“Peeps” and “quacks” fill the air these days. How is it that these frog calls travel so far? It’s all thanks to a thin membrane , or sac, that most frogs have. Note the single inflated sac of the spring peeper, and the paired sacs on either side of the wood frog’s head. These vocal sacs act as resonating chambers, causing the male frog’s mating call to be amplified and carried far (some species of frogs can be heard over half a mile away). Most frogs have one of three basic types of vocal sacs: a single throat sac (the most common), paired throat sacs (partially separated by connective tissue) and paired lateral sacs (completely separate chambers on either side of the head). Vocal sacs are outpocketings of the floor of the frog’s mouth. When calling, a frog closes its mouth and nostrils, and expels air from its lungs through the larynx and into the vocal sacs. The vibrations of the larynx emit a sound which resonates within the vocal sacs. The frog continues calling as muscles within its body wall force the air back and forth between the lungs and vocal sac. The thickness of the vocal sac wall in frogs varies. Typically, small frogs that call in the air (spring peeper) have thin vocal sac walls, whereas those that call in the water, particularly large species (green and bull frogs) often have thick-walled vocal sacs that appear swollen, not inflated like a balloon, when filled with air.
Although Spring Peepers emerged from hibernation about two months ago, on warm nights the males are still advertising for mates and will continue to do so into June. Let your ears guide you to the peepers as they call repeatedly, often while perched on low vegetation near water. Armed with a flashlight, look for the movement of their vocal sacs as they inflate and deflate as the peepers sing.
I heard my first peeper on March 18th, roughly two weeks earlier than in past years. These tiny members of the treefrog family begin mating rituals shortly after the end of hibernation. The males gather at small pools by the hundreds. Each male establishes a small territory and begins calling the familiar high-pitched “peep” quite frequently. The louder and faster he peeps, the better his chances are of attracting a receptive female. Males usually compete in trios, and the male with the lowest-pitched call usually starts the vocal competition. If you look closely at the peeper in the photograph you can see some snow fleas hitching a ride.
You may well have heard the single “peep” of a male spring peeper emanating from the woods recently. It does seem odd to hear this call now, often far from water, as we associate it with spring courtship. This phenomenon occurs so regularly in the fall that herpetologists have given it a name – “fall echo.” They speculate that peeper calling is spurred by light and temperature conditions, when fall climate conditions are similar to those of spring.