Graded Aggressive Calls in the African Painted ... - Wiley Online Library

Ethology 101, 67-81 (1995) @ 1995 Blackwell Wissenschafts-Verlag, Berlin ISSN 0179.1613

Section of Neurobiology and Behavior, Cornell University, Ithaca

Graded Aggressive Calls in the African Painted Reed Frog Hyperolius mawnoratus (Hyperoliidae) T. ULMAR GRAFE GRAFE,T. U. 1995: Graded aggressive calls in the African painted reed frog Hyperolius rnurrnorutus (Hyperoliidae). Ethology 101, 67-81.

Abstract The African painted reed frog, Hyperolius rnarrnordtus, has a potentially complex communication system. Advertisement calls and aggressive calls, although distinct from each other, are in fact two ends of a continuum of graded calls. Playback experiments using standard advertisement calls showed that males increased the proportion of aggressive calls as the stimulus intensity was increased. In addition, three characteristics of the aggressive calls changed in response to higher playback levels. Males increased the number of pulses /call, increased call duration, and decreased dominant frequency. Aggressive calling occurred primarily during the early hours of the night, with considerable overlap with times when females were searching for mates in the chorus. Females tested in two-choice arena trials discriminated against aggressive calls in favor of advertisement calls. It is suggested that aggressive calls reduce a male’s ability to attract a female and that a graded signalling system may enable males to escalate agonistic encounters with other males without rendering calls completely unattractive to females.

T. U. GRAFE,Zoologisches Institut 111, Biozentrum, Am Hubland, D-97074 Wiirzburg, Germany.

Introduction Many anurans have distinct advertisement and aggressive calls o r call components that are used in specific social contexts and directed to specific receivers. Some species, however, use multiple types of aggressive calls or graded signalling systems in which males are thought to escalate encounters in a continuous fashion

(ARAK1983b; LITTLEJOHN & HARRISON 1985; SCHWARTZ 1989; WELLS1989; WAGNER1992). U.S. Copyright Clearance Center Code Statement: 0179-1613/95/1011-0067$11.00/0

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The functional significance of graded signals continues to be of considerable interest to behavioral ecologists. It has been proposed that graded aggressive signals function to convey the signaler’s probability of escalating an agonistic encounter (e.g. ENQUIST1985). Graded signals would be especially useful for conveying motivation if motivation varied in a continuous fashion. Initial models based on game theory predicted that signals should not convey information about the probability of subsequent behaviour because such signals could easily be exaggerated and become unreliable (MAYNARD SMITH& PRICE1973). However, 1984; POOLE more recent models (GRAFEN1990) and empirical research (NELSON 1989; CAPD & SEARCY1991) have shown that graded aggressive signals can reveal the motivational state of individuals and are used to reveal information about & NELSON 1991). intentions (review: HAUSER Graded aggressive displays appear to be common in a wide array of animal taxa and across sensory systems (SEARCY& ANDERSON 1986; ARCHER1988; WELLS1988). Anurans are well suited for studies of acoustic communication because of their relatively limited vocal repertoire and the ease with which signals can be synthetically generated and played back in the field. However, studies of graded signalling systems have been limited in anurans since most research on anuran communication has focussed on the advertisement call. As a result, details of the aggressive call function are generally not well understood (review: WELLS 1988). The purpose of this study was to investigate the function of aggressive calling in the painted reed frog Hperolius marmoratus broadleyi (family Hyperoliidae) and the use of such calls in mediating male-male interactions. Specifically, I examined how the intensity of a stimulus call influences the level of aggressive response by males as they defend their nightly calling sites. In addition, I tested the preference of females for a standard advertisement call against a standard aggressive call. I predicted that females would discriminate against long aggressive calls and that males would use long aggressive calls only when shorter aggressive calls have not been successful in repelling males. Finally, preliminary recordings of advertisement and aggressive calls showed considerable variation in the number of pulses per call. Advertisement calls, although predominantly tonal (with an average duration of 83 ms, a dominant frequency of 2400 Hz, and a slight upward frequency modulation of 215 Hz; based on recordings from 68 males), often contained pulses in the initial portion of the call (Fig. la). By contrast, aggressive calls were completely pulsatile and longer in duration than advertisement calls (Fig. 1 b). The extent of the variation of the two call types was further investigated. Male H . m. broadleyi typically call in dense aggregations and defend elevated calling sites against other males. Encounters often escalate into intense grappling and kicking bouts that usually lead to the displacement of one of the males. The functions of both advertisement and aggressive calls have been extensively 1981; TELFORD & characterized and studied in H. m. marnorutus (PASSMORE PASSMORE 1981; TELFORD 1985; DYSON & PASSMORE 1992; DYSON et al. 1992), but none of those studies reported the kind of temporal variation in advertisement and aggressive calls that I report here for H. m. broadleyi.

