Frog Calls

Communication between animals can be achieved through using any one or a combination of senses, such as sight, touch, smell, taste, or hearing. Several groups of animals have developed the use of sound as the principle means of distant communication; among those groups are birds, insects, whales, and frogs. Most species of frogs can produce a minimum of two types of vocalizations, a distress call and an advertisement call. Distress calls can be produced by both males and females of many species when subjected to severe stress, such as when seized by a predator or when handled roughly. Advertisement calls are produced by males to convey information about location and breeding readiness to both sexes.

Advertisement calls can be used to identify the species of Ohio frogs and toads. With some practice and a little patience, a person with a "good" ear can soon learn to distinguish the calls of most species. For those with a less discriminating ear, a visual picture of the sound can be a useful teaching aid for learning calls and comparing them. A sound spectrogram is a graphic image of sound that can be produced by digitizing sound in a computer and then displaying it on a monitor. The spectrogram represents a plot of frequency (low to high pitch) against time; it depicts changes in frequency over the time duration (length) of the call. Time is measured in seconds, and frequency is measured in hertz (Hz), or cycles per second. A spectrogram of middle C on the piano has a frequency of 263 Hz (played here on an electric guitar, Fig. 2). The center of the lower light band, the dominant frequency, is 263 Hz, whereas that of the upper band, the overtone or harmonic, has a frequency of about four times (1052 Hz) that value. Shades of gray indicate levels of sound energy emitted at a particular frequency; the lighter the area the greater the energy present at that frequency.

Fig. 2. Spectrogram of middle C

Middle C on an electric guitar (7 seconds, 61 KB)

The image displayed above is a spectrogram of middle C played on an electric guitar. Frequency is displayed on the vertical y-axis, and time is displayed on the horizontal x-axis. As the frequency increases, the perceived sound, or pitch, gets higher. The frequency of middle C is 263 Hz and is located near the middle of the lower horizontal band, the dominant frequency band. Notice, the lower band is much lighter than the horizontal band above it. The lighter the band, or portion of a band, the more energy contained in the sound at that frequency. The higher band is the harmonic or overtone at a frequency of 1052 Hz. The length of time the string vibrated is measured on the x-axis and exceeded two seconds.

For purposes of comparison, a bullfrog calls at a frequency of about 300 Hz (Fig. 24) and a spring peeper at 3000 Hz (Fig. 17). Frog calls may continue for 10 or more seconds as in the American toad, or they may be very short and have a call duration of less than 0.1 second as in the cricket frog (Fig. 9). Calls may be given in a single pulse (cricket frog, Fig. 9); as a series of similar pulses termed a trill (gray treefrogs, Fig. 20 and Fig. 21); or as a sequence of dissimilar sounds (bullfrog, Fig. 24).

Bullfrog (6 seconds, 50 KB)
Spring peeper (5 seconds, 46 KB)
Cricket frog (7 seconds, 61 KB)
Gray treefrog (14 seconds, 110 KB)
Bullfrog (14 seconds, 114 KB)