It’s a warm evening and the only sign of snow are the eroding piles beside the driveway. The spring peepers are in stereo. Down in the valley, there is a chorus erupting and up on the hill in a pond, another is just beginning to call. Spring has finally sprung.
The cacophony of calling is made up of hundreds of male spring peepers. Each peep call is made by forcing air from their lungs over vocal cords in the larynx and into an air sac in their throat. The air enters the sac from openings on each side of the mouth cavity causing it to balloon outward with air. The inflated sac amplifies and transfers the sound energy from the frog to my ears across the valley.
What sounds like chaos to me, is not to a peeper. Several males may interact vocally by forming duets, trios or quartets, with alternating peep calls and individual pitches. When males alternate calls, one individual, the follower, usually calls within 40 to 70 milliseconds from the end of the leader's call.
Males tend to stay within an auditory threshold of each other. Spacing and timing of calls accentuates the distinctiveness of each male’s calls so that it is not lost in the din of the chorus and allows a female to zero in on a single calling male. When another calling frog is too close, they may use a second type of call, a trill. The trill seems to reflect a higher degree of aggression than the peep call and may stimulate another individual to move away or to trill back, causing a trill-off. A trill-off can escalate into a brief physical interaction with the winner staying and the loser moving on to quieter waters.
Each male peeper can pump out from 3,000 to 4,000 peeps an hour for several hours each night. So it is not surprising that male trunk muscles, which help propel air from the lungs, average 15 percent of their body mass compared to only 3 percent for the quiet females. Aerobic capacity of trunk muscle is six times that of leg muscles and 17 times greater than female trunk muscle. Peepers derive about 90% of their energy for call production from fat reserves in these muscles. Males weigh on average about the same as two dimes, yet their sound pressure is comparable to the song of a warbler (about 4 quarters in weight) or a blackbird (a whopping eight half dollars in weight). These little peepers have big bellows.
The peeper mating system is based on female choice. The louder and faster he peeps, the better his chances of attracting a receptive female. When females were presented with recorded peep calls, they preferred peep rates double to those found in the wild. So there is strong sexual selection for males that can peep fast. Because of the strong directional selection for fast calling rates, males perform at or close to their physiological limits. But one night of binge calling doesn’t seem to wear them out. Males that have the higher peep rates in one night tend to have higher peep rates every night.
The evening temperature also affects calling patterns. On warmer evenings peepers call much more frequently. The consumption of oxygen increases with calling rate, which in turn increases with temperature. At a balmy 60 degrees peep calls are repeated up to 13,500 times per night.
All males don’t sing though. The bigger the chorus the more silent males one might find hanging around. These shy guys tend to be smaller than the big singers. They quietly wait near a good singer watching for a female that may be attracted to the peeps. And when she comes along, they use their quickness and agility to beat the singer to the female.
According to the Vermont Amphibian and Reptile Atlas, peepers can begin calling as early as March 15th with peak activity in early May. Chances are good that the spring peeper chorus is now earlier than in the past, according to a study by James Gibbs, from the State University of New York in Syracuse, and Alvin Breisch of the New York Department of Environment Conservation.
From 1900 to 1912 Albert Wright, an instructor in zoology at Cornell University, visited ponds around the campus daily each spring to record the date of the first calls of frogs. Ninety years later volunteers collected the same kind of information for the New York Amphibian and Reptile Atlas, allowing Gibbs and Breisch a chance to compare. Wright heard his first peepers on April 4th. Atlas volunteers heard them on average around March 20th, about 13 days earlier.
For us, it is spring music to our ears. For the peepers, it is a singing battle of life and death. I hope their spring tradition goes on and on.