Solitary males radiate a high-intensity calling song that attracts receptive females. When a male senses the presence of an approaching female, he stops singing, turns around, and touches her with his antennae (38). Tree crickets and most other gryllids, once in contact with a female, switch from calling song to a lower-intensity courtship song (63). The courting male again turns away and courts the female through an elaborate series of courtship song and body vibrations termed tremulation (10).
SONG PREFERENCES
Song is the principal mode of communication in crickets. Tree cricket song is produced by a file-against-scraper mechanism on the specialized tegmina. Sound is produced as the wings are closed and each tooth of the file engages the scraper. Singing males close their wings 15–100 times a second, resulting in a series of pulses either strung together in a continuous trill, discontinued, and resumed at irregular intervals into a broken trill or broken into a regular sequence into a chirp (126).
Work on the function of cricket song has generally focused only on its use as a specific mate-recognition tool and consequently has focused on female choice at the level of differences among species. Compared with the vast amount of work that has been done on phonotactic preferences for song characters that vary at the species level, little is known about the capacity of females to discriminate between males within a population. To understand the role that sexual selection by female choice plays in structuring cricket song, we need to know whether females also choose among conspecific males, what characters they use to do so, and why they might show these preferences.
Orthoptera measure four physical aspects of sound: intensity (amplitude or power output), frequency, amplitude modulation (temporal changes in inten- sity), and direction of the source (41). In the following section, the song charac- ters chosen by females are reviewed. Clear evidence that calling songs contain information about species membership and that females are preferentially at- tracted to conspecific calls suggests that stabilizing female choice for average song characters may be particularly prevalent. But there is strong evidence that other forms of female choice occur. Other forms of female choice generally predict directional preferences, at least within the species-typical range. Below, the results of phonotactic preferences for the suborder Ensifera are summarized and—with the goal of identifying characters that may be subject to different forms of female choice—they are separated into those traits subject to stabiliz- ing preferences and those subject to directional preferences.
Stabilizing Preferences: Species Indicators
Tree crickets, like other acoustic Orthoptera, have a strong capacity to distin- guish sounds of different pulse rates and chirp rates (30, 86, 126), and like other acoustic Orthoptera, they may also be able to hear fine differences in pulse or chirp duration and duty cycle (pulse duration/pulse period) (26, 29, 33, 118). These temporal properties, particularly pulse rate in trilling species and chirp rate in chirping species, are generally the main characteristics in species-speci- fic preferences. Moreover, as Walker (126) demonstrated for eastern North American tree crickets, pulse rate tends to be the character that varies most among sympatric species (also see 81). Does this produce stabilizing sexual selection on the song characters within the population? [1]