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a. Sonogram and waveform of a typical advertisement call with one distinct pulse; b. Sonogram and waveform of a typical aggressive call by the same male with 16 pulses

Methods Study Area Field work was conducted in northeastern Zimbabwe at Claremont Orchards (1 800 m in elevation; 18"20' S, 32"42' E), just south of the Nyanga National Park. Complete nightly records of calling activity were recorded for 18 consecutive nights in Jan. and Feb. 1991 at a small natural pool (3 x 10 m) about 500 m along a small stream leading into Claremont Dam. Playback experiments were conducted in Feb. and Mar. 1993 along the same stream. In both years, males called in isolation and in small groups (up to 30 individuals) on vegetation in or near the water. Air temperatures ranged from 14 to 19 "C on nights when playback experiments were conducted in 1993 and never changed by more than 1 "C during any one trial. Thus, changes in temperature did not influence the results.

Patterns of Nightly Aggressive Calling I recorded continuous nightly activity of small choruses (2-18 males) in 1991 by placing an omnidirectional microphone (Vivanco EM-32) at the edge of the chorus and recording with a Sony WM-D6C stereo cassette recorder. Tapes were later analysed for the number of aggressive calls males gave to determine the timing of aggressive calling as a function of the time of night (8 nights analysed) and the total number of aggressive calls given as a function of chorus density (9 nights analysed). Aggressive calls could not be assigned to individual males from the single-channel recording because of the large number of chorusing males. Only those aggressive calls were counted that had at least 10 pulses per call and that I could easily audibly distinguish from advertisement calls. During recordings, male behavior was closely monitored by two observers and the time at which each male mated was determined to the nearest 5 min.

Playback Experiments Individual males were presented with synthetic advertisement calls and aggressive calls in an interactive fashion. Stimulus calls were generated using customized software in H. C. GERHARDT'S laboratory. Synthetic advertisement calls with properties modelled according to the population mean (hereafter referred to as a 'standard call': dominant frequency 2450 Hz, duration 80 ms, frequency modulation 200 H z , 4 ms rise time, 32 ms fall time, 2 pulses) were presented to males in the field from a Macintosh PowerBook, amplified by a Shure F P l l microphone amplifier and broadcast using a Realistic Minimus-0.6 amplified speaker mounted on a tripod. The speaker was placed an average distance of 41 cm (& 9 cm) from the calling male in the horizontal plane, unobstructed by vegetation. Discrete settings of the amplifier allowed for the stimulus amplitude to be increased in increments of 6 dB. For every call a male gave, either advertisement or aggressive, a single synthetic advertisement

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call was presented with the click of the PowerBook track-ball button. The vocal response of males was recorded using a Sony WM-D6C stereo cassette recorder and an Aiwa CM-Z3 electret condenser directional microphone placed near the male. Males were first recorded for a 2-min non-stimulus period (NS) after which they were confronted by broadcasts of calls for 2-min periods at increasing intensities, starting at the lowest intensity and ending with the highest intensity. Stimulus intensities were 84, 90, 96, and 99 dB (re 20 $a) peak sound pressure level (SPL) at 41 cm. The third intensity increment was only 3 dB due to the output limitations of the speaker. The design of the playback experiments did not exclude any possible bias from test order. However, the playback sequence was intended to simulate natural encounters between males in which call intensity increases with decreasing intruder distance. Playback intensities were determined using a Briiel and Kjaer impulse precision sound-level meter (Model 2204) set for flat weighting and a free field-response microphone (Bruel and Kjaer type 4163). Playback levels were similar to the natural range encountered in this species. Peak sound-pressure ,a)' varied between 87 and 91 dB measured at 50 cm in front of males (n = 15) levels (SPL; dB re 20 & with a hand-held SPL-meter (realistic meter set for flat weighting and fast response). Variation in the distance between speaker and males resulted in an intensity difference of the playback at the male calling sites of ? 2 dB. Both stimulus and response were recorded on the same channel of the tape recorder. At the end of the experiment, measurements were taken of the snout-vent length, tibia-fibula length, and the mass of males. I also toe-clipped males for later identification to prevent retesting. O u t of 36 males tested, 14 responded with numerous aggressive calls to at least one playback intensity. Most males either moved away from the speaker, stopped calling, or did not respond with aggressive calls, even at the highest playback level. Some males (n = 9) responded with aggressive calls only after synthetic aggressive calls were presented. For five males that responded with aggressive calls to advertisement-call stimuli, the playback series was extended to include 2 min of aggressive-call playback (160 ms duration, 2450 Hz dominant frequency, and 20 pulses) at the unchanged highest playback intensity of 99 dB at 41 cm. This sample was used to determine whether aggressive calls elicited a stronger response than advertisement calls. As in the playbacks with advertisement calls, playbacks using aggressive calls were interactive with a call presented for every call presented by the male.

