AMER.ZOOL., 41:1222±1228 (2001) Regulation of Chorusing in the Vibrational Communication System of the Leafhopper Graminella nigrifrons1 RANDY E. HUNT2 AND THOMAS L. MORTON3 Department of Biology, Centre College, Danville, Kentucky 40422 SYNOPSIS. Male Graminella nigrifrons participate in alternating choruses. Vibra- tional calls emitted by males consist of three sections (S1, S2, and S3) that differ in pattern of amplitude modulation. In this study we examined the response of single males to synthetic choruses and to isolated call components to gain insight into the regulation of chorus structure. Males initiated calls primarily during the silent periods within synthetic choruses. In all 15 trials the number of overlapping calls and the duration of overlap was signi®cantly less than expected if males call at random. Playback of S2, S3, or random noise while males emitted S1 caused males to interrupt calling, whereas males continued to call when S1 or no signal was played. In a related experiment, we played S2 or no signal while males were beginning to emit the S1, S2, or S3 phase of their calls. In response to this playback the duration of S1 and S3 was reduced, but the duration of S2 was not affected. These results suggest that an inhibitory-resetting mechanism may result in alter- nation of calls in this leafhopper. INTRODUCTION ing. Although chorusing is sometimes co- The evolution of communication systems operative (Green®eld, 1994), it is often a is strongly in¯uenced by the mode of com- re¯ection of competition among males for munication used by animals and the envi- mating opportunities and consists of males ronment through which signals are trans- that either overlap or alternate calls with mitted (Wiley and Richards, 1982; RoÈmer, neighboring males. In contrast, vibrational 1992; Endler, 1993; Robisson et al., 1993). communication is sometimes viewed as an Thus, knowledge of signal properties and alternative mode of communication which how the environment may facilitate or con- reduces the likelihood of eavesdropping by strain transfer of information has been an natural enemies or sexual competitors important guide in theoretical and empirical (Henry, 1994). The enemies hypothesis is research. Acoustic signals are transmitted supported by studies of certain Neotropical rapidly over great distances and the envi- katydids that have shifted from acoustic to ronment in¯uences signal ®delity in ways vibrational communication in response to that are often predictable. Thus, sexual se- bat predation (Belwood and Morris, 1987). lection acting on acoustic signals and re- The competition hypothesis suggests that ceiver mechanisms at long distances is pos- males avoid vibrational interactions or have sible and has led to male-male competition little potential to interact because of the and female preference in a broad array of limited range of their signals. Thus, cho- insect and vertebrate taxa (Andersson, rusing should be a rare or absent component 1994; Green®eld, 1994). The most conspic- of their mating system. uous outcome of such selection is chorus- Although the possible involvement of vi- brational signals in male-male competition or female preference has received little 1 From the Symposium Vibration as a Communi- cation Channel presented at the Annual Meeting of the study (Ott, 1994), vibrational signals have Society for Integrative and Comparative Biology, 3±7 been associated with interactions among January 2001, at Chicago, Illinois. males in a few species of planthoppers 2 Present address of Randy E. Hunt is Department (Delphacidae) and leafhoppers (Cicadelli- of Biology, Indiana University Southeast, New Alba- ny, Indiana 47150; E-mail: [email protected] dae) (Ichikawa, 1982; Heady et al., 1986; 3 Present address of Thomas L. Morton is P.O. Box Claridge and de Vrijer, 1994). These signals 152, Wilmore, Kentucky 40390. are often referred to as aggressive or rivalry 1222 REGULATION OF CHORUSING 1223 FIG. 1. Typical call emitted by a male G. nigrifrons. The call is composed of three main sections: section 1 (S1), 2 (S2), a transitional phase, and 3 (S3) (refer to text). S1 is composed of a series of low amplitude pulses, whereas sections 2 and 3 are composed of repeated chirps. S2 and S3 chirps differ in the number of lower amplitude pulses that precede the major pulse (see Hunt et al., 1992). signals. They differ structurally from ad- such as those reported here are common vertisement signals which elicit response (R.E.H. and T.L.M., unpublished data). calls from females (see below) and are This study examines how individual emitted when males are in close proximity male Graminella nigrifrons interact with re- or engaged in physical aggression. For ex- cordings of other males. Our goal was to ample, Ichikawa (1982) found that male determine whether vibrational signals in the rice brown