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Michigan's Frogs & Toads
There are more than 3400 species of frogs and toads worldwide, with the majority living in the humid tropics. Michigan can boast only 13 species, but they are an important part of the State's wildlife heritage.
Frogs and toads, along with the salamanders, are members of the class Amphibia. Amphibians are characterized by a life cycle which begins with an unshelled egg laid in water, the egg hatches into a fish like, gill breathing larva (called a tadpole or polliwog in frogs and toads), followed by the transformation of the larva into an adult. During this transformation, the larva gradually develops legs, lungs, and other modifications for life as an air breathing adult capable of living on land. Sometimes this transformation occurs inside the egg, with the animal hatching out as an "adult," but all of Michigan's frogs and toads, have a typical aquatic (water living) tadpole stage. Most amphibians have rather thin skins through which they can "breathe" and absorb or lose water. Because of this, they prefer to live in moist or wet habitats.
How can you tell a frog from a toad? It's been said that a toad is just a lumpy frog! Toads do have thicker, more warty skins compared to the smoother skin of most frogs. Toads are adapted for drier conditions than frogs, though they spend much of their time burrowed into moist soil during times when the air is drier. Toads have shorter hind legs than frogs, comparatively speaking, and move in short hops or simply walk instead of making long leaps.
Michigan has two species of "true" toads, the American Toad and Fowler's Toad. Some species are hard to fit into either category. For example, Cricket Frogs have a warty skin, and the Gray Tree Frog (often called a "tree toad") spends much of the summer high in the trees away from water.
Amphibians are "cold blooded" (ectothermic) animals, which means that they do not produce internal body heat. Instead, their body temperature is dependent upon that of their surroundings. Frogs are active at fairly cool temperatures in spring, but must seek protection from below freezing conditions in winter.
Most species pass the winter in a dormant ("sleep like") state underwater, either burrowing into mud, or just sitting on the bottom in ponds and swamps. Toads burrow deep into woodland soil. Recent studies revealed that a few kinds of frogs can stand being in below freezing conditions for short periods (like the Wood Frog and Spring Peeper), while others died under the same conditions (such as the Leopard and Mink Frog).
As temperatures rise in early spring, frogs begin to move to their breeding sites. The actual timing depends on the warmth of the air and water, and the humidity, but there is noticeable order in which the various Michigan species become active and begin voicing their breeding calls. For example, in southern Michigan the raspy voice of the Western Chorus Frog is usually heard first, often in late March, followed quickly by the highpitched peeps of the Spring Peeper. In a few days the woodland swamps are filled with the quack like calls of the male Wood Frogs, while in another week in open marshes the low snores of the Leopard Frog are barely heard over the squeaky songs of newly arrived Red Winged Blackbirds.
The first warm rains of April bring American Toads out of the woods to the breeding ponds, where the air is soon filled with their melodious trills. Several of our frogs postpone their breeding activities until later in spring, when air and water temperatures are higher. Included in this late group are the Gray Tree Frog, Blanchard's Cricket Frog, and Green and Bull Frogs.
Frogs are far more often heard than seen. Most frog sounds are the advertisement calls of the males, intended to attract the females for breeding. Frog voices may carry for long distances, especially the higher pitched calls of the smaller species. The males increase the loudness of their calls by ballooning out their throats or special sacs at the sides of their throats, creating a kind of resonating chamber. Only males produce advertisement calls, but both sexes may give shorter warning calls or screams when danger threatens. Males can also produce distinct calls that warn away rival males that approach their calling or breeding sites.
Female frogs and toads may lay hundreds or even thousands of eggs. These are usually attached to underwater vegetation or left floating in large masses at the surface. During egg laying, the male clings to the female's back and fertilizes the eggs. The small, dark eggs are protected by layers of a jelly like substance. They may be in rounded masses (as in Wood and Leopard Frogs), loose clusters (Gray Tree Frogs), long necklace like strings (Toads), thin surface films (Bull and Green Frogs), or deposited singly or in small clusters (Spring Peeper). Many frog eggs are eaten by predators such as fish, turtles, and aquatic insects, or are lost to drying or destruction by micro organisms.
Tadpoles hatch in a few days or weeks, depending on temperature and species. They feed mostly on algae and other plant life, but also scavenge on animal remains. Eventually they reach a certain size and age when chemicals inside their bodies, called hormones, will trigger the change to adult form. First the hind legs appear, then the front legs. Feeding may stop as their jaws and digestive systems change to prepare for the adult diet of insects and other small animals.
