Spatial Distribution and Male-Male Communication in the Northern Cricket Frog, Acris crepitans blanchardi

Abstract

-Field observations, coupled with displacement experiments, suggest that male Acris crepitans are site specific. In two experimental manipulations, when conspecific mating calls were broadcast to resident calling males, there was a significant reduction in the number of notes produced by the test subjects. In another series of playback experiments, a subject's response call was modified by an increase in note duration. We suggest that call modification resulting from the stimulus of a conspecific mating call is a mechanism designed to spatially spread out males. Starting with Martofs study on the green frog, Rana clamitans (1953), territoriality has been described for a number of anuran families (see Wells, 1977, for review) and it is clear that in most frog species, some kind of spatial separation is achieved during reproduction. In those species that defend resources such as oviposition sites, physical combat plays a role in agonistic encounters (Wells and Schwartz, 1984). However, in many hylid frogs, spacing patterns are maintained with minimal physical fighting (Fellers, 1979). Playbacks have been used to demonstrate that male anurans are responsive to conspecific auditory signals (e.g., Wells and Greer, 1981; Perrill et al., 1982; Ramer et al., 1983; Schwartz and Wells, 1984; Sullivan, 1985; Sullivan and Leek, 1986; Wells and Taigen, 1986). The use of acoustical playback techniques is powerful in that it allows for easy manipulation and assessment in the natural habitat of the animal. Our study, designed to examine the dynamic nature of anuran communication, demonstrates that calling male cricket frogs, when presented with a conspecific advertisement call, modify their calls giving what we take to be an aggressive response. Distinct aggressive calls are used by many frogs to maintain appropriate separations through auditory signals. The northern cricket frog, Acris crepitans blanchardi, is a hylid frog whose advertisement call varies within and among individuals. This call is a series of notes, varying both in number of notes per call and i g ith Martofs study on the green frog, cla itans (1953), territoriality has been defor a number of anuran familie (see l , 1977, for review) and it is clear that in rog species, some kind of spatial separais achieved during reproduction. In those i s that defend resource such as oviposisites, physical combat plays a role in agoi encounters (Wells and Schwartz, 1984). er, in many hylid frogs, spacing patterns aintained with minimal physical fighting l s, 1979). Playbacks have been used to strate that male anurans are responsive specific auditory signals (e.g., Wells and note duration (see Nevo and Capranica, 1985). An increase in note duration is the result of a greater number of pulses per note. These pulses are not resolvable by the human ear. Of particul interest to our study is note duration which, in a typical advertisement call, is short at the beginning of the call and increases toward the end of the call. It has been shown in several recent studies with anurans that the male call consi ts of a graded signal with different functio s associated with different parts of the call (e.g., Littlejohn and Harrison, 1985). Although a di tinct male aggressive call used during agonistic encounters with other conspecific males has been described for other anurans, the call repertoire of A. crepitans does not include a separate aggressive call. Rather, we demonstrate that the note duration of the advertisement call is modified to serve as an encounter call designed to space out males. MATERIALS AND METHODS The northern cricket frog is distributed through the central U.S. from Michigan to Texas (Conant, 1975). Males call from mats of vegetation in the water and along the mud banks of ponds. Calling begins before sunset and continues until 0200 or 0300 hours EST. Early in the evening, when calling first begins, males are often found calling within 1 m of one another. However, as the evening progresses, males spread out and are typically separated by more than 1 m. This study was performed in a te duration (see Nevo and Capranica, 1985). 237 This content downloaded from 207.46.13.120 on Wed, 14 Sep 2016 05:48:03 UTC All use subject to http://about.jstor.org/terms S. A. PERRILL AND W. J. SHEPHERD series of shallow ponds in central Indiana during the springs and summers of 1984-1987. The ponds are located in an Indianapolis city park where the animal populations are protected from extensive human disturbance. Spatial Distribution. -When first captured, snout-vent length (SVL) measurements were taken on all frogs and male frogs were toeclipped and fixed with embroidery floss waistbands of various colors for individual recognition. The waistbands had no apparent effect on the frogs' activities. In order to identify an individual's position and monitor it throughout the night, a small white plastic marker with the number assigned to that frog was placed in the mud bank at a distance of 10 cm from the frog. Over two field seasons (1985 and 1986), 35 male frogs on 2 or more nights were identified as to their calling position at the beginning of the evening (0830 to 0930) and censused at their marked positions at the end of the evening (1130 to 1230). In 1984, a displacement experiment was conducted on 10 males that had been observed calling from the same location on at least 3 previous nights. Five of the 10 were moved 15 m down the bank and the other 5 were moved 25 m. Playback Experiments.-The stimulus call used in this study was a recorded natural call of 22 sec consisting of 70 notes; we repeatedly dubbed this onto an experimental tape. Two Uher tape recorders (report monitor models 4200 and 4400) were used for recording and broadcasting calls. Calls were recorded with a Sony Electret Condenser Microphone (model ECM-33F) handheld at 30 cm from the subject. Calls were broadcasted with a Sony Amplifier Powered Monitor (APM-090) whose face measured 14 cm by 9.5 cm. A tape speed of 9.5 cm/sec was used for all broadcasting and recording. When broadcasting calls, we placed the speaker either directly on the mud bank or on an elevated platform consisting of a 3-legged ringstand with a wooden block on top for speaker placement. Since male cricket frogs in our populations are typically seen calling at 50 cm or more, an intruder effect was achieved by broadcasting all calls at a distance of 30 cm from the subject at a sound pressure level of 89 dB (SPL re 20 ,iPa). This sound pressure level is comparable to what we recorded from calling males (see Table 4). Sound level readings were taken with a Bruel and Kjaer integrating sound level meter (model 2230). We observed potential subjects for a minimum of 5 min before testing them to be sure that they were stable calling males. All testing was done between 2100 and 0100 hours EST. Calls were analyzed with a Kay Sonagraph (model 7029A) and a Tektronix Storage Oscilloscope (model 5111). Call notes were counted with a handheld counter and data were analyzed with the one-tailed sign test (Siegel, 1956). Two experiments were conducted to determine whether a calling male changes his note rate when presented with a broadcasted conspecific call. In the first experiment (N = 17 males), a frog was observed calling for a minimum of 5 min, the speaker was placed 30 cm from him, and the number of notes emitted during a 2 min interval were counted. This was ollowed by one of two broadcasted calls: 1) 2 mi of the conspecific call with each call separated by an 8 sec pause (total of 4 calls) or 2) serving as a control, a Hyla versicolor call, each call separated by an 8 sec pause. The order of call presentation was reversed for each subject. During the broadcasting intervals, the number of notes emitted by each subject were counted. A similar approach was used in the second experiment (N = 10 males), but both experimental stimuli were conspecific calls, one with the calls separated by no pause and the other with each call separated by an 8 sec pause. Two trials were run on each subject with the order f call presentation reversed for each trial. A third experiment was designed to analyze the fine features of a subject's response call to a conspecific broadcast. A male was observed calling for at least 5 min and the speaker was placed on an elevated platform at 30 cm. With a hand-held microphone, we recorded an unsolicited advertisement call at 30 cm; we followed this with a broadcast of a 4 sec interval c mprised of 20 notes from the middle portion of the dubbed call. The subject's response to the broadcast was recorded. Twelve frogs were tested in this design and all 12 responded immediately. These experiments allowed for diect comparisons between the A. crepitans unsolicited call (recorded prior to broadcast) and the solicited response (recorded immediately after the broadcast). A 4 sec interval of three H. versicolor advertisement calls served as a con-