planthoppers (Nilaparvata lu- absence of other stimuli (physical or visual) gens) reared at high density emit aggressive would result in chorusing. We also report calls that appear to suppress calling activity on a series of playback experiments that of males reared at low density. However, contribute to an understanding of how cho- experiments designed to isolate aggressive rus structure is regulated in this leafhopper. signals as the cause of calling suppression and the relationship between aggressive sig- METHODS naling and mating success were not con- G. nigrifrons were reared indoors on oats ducted (see Ott, 1994). More recently, how- and maize at 24±268C on a 15:9 light:dark ever, studies of the leafhopper Graminella cycle following procedures described by nigrifrons show that competition for mating Hunt and Nault (1991). Males were housed opportunities involves chorusing (R.E.H. separately from females and used in exper- and T.L.M., unpublished data). iments 1±2 wk after adult eclosion. G. nigrifrons is one of the most common Male playback signals were constructed leafhoppers found in the eastern United from recordings of calls emitted by ®ve in- States (Whitcomb et al., 1987). This small dividual males (exemplars) (Fig. 1). Each insect (3.5±4.5 mm) feeds on many annual call was recorded by resting a BSR-X5H and perennial grasses, but it is especially phonograph cartridge on a leaf near a male well adapted to exploit annuals, including (ca. 2 cm). Signals were ampli®ed 100-fold many grain crops (Whitcomb, 1987; Hunt using a Stanford Research Systems Model and Nault, 1990). Although ®eld or labo- 560 preampli®er and recorded on a Sony ratory studies have not determined the re- DTC 700 or DTC A7 digital tape recorder lationship between population density and (DAT) at 48,000 samples sec21. Recordings the potential for males and/or females to in- were transferred to a Macintosh computer teract, monitoring of male signaling in ur- equipped with a Digidesign Audiomedia II ban landscapes indicates that interactions soundboard. Editing of calls was done us- 1224 R. E. HUNT AND T. L. MORTON ing Alchemy (Passport Designs) software. dividual ¯ew or when the observer grew Band-limited white noise (0.0±20.0 kHz) weary (ca. 1 hr). An acceptable trial re- was created using SoundEdit Pro (Macro- quired a sample size of at least six calls media) software. It was then subjected to a which included four calls from the male and 240.0 dB low-pass ®lter with the cut-off two playback calls. frequency set at 0.8 kHz using Alchemy A sampled randomization test was used software. This procedure resulted in band- to determine whether the number and over- limited noise that covers the frequency lap time of calls emitted by a male inter- range of male signals (Hunt et al., 1992). acting with the playback stimulus was sig- Playback signals were fed from the com- ni®cantly less than the random expectation. puter to the DAT recorder and played into The test was implemented using a program a leaf using a BruÈel and Kjaer Model 4810 developed by one of us (T.L.M). This pro- mini-shaker connected to the headphone gram calculated descriptive statistics for jack of the DAT recorder. Each call was each sequence: average call length, total ®ltered using Q10 (Waves) software to number of overlapping calls (NUMBER) compensate for the 212 dB/octave rolloff and the total time of call overlap (TIME). associated with the minishaker. We determined the time of call overlap of We determined whether chorusing could individual i by individual j by subtracting be elicited solely by calls of other males by the starting time of individual j from the playing recordings of male calls to individ- ending time of individual i (see Brush and ual males and recording their response. The Narins, 1989). We then randomized the ®ve exemplar calls ranged in length from starting times of each call emitted by a male 8±32 sec. We added 40 sec of silence to within the total time of the trial while each call to provide adequate time for a avoiding the overlap of calls by the same male to respond. The sequence of exem- individual. The randomization was repeated plars used within each block (replicate) of 10,000 times for each trial. We compared the experiment was randomized. Although each value of NUMBER and TIME to the the response of each male during a trial rep- appropriate matching distribution from the resents an independent test with an associ- randomization output. The probability of ated P value (see below), we used a ran- receiving a value higher than or equal to the domized complete block design so that we observed value was the number of values could detect possible changes in the re- less than or equal to the observed value di- sponse of males over the course of the ex- vided by the total number of values or ran- periment which lasted several weeks.