by Whit Gibbons

February 14, 2010

A cold rainy winter has brought forth some unusual questions. I received the following within the last two weeks.

Q. I thought frogs were cold-blooded, but I live in Alabama and have heard some kind of frog calling during some near-freezing weather. What's going on?

Q. I have heard frogs that make a peeping sound calling in fairly large numbers and from several locations in both central Georgia and South Carolina during both day and night. I have heard of a little frog called a spring peeper, but this is winter and the weather has been cold. Any idea what these frogs might be? They are going crazy in the evenings outside where I work.

A. The frogs you have heard calling are most likely breeding choruses of spring peepers, a species found from northern Florida throughout the eastern United States and well into Canada. They are called "spring" peepers because the people who first noticed them and named them lived in northern states. In the North, these frogs begin breeding in early spring. I have seen them calling from a pond in Michigan in April when the edges of the pond were frozen, and the air temperature was a toasty 36 degrees. In the South conditions similar to a northern spring are more likely to occur in winter, so that's when they breed down here. A southern winter night around Valentine's Day is ideal for these little creatures to start their mating activity.

Why would an animal come out at night, at such a miserable time of year for such an important biological activity? The answer is in the question itself: it is a "miserable time," which means fewer predators (no snakes or turtles will be active) to eat the adults or their eggs. Also, the late spring- and summer-breeding frogs will not have tadpoles in the pond to compete with spring peeper tadpoles.

I mentioned that the frogs calling right now are "most likely" spring peepers (because that is what I have been hearing, and they are the most common), but other kinds of chorus frogs also breed in the winter and could be calling. Most produce rather musical notes. Spring peepers sound kind of like baby chicks; the southern chorus frogs sound somewhat like rubbing a thumb over the teeth of a comb; the ornate chorus frog makes a sound like someone hitting a steel spike with a hammer. All of these species breed in the winter or early spring. In most parts of the Southeast, that means one or more species of frogs is calling every month of the year.

The question above about frogs being "cold-blooded" (or ectothermic, to use the scientific term) means their body temperature, and therefore their metabolism, reflects that of the surrounding environment. The phenomenon is complicated, however, because some animals, including certain frogs can remain active at cold temperatures approaching freezing.

I recently saw dramatic evidence of the ability of some frogs to withstand much colder temperatures than most animals. Cris Hagen with the Savannah River Ecology Lab showed me a photograph he had taken of a cascades frog, which looks like a miniature version of the common leopard frog, as it was moving to a breeding site on Mt. Rainier, Wash. The dramatic part was not that the frog was moving overland but that it was doing so before the snow had melted. That was one cold little frog hopping across a sheet of ice and snow. But it had found a sure way to avoid predators that can't tolerate the cold.

Incidentally, some tadpoles besides those of the chorus frogs manage to make it through the winter, the most notable being the bullfrog. Bullfrogs have been known to lay eggs in the summer that develop into tadpoles but do not have enough warm weather to completely metamorphose into frogs. Hence the tadpoles spend the winter in the pond, staying safely in the mud and metamorphosing a year or more later.

Frogs are amazing creatures, and people living in the Southeast are fortunate in being able to enjoy them even in the winter.

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