Female-choice Experiments I collected gravid females at Claremont Dam and tested them within 3 h in a nearby indoor arena (1.0 x 2.0 m). Females were given the choice between a standard advertisement call with two pulses and an aggressive call with 20 pulses. Both calls were identical to those used in playback experiments with males. T w o speakers (Realistic Minimus-0.6) were set 2 m apart, one at each end of the arena. The stimulus tapes were played with a Sony WM-D6C stereo cassette recorder at a rate of 52 calls per min. Females were placed under a small plastic container in the center of the arena midway between the speakers where they were exposed to the alternating test stimuli for 30 s before the container was removed. A positive response was recorded if females phonotactically approached within 10 cm of a speaker within 6 min. Most females made physical contact with one of the speakers within 2 min. Playback levels were equalized at 88 dB SPL (re 20 PPa) at the female release point using a Realistic sound-level meter (flat weighted and fast-response setting). The temperature during experiments was 17-20°C. For further details of the experimental setup and the rest protocol see GRAPE(1995).

Analysis of Recordings Recordings were digitized using a MacRecorder (Macromedia Inc.) at a sampling rate of 10 k H z and analysed using SoundEdit and Canary software run on the Macintosh. The number of advertisement and aggressive calls given by males during playback experiments was counted and the number of pulses in each advertisement and aggressive call determined. Pulses were defined as cycles of amplitude modulation with a depth of at least 50 %. Calls that were completely tonal will be referred to as unmodulated (UM). The frequency of maximum amplitude was used as a measure of dominant frequency. The call duration of advertisement and aggressive calls was measured to the nearest 1 ms.

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Fig. 2: Relationship between chorus size and total number of aggressive calls given per night over 9 nights

All aggressive calls were measured, whereas a sample of 10 randomly selected calls was used to describe advertisement calls.

Statistical Analyses Analyses were conducted using SYSTAT (WILKINSON 1992). The nonparametric Friedman twoway ANOVA was utilized to analyse any effects of playback intensity on male vocal behavior. The Wilcoxon matched-pairs signed-ranks test was used for paired comparisons. To control experimentwide type-I error rates, the sequential Bonferroni method (RICE1989) was used and probabilities adjusted accordingly. Mean values 4 SD were used as descriptive statistics unless stated otherwise.

Results Pattern of Nightly Aggressive Calling The total number of aggressive calls (pooled over all males) given on any night depended on the number of males present in the chorus (Fig. 2). A second order polynomial best fits the data, suggesting that aggressive calling increases exponentially as chorus size increases. However, the data could also be interpreted as a linear increase in aggressive calling once a threshold chorus size of eight males is reached. Aggressive calls were given predominantly during the early hours of chorusing activity prior to the appearance of gravid females. Fig. 3a shows one representative example of the distribution of aggressive calls over the course of a night in a chorus of 11 males. O n this particular night, in which the chorus lasted close to 4 h, 69 YO of all aggressive calls were given within the first h, 97 YO within the first 2 h. Overall, in a sample of 8 nights, the average percentage of aggressive calls given was 68.1 2 22.4 % within the first h and 89.7 12.6 Yo within the first 2 h. A total of 10 males mated during the 18-night observation period. Fig. 3b shows at what time of night these males mated. Even though most aggressive calls were given early in the evening, some aggressive calling also occurred at a time when females were present in the chorus selecting mates.

