Passage VI

Females of the New World tree frog Hyla ebraccata are strongly influenced in their choice of a mate by advertisement calls (calls made by males to attract females). Thus, the reproductive fitness of a male is dependent on the characteristics of his advertisement call.

Advertisement calls differ in their pulse-repetition rate (PRR), measured in hertz (Hz). Natural populations of Hyla ebraccata show low variability for PRR. A researcher hypothesized that a greater percent of female frogs would be attracted to a call with a PRR equal to the mean PRR of the population than would be attracted to a call with an alternative PRR.

The researcher located and recorded the advertisement call of a male frog, then captured, marked, and released the frog. This procedure was repeated with other frogs from the same population until 60 recordings were obtained, each from a different frog. The mean PRR was found to be 100 Hz. Next, the researcher collected 25 gravid frogs (frogs carrying unfertilized eggs). Then the following experiment was conducted.

Experiment

In each trial, a single gravid frog was placed in a small arena (built from foam padding) with 2 speakers at opposite ends (see Figure 1).

One speaker played a call (at an intensity level of 80 decibels) with a PRR equal to the mean PRR of the population. The other speaker played a call (at 80 decibels) with an alternative PRR. The frog was monitored until it moved to within 2.5 cm of a speaker, and was then removed from the arena.Every gravid frog participated in 6 trials, each with a different alternative PRR. Figure 2 shows the percent of frogs that moved to within 2.5 cm of the speaker playing an alternative PRR.

Figure 2 adapted from Lori Wollerman, "Stabilizing and Directional Preferences of Female Hyla ebraccata for Calls Differing in Static Properties." ©1998 by the Association for tho Study of Animal Behaviour.