Calling songs of male crickets attract sexually mature, conspecific females for mating (for recent reviews see Elsner & Popov, 1978; Huber & Thorson, 1985; Doherty & Hoy, 1985). This communication system has been the subject of many behavioural studies on the relevant properties of the male’s calling song for recognition by females (e.g. Walker, 1957; Popov & Shuvalov, 1977; Pollack & Hoy, 1979, 1981; Thorson, Weber & Huber, 1982; Stout, DeHaan & McGhee, 1983; Doherty, 1985b; Doolan & Pollack, 1985). The behavioural investigations have required some means of measuring the female’s phonotaxis (her locomotion or turning towards a sound source). These include ‘closed-loop’ methods in which the animal moves freely in acoustic space and ‘open-loop’ methods in which the animal is tethered and is not allowed to experience changes in sound intensities as it runs in the direction of the sound source. Closed-loop methods include free walking in arenas (e.g. Murphey & Zaretsky, 1972; Paul, 1976; Hoy, Hahn & Paul, 1977; Pollack & Hoy, 1979; Stout et al. 1983); open-loop methods include tethered flight (Moiseff, Pollack & Hoy, 1978; Pollack & Hoy, 1979, 1981; Pollack, Huber & Weber, 1984), tethered walking on Y-maze globes (Hoy & Paul, 1973) and free walking on a spherical locomotion compensator (Wendler, Dambach, Schmitz & Scharstein, 1980; Thorson & Huber, 1981; Thorson et al. 1982; Pollack et al. 1984; Doherty, 1985a,b,c; Schmitz, 1985).
Here we describe a new open-loop method for quantifying cricket phonotaxis. This method uses inexpensive microcomputer technology, is completely automated and therefore rapid and objective, and could be adapted for studying locomotory movements of other animals. The data obtained are in such a form that they are easily compared to data generated by the spherical locomotion compensator (Kramer treadmill, see Kramer, 1976; Wendler et al. 1980; Weber et al. 1981), which has been used for quantifying phonotaxis in the field cricket, Gryllus bimaculatus (Doherty, 1985a, b, c).
The spherical locomotion compensator or Kramer treadmill was first developed by scientists in West Germany (Kramer, 1976; Wendler et al. 1980; Weber et al. 1981; Thorson et al. 1982). It uses an infrared detection system to monitor movements of an untethered cricket on top of a sphere. This positional information feeds back to servomotors that move the sphere in the opposite direction. These compensatory movements ‘fix’ the cricket at the top of the sphere as it performs walking phonotaxis. The colloquial name for this locomotion compensator is ‘Kugel’ (German translation of the word ‘sphere’). Because our device for measuring walking phonotaxis utilizes the Apple Macintosh computer and is similar in appearance to the German Kugel, we call our new device the ‘MacKugel’. The main mechanical difference between these two devices is that in the MacKugel system the cricket is tethered and its own walking movements provide the power to rotate the sphere.
The MacKugel is a complete stimulus control and data acquisition system for online studies of cricket phonotaxis. Data on cricket movements during phonotaxis experiments are passed directly to an Apple Macintosh microcomputer, which can also synthesize acoustic stimuli or send control signals to an external acoustic synthesizer. In designing this system, we took advantage of ROM routines in the Macintosh for quantifying movements of the Apple ‘mouse’.
The mouse is a mechanical, optoelectronic device that converts rotational movements of a rubber ball to changes in the coordinates of a cursor displayed on the computer’s CRT monitor. The ball bears against the operator’s desk surface, and against three rollers inside the ball’s housing. One roller is for mechanical support only. The other two, representing orthogonal x- and y-axes, are connected to rotating vanes which interrupt light beams between LED–phototransistor pairs. The phototransistors produce electronic signals which are sent to the computer.
We adapted the mouse to study cricket phonotaxis by expanding the distance between the rollers and replacing the small rubber ball (2·5 cm diameter) with a larger and much lighter sponge-rubber ball (10 cm diameter, 8·7g). This larger ball (a hollowed-out, toy ‘Nerf ball’ from Parker Bros, Beverly, MA, USA) was mounted in a frame along with the roller/phototransistor modules from the mouse (Fig. 1). In this way the rollers could be actuated by a tethered cricket walking on top of the ball. As the cricket ran in one direction (i.e. towards an attractive acoustic stimulus), the ball was moved in the opposite direction and the movements were transduced into changing pixel coordinates on the Macintosh screen. The minimum force required to move the Nerf ball ranged from 1·117 to 2·793×10−2N. A plastic sphere (10 cm diameter, 12·8 g) is currently under development and this improved version only requires forces ranging from 0·69 to 1·05×10−2N. High-level computer languages (Macintosh Pascal®) were used for data acquisition and analysis (program available from author). By sampling pixel coordinates once every second, we were able to calculate instantaneous velocity and direction profiles for the cricket’s movements. These profiles, along with their corresponding ‘vector plots’ (see below), were displayed simultaneously on the Macintosh screen, and the data were saved on micro-floppy disks for subsequent data analysis.
