"A bullfrog croaks either to attract a companion or to warn away other males. How can such a small animal, with a small mouth, make such a deep, booming noise?"
Watch and listen to this loud creature following this link! Bullfrog Blues
"A bullfrog emits most of its croak through its eardrums, not through its mouth. Reportedly, a researcher discovered this by (gently) pressing his fingers over the ears of a bullfrog and noticing that the bullfrog’s sound level was greatly reduced. Later the demonstration was repeated with “frog earmuffs,” which were pieces of foam held in place over the eardrums by spring!
The sound originates in the vocal folds of the frog, just as it does in mammals. But it is then sent to the eardrums, which resonate at certain frequencies like drumheads. This resonance greatly increases the sound level at those frequencies and broadcasts the sound through the frog’s environment. Before this role of the eardrums was discovered, many people believed that the resonance occurred in the vocal sac, the region of the throat that the frog inflates when croaking. Although gibbons, and some types of frogs and toads, might set up resonance in their vocal sac in order to be louder, the bullfrog doesn’t” (Walker, 151).
According to Alejandro P. Purgue (1998), from the University of California, Los Angeles, approximately 90% of the sound that comes from a bullfrog comes from their ear drums. He found this out by placing his fingers over the ears of a bullfrog and listening to the frog. It sounded "muffled, like someone trying to shout through a gag” (S. M., 1998, p. 12). He also discovered that for the same size female and male frog, the male has eardrums that are twice as big as the female’s eardrums.
I believe that there are still some people who are not clear on the facts of where the real noise comes from when a bullfrog croaks. Many websites and articles I found on the internet were talking about the resonance from the vocal sac. That is where some of the noise comes from within the anatomy of the frog, but is not the primary source in the bullfrog. Other species of frogs however do in fact have most of their noise come from the resonance of the vocal sac, just not the bullfrog. The muscles in the abdomen of the bullfrog and the inner lining of the vocal sac play a large role in the making of the noise but not the loudness. “Air flowing through the larynx inflates a highly extensible vocal sac lined with muscle and a network of elastic fibers” (Jaramillo, C., A. S. Rand, R. Ibanez, and R. Dudley, 287). This is how the air gets flowing through the frog, but the actual loudness and noise comes out of the eardrums of the bullfrog.
Glass (2003) explained that bullfrogs have membranes on the side of their heads that resonate with the frequency of the croak. These membranes amplify the croak by vibrations of these membranes and then the vibrations come out of the eardrums in sound waves being longitudinal waves. Whichever frequency the bullfrog makes in his throat is projected out through the resonating ear traveling in the form of sound waves then reaching our ears to where we can hear the croaking.
Glass, D. (2003, September 27). Talk out your ears. Retrieved from http://indianapublicmedia.org/amomentofscience/talk-out-your-ears/
This was a small article written about how the croak of a bullfrog develops in their throat but the sound is produced out of their eardrums. This was helpful for me because it was short and straight forward with much of the information I was looking for. This article was a small article written in the Indiana public newspaper, therefore it may not be as reliable as a scholarly journal or article. It’s probably not the best source to use.
Jaramillo, C., Rand, A. S., Ibáñez, R. and Dudley, R. (1997). Elastic structures in the vocalization apparatus of the Túngara Frog Physalaemus pustulosus (Leptodactylidae): An annotated bibliography Journal of Morphology, 233(3), 287-295. doi: 10.1002/(SICI)1097-4687(199709)233:3<287::AID-JMOR7>3.0.CO;2-0
This group of authors put together this piece to explain the different parts of the frog that are involved in the croak. They show with diagrams and pictures of the frog’s necessary features for the croaking to take place. This was somewhat helpful for me. It was hard to get through because of the large vocabulary in the article, but the information I could understand was useful. This is a scholarly article published in a reliable journal. This is a reliable source.
Milius, S. (1998, January). Frogs that talk through their ears. Science News, 153(1), 12. Retrieved October 25, 2011, from Research Library.
This is an article Susan Milius wrote about different research that has been done on bullfrogs and why they make such loud noises and where it comes from. The article was awesome for me because it helped give me a clear understanding of how they discovered where the croak of a bullfrog comes from. I trust this article because of all of the scientists sourced in it and Science News is a well-known reliable magazine.
Photo by Chris Brown, USGS.