Thursday, June 2, 2011

THE CHORUS OF THE CHORUS FROGS

Early every year in the Pacific Northwest a familiar sound rings out, telling us it is spring whether it seems that way or not. this is the “song” or advertisement call of the male Pacific Chorus Frog, Pseudacris regilla.

Most people recognize the call immediately as the generic frog call of movies in decades past. As this is a common frog (or at least used to be) in Hollywood, California, its call was incorporated into many a movie that needed frog calls as ambience. In fact the “ribbit, ribbit” sound has become the stereotype of frog calls.

These frogs come out of their winter dormancy very early in spring, when, to paraphrase Robert Burns, a young frog’s fancy lightly turns to thoughts of love. The only way a frog has to express itself in such a situation is to call . . . . and call . . . . and call. And that they do, with surprising strength.

When a male frog feels these stirrings, he heads for the nearest pond or marsh, usually in the evening but sometimes even during the day. On arrival, he jumps in the water and swims to what he considers a good position. Only the frog knows why it is a good position, but it probably provides a place to hold onto the vegetation and a place where he can be easily seen as well as heard. Interestingly, it is often the same male that calls first each evening.

He begins to call: ribbit, ribbit, ribbit, a creaky two syllables that carries at least a hundred yards or more on a quiet night. Another frog heads for the pond, either because hopping downhill in a moist environment will lead to water or because it homes in on the first frog. The second frog begins to call, perfectly insinuating its calls between those of the first. The two may sound quite different, so we hear ribbit, rabbit, ribbit, rabbit, ribbit, rabbit.

A third male begins to call, amazingly also able to insert its calls into the soundscape so they can be heard as distinct: ribbit, rabbit, robbit, ribbit, rabbit, robbit . . . the pace is speeding up, and there is no room for a fourth frog, but that one calls anyway. As the chorus swells, the individual voices become less apparent, even though the structure may still be there, but a female approaching the pond can easily distinguish the individual voices.

Females apparently choose males based on the vigor of their songs, and as the evening progresses, more and more males acquire a mate. The male clasps the female and stays with her while she looks for a place to lay her eggs. She finds such a place, lays a cluster of eggs, and the male fertilizes them.

The eggs hatch in a few days, and the tadpoles grow quickly on a diet of plant matter. After a few months, they finally absorb their tails and grow a set of limbs and are then ready to leave the pond. If they survive the year, they will return the following spring, and the pond will resound again with the chorus frog chorus.

Dennis Paulson

2 comments:

Mike Benard said...

Very enjoyable post! Pacific Chorus Frogs are fascinating animals.

I would be curious to know what evidence supports the statement that "... it is often the same male that calls first each evening."

I don't know of any study that has tested whether the same male normally starts calling each evening. It seems like it would be a very difficult hypothesis to test, especially in populations with more than a dozen males. Can you provide a citation?

It is also worth noting that larger males pacific chorus frogs are more likely to mate with females than smaller males.

Mike

Slater Museum of Natural History said...

Mike, I make this claim because I have two male chorus frogs calling every night at my pond, and it is the same one that begins to call every evening; then the other one chimes in, and they alternate as described. I have watched them both calling and can easily recognize either one by its appearance or call. In no way is this science, with such a small sample size, but I thought it was interesting enough to make the statement. Originally I intended just to describe the situation in my own pond, but then I decided to expand it to a more general account. Thanks for questioning it.