Crickets: Nature's Thermometer

In 1897, a physics professor at Tufts University named Amos Dolbear spent time counting cricket chirps. He noticed the rate changed with the temperature. Being a physicist, he wrote down the numbers and looked for the relationship.

He published what he found in The American Naturalist. Forty chirps a minute means fifty degrees Fahrenheit, and every four chirps above that is one degree more. Counted in a quarter of a minute instead, it collapses into arithmetic you can do standing still.

He gave it no name at all. The paper is two pages and ends on a worked example. The name arrived thirty-eight years after he died, in a book by someone else, and Dolbear's Law is not named after an entomologist or a naturalist. It is named after a physicist who was paying attention to the wrong thing and found something real.

The formula works because crickets are ectotherms. Their body temperature tracks the air around them, and the processes that drive muscle contraction, the enzyme-catalyzed reactions that power each movement, run faster as temperature rises. A cricket chirp is a muscle event: the male lifts his forewings and closes them, dragging a hardened edge on one wing across a file of teeth on the other. Warmer air, warmer muscles, faster reactions, more chirps per second. For this group of crickets, chirp rate rises almost exactly in step with temperature across the range they sing in, which is why a simple formula holds at all.

The species the formula fits best is the snowy tree cricket, a pale, almost translucent insect that lives in shrubs and low trees. It did not have a name of its own in 1897; Thomas Walker separated it out as Oecanthus fultoni in 1962. Dolbear never said which cricket he was listening to. Its song is a run of separate, evenly spaced chirps, close to three a second on a warm night, rather than the buzzy pulse of the field cricket most people picture. The regularity is what makes it countable, and countable is what made Dolbear's observation possible in the first place.

In September, across most of the eastern United States, the chorus is at full strength, and for one species that is a measured fact rather than an impression: fall field crickets were counted through a whole season in southeastern Michigan and peaked around the tenth of September. Snowy tree crickets sing from early August into late October. Frost does not switch them off the way you would expect. Field crickets have been recorded singing past heavy frosts into the second week of November, moving their song into the daytime once the nights turn cold. It is easy to stop noticing any of it, the kind of background noise that becomes furniture. It is also a real-time temperature measurement, running continuously, broadcast at whatever volume the night requires.

The test is simple and takes thirteen seconds. Find a cricket, an open window will work. Count its chirps for thirteen seconds. Add forty. That is roughly the temperature in Fahrenheit where the cricket is sitting. Dolbear's own count was a quarter of a minute, and a century of measurement has since trimmed it to thirteen seconds. The cricket was right the whole time; we got better at listening.

He published that formula in a natural history journal and moved on. The cricket kept reporting.

Going outside on a warm late-summer night and standing still enough to count is a way to use your attention on something specific. The ambient sound that reads as background is a measurement. Rewyld exists for moments like this: when what the living world is doing and what you are actually paying attention to become the same thing.