From tadpole to frog -- the life of a Minnesota amphibian
Several weeks ago a lady from the Detroit Lakes area told me that her husband had found tadpoles in a pond. She wondered how this could be since she had believed that frogs became adults in the late summer or early fall after hatching as tadpoles from eggs laid by adults in the springtime.
To tell you the truth, I had mistakenly believed that only bullfrog tadpoles overwintered (they actually go as long as two years in the larval, tadpole stage), but it turns out that other common frogs here in northern Minnesota also overwinter.
Indeed, tadpole green frogs and mink frogs, two species often found in large wetlands and lakes, remain in their larval state for an entire year. So in the case of the lady from Detroit Lakes who had also mentioned the type of habitat the tadpoles were found in, as well as providing me a good description of the tadpoles' physical characteristics, they were likely green frog tadpoles.
Most species of frogs, however, tend to develop from tadpole to adult frog in one growing season, as I will soon illustrate.
Five salamanders and fourteen frogs and toads call Minnesota's bountiful aquatic and woodland habitats home. These amazing amphibians come in two distinct orders: salamanders (Caudata) and frogs and toads (Anura). The word amphibians is derived from the Greek word amphibious, which means "living a double life" (amphi) and "mode of life" (bios).
What is fascinating about amphibians is that parts of their lives occur entirely in water. Yet, as with so many things in nature, there are exceptions. In the case of the mudpuppy, their whole lives are spent in the water as they breathe oxygen through their gills. Conversely, tiger salamanders emerge from their aquatic nurseries and spend their adult lives primarily on land.
Like salamanders, all frogs and toads lay eggs in water. In the case of anurans, the hatched offspring are called tadpoles. The fish-like, gilled tadpoles eventually undergo the amazing transformation, or metamorphosis, from baby tadpoles to adult frogs or toads. Can you imagine what early naturalists thought as they discovered and described frogs and tadpoles? They likely assumed they were observing entirely different species!
As a boy growing up near a variety of wetlands, creeks, rivers and lakes, I spent countless hours hunting and capturing frogs and toads. But what was especially alluring to me was collecting tadpoles. Back then I never knew for sure which species of frog or toad they were, yet, consistently, the tadpoles captured represented various developmental stages. Some tadpoles were nearly fully developed frogs while others showed no leg appendages at all.
In the case of, say, a tadpole leopard frog, it begins its life outside the egg as a very delicate and vulnerable organism. They are easy prey for a host of other creatures including predaceous aquatic insects, fishes, birds, and mammals. For the first week to ten days of their lives, tadpoles feed on their own yolk sacs. During this early period tadpoles have poorly developed mouths, tails, and gills. In fact, tadpoles will often attach themselves to aquatic vegetation by aid of special organs on their bodies.
Following this brief time of semi-sedentary living, the young tadpoles detach from plants and begin swimming freely while feeding on mostly algae. And as they continue to feed and grow, tadpoles begin taking on the appearance of what they will eventually become -- a frog or toad.
As their first month of life draws to a close, a tadpole's gills begin to gradually disappear as new skin-growth covers their gills. Tiny teeth develop inside their mouths to help them better masticate their food and, coupled with long, coiled guts, contributes to maximizing nutrients needed for their fast-changing bodies.
Over the next several weeks (week six to nine) leg development begins in earnest. First to reveal themselves are the hind legs, followed shortly thereafter by the front legs. In due time, the long-tailed tadpole sports fully developed front and hind legs as well as a head that is more similar to that of an adult frog or toad.
By the end of their third month of life (twelve or so weeks), a tadpole really isn't a tadpole anymore. Aside from a stubby vestige of a tail to remind us of its former self, the tadpole is nearly ready to emerge from the water to live the remainder of its life in its adult form. Depending on the species and sometimes temperature, food, and water supply, about 16 weeks typically has to pass before a tadpole has fully transformed into an adult frog and toad.
Come autumn, adult frogs and toads migrate from wet meadows to more permanent water sources or other protective places where they can hibernate in mud. Some frogs, like the wood frog, simply hibernate beneath logs, woodland leaf litter, or other forest debris. To prevent their body's cells from freezing, wood frogs have the astonishing ability to produce glycerol, which is an organic anti-freeze.
And come springtime, the over-wintering adult frogs and toads emerge one by one and species by species to mate and begin life anew. The first to vocalize are the wood frogs and chorus frogs, followed by leopard frogs and spring peepers. Soon after, the trills of American toads and the raccoon-like calls of gray tree frogs can be heard.
As well, other calls to listen for are the guitar string plunks of green frogs and, further south in Minnesota, the low "rumm, rumm, rumm" notes from bullfrogs -- all of which, from tadpole to frog or toad, are aquatic marvels to see and hear as we get out and enjoy the great outdoors.
(Klemek is the DNR Area Wildlife Supervisor in Detroit Lakes. You can contact him at firstname.lastname@example.org)