In A Frog’s Ear?

August 25, 2010

Nature Notes by Bill Dunson


As a long-time herpetologist I have always enjoyed listening to the calls of frogs and toads and trying to figure out which species are which and what triggers calling. Indeed one of my “bucket list” items was to acquire a house from which I could lie in bed at night and hear frogs calling. I finally achieved this late in life when we bought a farm in SW VA and I built a series of 8 small ponds during restoration of the degraded spring-fed marshes. This has enabled me to enjoy many nights of frog sounds, mainly from bullfrogs, green frogs, American toads and Spring peepers.

Now I am always admonishing folks to not only enjoy Nature but to carefully observe patterns and try to decipher the meaning of these patterns. One such froggy pattern is the breeding biology of the bullfrog (see photo of male). The male occupies a defended territory and announces this fact by booming “jug-o-rum” calls. Apparently females tend to choose males as partners with larger body size, deeper calls and better territories for raising babies. The ear-drum of the male is huge- bigger than the eye and much larger than the female ear-drum which is considerably smaller than the eye. Now bullfrogs are home-bodies and remain in and very near their ponds. Eggs are laid in early Summer in shallow water near the surface (see photo of embryos in such an egg mass) possibly in response to the fact that they breed during warmer periods when low oxygen could be a problem. Even after metamorphosis young frogs remain in or near the pond.
Another amphibian breeding in these ponds is the American toad (see photo) which is mainly terrestrial but comes to ponds in Spring to breed. Males (see photo) give a distinctive trill which attracts females for mating. Eggs are laid in strings underwater (see photo), grow rapidly to a size where they metamorphose, and then leave the pond.

So have you noticed that the ear-drum of the male toad is much smaller than that of the male bullfrog? Why do you suppose this is?

Consider what the purpose of calling/singing and listening might be- presumably to defend a territory against other males and/or to attract females. Do you think both of these functions might apply equally well to bullfrogs and American toads? Not likely since the toads enter the ponds only to breed and leave immediately thereafter. Bullfrogs on the other hand remain all year in the ponds. Thus bullfrogs likely use their loud call to both repel male intruders and to attract mates, whereas the toad would primarily be calling to attract a mate. Thus the larger ear-drum of the male bullfrog probably reflects his greater need to discriminate sound frequencies and volume. At least this seems to be a reasonable hypothesis to explain the huge ear-drum of the male bullfrog..

So when listening to the wonderful sounds of the night, including those of amphibians, reflect upon the possible meaning of these sounds to the animals themselves. There may be more information communicated than you might have thought; I am here, I am big, I am ready to breed and have great habitat for your babies if you are female, I will fight you if you are a male, etc.

Bill Dunson, Englewood, FL & Galax, VA

Bill Dunson

About Bill Dunson

Bill Dunson, born in rural Georgia, skipped 12th grade and went directly to Yale. Bill subsequent-ly received a PhD in Zoology from the University of Michigan, studying softshell turtles. Bill is Professor Emeritus of Pennsylvania State University thanks to a career spent entirely at that institution, teaching and doing research on the physio-logical ecology and ecotoxiciology of reptiles, amphibians and fish. Always curious about nature, Bill has dedicated his life to learning and sharing his knowledge with others. He has served on many advisory boards here in Southwest Florida to preserve the water that gives life to our region.

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