To establish the usefulness and validity of our method, we repeated some locomotion compensator experiments run previously on the field cricket, G. bimaculatus (Doherty, 1985a,b). In both methods, walking phonotaxis in crickets can be quantified by measuring time profiles of the cricket’s walking velocity and direction. Fig. 2 shows these time profiles for one MacKugel experiment in which a synthetic calling song was presented to a female G. bimaculatus from one of two loudspeakers separated by 180°. This calling song was played from the left speaker for 2 min and then from the right speaker for another 2min. A 30-s silent period preceded and followed this playback. The female tracked the stimulus, as shown by her narrow meandering about the direction of the active speaker. Her walking velocity also increased when the stimulus was presented, compared to the preceding silent period.
The time profiles shown in Fig. 2 were generated by sampling and storing the position of the animal once each second during an experiment. This method of data collection easily lent itself to another way of representing phonotactic movements visually. For each 1-s interval, a vector was calculated that showed the animal’s walking direction relative to the direction of the active speaker (vector angle) and its velocity (vector length). These vectors were cumulatively plotted as shown in Fig. 2. These ‘vector plots’ show that when the stimulus was presented from either the left or right speaker, the vectors were clustered about the direction of the active speaker.
The direction component of cricket walking phonotaxis is a sensitive indicator of the attractiveness of an acoustic stimulus. To investigate this sensitivity further, we ran phonotaxis experiments in which acoustic stimuli with different pulse periods were presented in a sequential, to-and-fro paradigm. As in earlier experiments of cricket phonotaxis on a locomotion compensator (Doherty, 1985c), pulse periods ranging from 30 to 50 ms yielded the best tracking on the MacKugel (i.e. best orientation to active speaker direction, see Fig. 3).
In these same phonotaxis experiments on the MacKugel we found slight effects of pulse period on the walking velocity profiles. Females walked for a greater percentage of the stimulus presentation time when pulse periods were optimal (i.e. 30–50ms) and walked less in response to stimuli with pulse periods outside this range. Furthermore, mean walking velocity was highest in response to pulse periods of 35 and 40 ms, whether or not pauses in walking were included in the calculation.
The vector score is comparable to other measures of stimulus efficacy in cricket phonotaxis. In earlier studies of G. bimaculatus phonotaxis on a locomotion compensator, the attractiveness or efficacy of an auditory stimulus was quantified by measuring the percentage of the stimulus presentation time that the female clearly ‘tracked’ the stimulus. Tracking has been defined as meandering within a certain angle ‘window’ (±60°) about the angular direction of the active speaker (see Thorson et al. 1982; Doherty, 1985a,b,c). These same criteria were used in our study for measuring the percentage of time spent tracking in a group of nine G. bimaculatus females. In to-and-fro sequential experiments, tracking scores and vector scores in response to calling songs with different pulse periods were comparable (Fig. 4). Both scores showed that songs with pulse periods between 35 and 50 ms were most effective in eliciting positive phonotaxis.
The MacKugel system is also useful for quantifying cricket phonotaxis in two-stimulus (choice) playback experiments. The total vector score was a sensitive, objective measure of the relative attractiveness of alternative acoustic stimuli. When given a choice between alternating chirps with different pulse periods (PP), female G. bimaculatus clearly preferred the standard chirp stimulus (40 ms PP) to the alternative chirp stimuli with pulse periods of 20, 30, 50 and 60 ms. The strength of the female’s preference for the standard over the alternative pulse period was also reflected in the mean vector score. These results were consistent with those of previous choice experiments on a locomotion compensator (Doherty, 1985c).
The MacKugel system is an efficient, rapid and objective method for measuring the efficacy of different acoustic stimuli in eliciting walking phonotaxis of crickets. Using this system the data can be displayed to give a qualitative, visual representation of an individual cricket’s movements. Complicated time profiles of movement velocity and direction can be reduced objectively to a numerical score that can be used as a rough basis of comparison between individuals and between experimental treatments. Data analysis can be done ‘on-line’ as the animal is behaving. Other strengths of the MacKugel are its low price, simplicity of design, and efficient implementation of existing microcomputer technology.
This research was supported by grants from NSF and NIH to JAD, AP and Ronald R. Hoy. We thank Alan Cohen and Rick Nicoletti for technical assistance and Wendy Sussdorf for preparing Fig. 1. Ronald Hoy, Peter Brodfuehrer and Jud Crawford read earlier drafts of this manuscript.