The warm rains of spring awaken an explosion of activity in our nearby pools, ponds, and muddy puddles. If you visit at dusk, you are sure to be greeted by a chorus of many and varied voices as one or more species of frogs comes a-courting, in a celebration of sound. This singing is an important part of their yearly breeding cycle and provides the music for spring and summer nights.
Frogs and toads, along with salamanders, are classified as amphibians. The word amphibian is from the Greek words meaning double life and refers to the fact that these animals have “two lives” – a larval stage and an adult stage that are often very different from each other. Most amphibians begin their lives in water but as adults are able to live on land, often returning to water to breed. They undergo an amazing transformation, or metamorphosis, from aquatic larva to terrestrial adult, involving physiological, anatomical, and behavioral changes.
With very few exceptions, amphibians lay their eggs in water. The eggs are encased in a clear, translucent jelly that protects the developing embryo. The number of eggs laid, their appearance, and the shape of the egg masses vary from species to species and can be used as an aid to identification. The round, gelatinous egg masses of the wood frog can contain 2,000 to 3,000 eggs while a bullfrog lays as many as 20,000 eggs in a surface-floating mass. Spring peepers can lay up to 900 individual eggs, attaching them to submerged vegetation, whereas the American toad lays long, double spiral strings of eggs encased in jelly, which can contain over 1,500 eggs each. Spotted salamanders attach their oval egg mass to twigs floating in the water while red-spotted newts lay their tiny eggs singly on underwater plants and sunken leaf litter. The time it takes for these eggs to hatch varies by species and ranges from days to weeks.
The larvae that hatch from the frogs’ eggs are called tadpoles or polliwogs, bearing little resemblance to their parents. A tadpole’s all-in-one head and body are undifferentiated, forming a round or oblong shape ending in strong, keeled tail to aid in swimming. They have small eyes on the sides of their head and gills for breathing underwater. Tadpoles have tiny mouths with specialized mouthparts to help them gather minute algae and bacteria that are either free-floating or scraped off rock surfaces or leaf litter on the pond bottom. The rate at which they grow and the length of time it takes to develop depends on the species as well as food availability and water temperature. Wood frog tadpoles may transform in a little more than two months, whereas a bullfrog can spend two winters as a tadpole, transforming in its third summer.
Over time, these tadpoles undergo an amazing transformation, which includes a major revamping of body shape along with incredible internal and external changes. First hind legs form and begin to grow. Their eyes develop lids, start to bulge and migrate to the top of their heads. The tiny mouth broadens to hold a long sticky tongue. The whole digestive tract changes to match the switch in diet from herbivore to carnivore. Lungs develop to replace gills, allowing for life and breath out of water. Lung development is complete once front legs appear, as they often erupt through the old gill openings! Gradually the tail is absorbed into the body and disappears. The transformation from tadpole to frog is then complete.
There are nine families of frogs in North America, each with unique physical and behavioral adaptations that distinguish one group from another. The family of true frogs (Ranidae) includes many of the “pond frogs” commonly encountered by children (bullfrog, green frog, pickerel frog, leopard frog). Most have large, jumping hind legs and hind toes that are connected by webbing. Members of the tree frog family (Hylidae), on the other hand, are relatively small, and most species have rounded sticky discs on their unwebbed toes that make their life in the trees easier (spring peepers, gray treefrogs). And the true toads (Bufonidae), like the familiar American toad, have thick skin and short hind legs.
Certain physical characteristics can be used to identify frogs. Look for dorso-lateral ridges, two raised ridges of skin running down a frog’s back. The presence or absence of these can be used to separate look-alike species – green frogs have a ridge running down their back; bullfrogs do not. Distinctive patterns or marking can also help distinguish one species of frog from another. Eye patches, the shape and color of spots, striped versus spotted legs, even a hidden flash of color underneath hind legs are field marks that can aid in identification.
The various sounds frogs make is another important clue to identification, as each species of frog has its own distinct call. In spring and early summer, males gather in pools and ponds to breed and make their presence known with loud advertisement calls to attract females. Males have vocal sacs that they inflate when calling to amplify their songs. They make an impressive assortment of sounds: croaks, peeps, trills, barks, and snores. The duck-like “quack” of wood frogs is usually the first call to be heard in the spring, followed by the sharp, piercing peeps of the spring peeper. Later in spring, the American toad’s long, high trill can be heard, lasting up to a minute. Green frogs are recognized by a dull “katunk,” reminiscent of the plucking of a loose banjo string, while bullfrogs make a deep, throaty “jug-o-rum” call. Once breeding is complete, the chorus of frog voices quiets. Intermittent calls may come from the pond and surrounding vegetation, but the symphony of frog voices is silent until courting season comes again next spring.
The symphony of spring is sung in a thousand small voices, calling you outside to explore pools, ponds, and wetlands.
Suggested Reading
http://www.naturesound.com/frogs/frogs.html
Elliott, Lang. The Calls of Frogs and Toads. Stackpole Books, 2004.
Elliott, L., Gerhardt, C. and C. Davidson. The Frogs and Toads of North America. New York: Houghton Mifflin Harcourt, 2009.
Tyning, Thomas F. A Guide to Amphibians and Reptiles, Stokes Nature Guides Series. Boston: Little, Brown and Co., 1990
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