Soon the gills which allowed the tadpoles to breathe in water are replaced by lungs, and the "frogs to be" must start surfacing for air. Lastly, the tail is absorbed, and the new froglets or toadlets can begin to move about on land, partly free of their dependence on water. As with the eggs, many tadpoles and young frogs are lost to predators. It would not be surprising to find that a thousand eggs laid in spring might produce only a dozen or fewer adult frogs by late summer. Obviously, one importance of frogs in nature is providing food for other animals!
Most Michigan frogs grow from egg to tadpole to adult in one summer. Those species that breed in temporary ponds typically have very rapid development. The Spring Peeper's eggs may hatch in three days, and the tadpoles can become frogs within six weeks often in a close race with the evaporation of the pond! In some years, small ponds dry up before the tadpoles can mature. However, the risk of breeding in these shallow places may be balanced by the lack of fish and other large predators that require permanent water. Some of the largest frogs (such as Green and Bull Frogs) have tadpoles that need two or three years to mature, and they are dependent on the deeper ponds, lakes, streams, and marshes.
From a human standpoint, frogs and toads are extremely beneficial animals. Their springtime choruses are a pleasure to hear, and most people enjoy seeing them, whether in wetlands or suburban gardens. Adult frogs and toads feed largely on insects, and destroy vast numbers of insect pests every year. It has been estimated, for example, that a single Cricket Frog an increasingly rare species in Michigan can consume about 4800 insects in one year. A hundred frogs would thus eat about 480,000 insects, a thousand frogs perhaps 4.8 million insects, and so on. It is clear that the loss of these tiny frogs in a wetland ecosystem would be no small matter! Larger frogs would eat even greater quantities, and the benefits of a backyard toad population are known to many gardeners.
The Bull Frog is the only species that might eat creatures favored by people, such as fish, young ducks, or other frogs. But their impact is very minor. The Bull Frog is valued as a game animal itself as the source of "frog legs" used for human food. To help prevent overharvest of local frog populations, Michigan protects all frogs during the spring breeding season. Frogs can be legally harvested only during the "frog season" which opens the Saturday before Memorial Day and closes November 15.
Unfortunately, many human activities are harmful to frog and toad populations. Valuable wetland habitats are often drained for agriculture or urban developments. Water pollution may destroy or degrade the ponds, lakes, marshes, and streams that remain. The problem of acid rain has been found to damage amphibian populations in affected areas. And the often unwise use of chemical pesticides is harmful to frogs and toads, both by killing them directly and also by reducing their insect food supply. Thousands of these creatures are killed every year by automobiles, often while crossing roads during migration to breeding sites in spring. An increasing problem is the destruction of wetland environments by the misuse of off road vehicles.
There is often much that can be done to protect existing frog and toad populations and to restore habitat that has been lost or degraded. Years ago many small wetlands were drained or filled for agricultural purposes. Where this "reclaimed" land is no longer needed for crops, steps can often be taken to restore a small wetland. Sometimes this may be as simple as blocking old drainage ditches or removing a tile system if this will affect drainage only on your property. Land developers should avoid filling ponds and marshes; alternatives can often be found. (Michigan does have wetlands protection legislation which controls and licenses the use and modification of ponds, lakes, streams, swamps, and marshes. Be sure to contact the Department of Natural Resources before attempting any modification of wetlands or drainage ways.)
Frog habitats are vulnerable to contamination by run off from farmland, residential yards and industrial sites. While the larger problem must be dealt with via governmental zoning ordinances, we can all exercise restraint in the use of fertilizers and pesticides, particularly where water contamination is possible. There are many alternatives to the use of pesticides to control pest insects. Backyard mosquitoes are more likely to breed in clogged rain gutters and discarded trash than in a healthy pond environment. Filling in or poisoning the pond will destroy frogs and toads and other predators that would otherwise be eating many insect pests. Sometimes a second look and common sense are all that is needed to assure that one problem will not be solved by creating another problem.
Frogs and toads are important contributors to a healthy ecosystem, particularly in the fragile wetland areas. Efforts to protect these animals and their valuable wetland habitats will produce many benefits, both for ourselves and for future generations.
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