahrenheit (F) and N is the number of cricket chirps per minute.  

Folklore?

So I recently recorded a cooperative specimen and decided to put it to the test.  

I read my audio recording into my sound spectrogram program producing the image above that displays sound frequency in kHz (thousand cycles per second) on the vertical axis as a function of time (seconds) on the horizontal axis, with various colors representing volume (sound amplitude or dBA, decibel absolute), the brighter the color, the louder the sound.  You can clearly see the chirps.

And can count them.

I count 55 chirps over 22.7 second (using the computer) which = 2.4229 chirps/second = 145.4 chirps per minute.  So what's the temperature?  Plugging this chirp rate into Dolbear's Law results in T = 50 + (145.4 - 40)/4 = 76.4 degrees F.

However, my outdoor thermometer at the time read 68.1 degrees F.

Not totally precise.

Solving Dolbear's Law the other way, if the temperature was the predicted 76.4 degrees F, then the chirp rate must be  N  =  4T - 160  =  112.4 chirps / minute, far slower than the 145.4 I recorded.  The relationship does not seem to predict very accurately, at least for this individual cricket on this specific night in this particular back yard with this moon phase, as a pedantic logician might argue ... 

Apparently, Dolbear created his Law using the snowy tree cricket (Oecanthus niveus), not a field cricket (Gryllus sp.), and it is known that field crickets do not seem to follow chirp rate-temperature relationships nearly as consistently.  

As to why crickets chirp faster, in general, when warmer ... that's the topic of a future exploration into biomechanics and biochemistry.  Until then, enjoy the chirps.