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Pleasant music like tiny sleigh bells breaks the nocturnal silence of a flooded woodland. First one bell is heard, soon accompanied by another and another until hundreds have joined the chorus. Spring has barely begun to warm the land and yet, on this early April evening, northern spring peepers, Wisconsin's smallest treefrogs, have emerged from hibernation and returned to the temporary ponds to renew the rites of spring and courtship.
Northern spring peepers, Hyla crucifer crucifer, measure just over an inch. Females are slightly larger. Their smooth dorsal skin may be light tan, gray, olive, or dark brown with a large characteristic X pattern on their back. The legs have a few dark cross bands, the belly is white, and the vocal sac is dark. Because of their small size and their ability to change skin color to blend in with the background, treefrogs are difficult to find. More often they are heard and not seen.
As one of the first spring sounds in the wetland, males begin calling when the water temperature warms to about 45°F. When singing, the males typically climb on and cling to vegetation overhanging the water. To produce the shrill, clear, high-pitched notes that sound like jingle bells, spring peepers push air from their lungs through two slits in the mouth to inflate a vocal sac. The inflated sac, resembling a thin sphere of bubblegum about to burst, seems much too large for the size of the tiny treefrog. And the peep seems much too loud to come from such a small creature. A full chorus of northern spring peepers has great carrying capacity and on still nights may be heard a quarter- to a half-mile away. At close range, the sound seems deafening. Spring peepers may continue to call until late May or early June when courtship ceases for another year.
Males call to attract females. Actually there is very little, if any, courtship. As soon as a gravid female arrives at the pond, an eager male clasps her from on top and behind her forelegs, a position termed amplexus. The clasping may stimulate the female to lay her eggs. As she deposits eggs singly or in clusters of two to three on submerged vegetation, the male is stimulated to release sperm, a form of external fertilization. Amplexus continues until she has laid 800-1,000 eggs. Only spring peepers know how long this egg-laying activity takes.
Each one-millimeter egg, protected by a clear, gelatinous membrane, hatches in two to three days into a three-millimeter tadpole. The young tadpoles feed on plant material, increasing in size, growing legs, and absorbing their tails. In June when the transformation is about complete, the froglets leave the pond for the woodland and their diet switches to delectable insects such as mosquitoes and flies.
Treefrogs are typically walkers and climbers, not jumpers. They've adapted to a terrestrial and arboreal existence aided by discs on their toes. Just below the disc surface are special glands that secrete a sticky, adhesive fluid that enables treefrogs to adhere to slippery, vertical surfaces. That's why you may have been surprised to see a treefrog clinging to a cabin window. In autumn, males may occasionally be heard calling from the trees, perhaps to practice, for no mating occurs at that time. For winter hibernation, spring peepers select secure spots under logs or tree bark.
Enjoy the spring music as the amorous chorus of tiny songsters heralds winter's retreat from Wisconsin's swampy woodlands.
Anita Carpenter listens for nature's lovesick crooners on walks near her Oshkosh home.