So you want to be a Spring Peeper, an amphibious, one-inch long, beloved herald of spring? I get it! Why wouldn’t you want to be a tiny tree-dwelling frog singing your heart out on a warm spring evening? In honor of Maine “Big Nights,” (warm, rainy spring nights during which frogs and salamanders migrate from winter habitats to breeding pools), here is what you’ll need to become the smallest, noisiest frog in Maine: the Northern Spring Peeper.
The Voice
Spring Peepers are famously loud—their genus name Pseudacris means “false locust.” The peeps of many males fill the air around breeding pools, creating a chorus that sounds like sleighbells. Small groups of two to four males often coordinate their calls, alternating peeps so that potential mates can hear and evaluate each frog. Females are attracted to volume and speed, and female choice pushes males to their limits; males can peep between 3,000 and 4,000 times per hour for several hours. This can add up to a staggering 13,500 peeps per night. They accomplish this with some serious oblique muscles and energy from fat stored the previous fall.
So what sort of mechanism do you need to join this deafening choir of determined frogs? Peeps are produced when the frogs close their nostrils and force air from their lungs over their vocal chords, so you’re off to a great start if you already have those! That sound is amplified by an air sac almost the length of the frog’s body that expands under the throat, allowing the sound to resonate. Choruses of spring peepers can be heard up to two miles away.
If all of this sounds like a bit much, don’t worry. You could always be a female Spring Peeper, which doesn’t sing. Or you could be a “satellite male.” These smaller males compete by using wits rather than songs. They wait near a loud, fast-peeping male, and when a female comes to check him out, the satellite male hops over and introduces himself before the singing male has a chance.
The Toes
If you want your peeps to travel, you’re going to need some elevation (since soft ground absorbs sound). Spring Peepers climb vegetation with specialized feet. The magic is in the toe pads: the surface of climbing frogs’ toe pads is extremely soft and pliable, allowing for high contact with uneven surfaces. Under a microscope, the structure of this surface is complex; tiny epithelial cells are surrounded by fluid-filled channels. These channels spread a mucous-like fluid produced by glands in the toes evenly over the whole toe pad.
This fluid results in wet adhesion. Wet adhesion is made possible by a combination of forces: surface tension (the property of water which makes denser objects, like a carefully-placed paperclip, float), the high cohesion of water (it likes to stick to itself), and capillary action (the same force which draws water from tree roots up into leaves and causes liquid to creep up a paper towel).
Those fluid mechanics, plus some help from good old fashioned friction, result in intense sticking power; one species of tree frog in the Amazon rainforest can hold fourteen times its body weight with its toe pads. This makes hauling their bodies up a tree trunk a breeze. Think of all the time you’ll save not waiting for elevators after becoming a Spring Peeper!
The Camouflage
One of your reservations about being a Spring Peeper might be vulnerability. Frogs are indeed a tasty little morsel for many a bird and mammal. As a species that spends most of its time in leaf litter, the ability to blend in is crucial. An identifying feature of Spring Peepers is the dark ‘X’ mark on their back. The lines are about the width of a blade of grass or leaf petiole (stalk), aka their typical backdrop on the forest floor. The wide, dark stripe through their eyes prevents their large eyeballs from standing out and revealing the frog as an animal. Visually, they melt into the leaf litter.
Since their lives depend on blending in, Spring Peepers have to adapt to various backgrounds, moisture levels, and lighting conditions. They accomplish this with dynamic coloration, a fancy way to say that they change color! An individual frog can become unrecognizable in about forty-five minutes, switching from light tan with very pale markings to dark yellowish-brown with a strongly pronounced ‘X.’ This change might follow the transition from dry leaves to wet leaves, full sun to shade, or a climb from the damp forest floor to a bright green leaf. Lab experiments show that Spring Peepers are attracted to certain visual textures and usually settle themselves on top of a stripe when available. The preference for variegated surfaces puts those ‘X’ markings to work, breaking up the outline of the frog’s body.
The Antifreeze
Your body is already full of glucose—it’s animals’ primary source of energy, and, together with oxygen molecules, fuels cellular respiration. It also happens to be one of this amphibian’s tricks to surviving winter without baseboard heating. That’s right—your heating bill will be $0 when you’re a Spring Peeper! Glucose is a sugar, so it lowers the freezing point of water. This is why maple syrup doesn’t freeze solid. It does the same thing inside the bodies of some ectotherms, (animals that don’t produce their own body heat).
Freezing presents some obvious complications to life; for example, essential functions like breathing and heart activity stop. Assuming that those functions could resume after thawing, however, the greater danger is ice. When water freezes in cells, the crystalline structures that form are sharp and can damage delicate structures like the nuclei. As if that wasn’t frightening enough, ice that forms between cells can rapidly draw water from normal, fully hydrated cells, rupturing their membranes. To protect precious cells, especially those in internal organs, water is pumped out of cells slowly and glucose is pumped in, turning cells into syrupy islands of concentrated glucose solution. Glucose therefore functions as a cryoprotectant, mitigating dangerous rapid dehydration and preventing the inside of the cell from freezing.
Spring Peepers can usually survive several days in this mostly-frozen state. They’ll freeze and thaw all winter as the ambient temperature under the leaf litter fluctuates. Functioning of major organs is regained within several hours after thawing, and complex motor functioning fully returns after one or two days. The three freeze-tolerant frog species in Maine, Spring Peepers, Gray Tree Frogs, and Wood Frogs, burrow only a few inches under the soil or leaf litter, well above the frost line. Surprisingly, the temperature remains relatively stable, and can be thirty degrees warmer than above ground. Snow adds important insulation. You might be over all this snow we’ve been getting, but if you were a Spring Peeper, you’d be exceedingly grateful! While sinking to unfrozen layers in ponds is a preferred strategy for some frog species, this strategy has served Spring Peepers well. They’re believed to have been one of the first amphibians to move north after the last ice age.
There you have it! That’s what you’ll need to become a Spring Peeper, the official State amphibian of Maine (as of last year!). I wish you all the best in your next adventure, and maybe I’ll see you on a warm, rainy evening this spring as you make your way to a pond or vernal pool to join a chorus of your peers in one of the most iconic sounds of the season.
If your human friends and family would like to help you and other amphibians cross roads safely during your nighttime migration, they can find out how to participate in a Maine Big Night here (hosted by Maine Big Night).
Maine Audubon is a Big Night kit host! To request a kit, email naturalist@maineaudubon.org.