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Time of night (h)

Fig. 3: a. Percentage of aggressive calls out of the total number of aggressive calls given in the course of 1 representative night in a chorus of 11 males; b. Time of mating for all males seen to mate during 18 consecutive nights of observations (n = 10). Width of bars is 5 min

Advertisement call I

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Fig. 4: Waveforms of calls given by a single male within a 2-min period in response to playback of advertisement calls. The calls are arranged in order of increasing number of pulses per call and call duration

Playback Experiments

Calls given by males in response to playbacks of advertisement calls varied considerably in the number of pulses per call and in call duration. The number of pulses per call and call duration varied in a graded fashion with the typical advertisement and aggressive call as two ends of a continuum of calls. Fig. 4 shows a representative example of calls given by one male during a 2-min playback period, sorted by number of pulses per call. A total of 14 males responded with aggressive calls to advertisement-call playbacks. During the non-stimulus period, all calls were either unmodulated (UM) or contained 1 4 pulses (Fig. 5a). The mode was unmodulated. During the playback of advertisement calls, calls given by males showed a bimodal distribution, with one mode at two pulses per call

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Fig. I: Histograms of the number of pulses per call (UM = unmodulated) of all calls given by 14 males: a. During the non-stimulus period preceding the playbacks (n = 947); and b. To playbacks of standard advertisement calls (n = 4381). Number of calls above 100 are reduced in scale by 1 :10 and are shown in gray

and the other at I8 pulses per call (Fig. 5b). Despite the graded nature of advertisement and aggressive calls, it seems useful, based on Fig. 5b, to define advertisement calls as calls with four pulses or less and aggressive calls as calls with more than four pulses. This cut-off point was used as an operational definition to distinguish advertisement from aggressive calls. Males switched to aggressive calling as the intensity of the advertisement call playback increased and gave proportionately more aggressive calls at higher playback intensities (Fig. 6a). A Friedman two-way ANOVA showed significant differences in the proportion of aggressive calls given to different playback intensities (2= 15.7, 3 df, p < 0.001). At most playback intensities, males gave a combination of advertisement and aggressive calls. Both the number of pulses per call and call duration increased significantly with stimulus intensity (Fig. 6b, c; Friedman two-way ANOVA: J = 16.6, df3, p < 0.001 and J = 16.0, df3, p < 0.001, respectively). Likewise, the average dominant frequency of a male’s call was significantly lower at higher stimulus intensities (Fig. 6d; Friedman twoway ANOVA: J = 9.9, df 3, p = 0.02). The drop in mean dominant frequency between the first and last playback and between the non-stimulus period and the last playback was 120 Hz and 136 Hz, respectively (n = 14). Changes in the average number of pulses per call, call duration, and dominant frequency over all calls, irrespective of whether advertisement or aggressive, may

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Fig. 6: Response to playbacks of advertisement calls at four different intensities showing: a. % aggressive calls;b. Number of pulses per call; c. Call duration; and d. Dominant frequency. Data are presented as X + SE. Numbers above bars: number of males tested; N S = non-stimulus period

simply be due to the increasing proportion of aggressive calls given. I therefore evaluated changes in advertisement calls (C four pulses /call) and aggressive calls (> four pulses/call) separately. Advertisement calls did not change significantly with playback intensity in any of the three variables measured (Friedman two-way ANOVA: p < 0.05 for all tests). However, the number of pulses per call and call duration increased significantly between the non-stimulus period and the average response to playbacks of advertisement calls (two-tailed Wilcoxon test, p < 0.01 for both tests). Aggressive calls were given to more than one playback level by 11 males. As males gave their first aggressive calls at different intensities, the first aggressive response was compared to the second such response. Table 1 shows the changes in the three call variables measured. Both number of pulses per call and call duration showed a significant increase with stimulus intensity (Friedman two-way ANOVA:? = 7.4, df 1, p < 0.01 and? = 5.8, df 1, p < 0.05, respectively), whereas dominant frequency decreased significantly (Friedman two-way ANOVA; 2 = 4.4, df 1, p < 0.05). Playbacks using aggressive calls elicited a stronger response than the previous advertisement-call playback both in the number of pulses per call and in dominant

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Table I: X '-c SD of number of pulses per call, call duration (ms), and dominant frequency (kHz) of aggressive calls given to synthetic advertisement calls at the first two playback levels that caused an aggressive call response. Number of pulses/call, call duration and dominant frequency showed a significant change (p > 0.05 in all three cases) from first to second response

Frog

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First response Call Dominant Pulses /call duration frequency

1 2 3 4 5 6 7 8 9 11 21

7 13 24 4 11 13 2 5 5 1 3

8.721.9 17.723.9 2 1 . 3 2 5.5 15.5 5 3.4 9.6 2 3.8 17.023.3 8.524.9 16.826.9 7.1 2 2.5 6.0 5.3 2 0.6

138233 255261 251 2 71 279 2 26 112 k 30 204240 121273 230582 94 2 7 79 80 2 10

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2.22kO 1.83-tO.13 1.93 2 0 . 0 9 2.11 f 0.06 1.85 k 0.14 1.9820.13 2.2820.16 1.95i0.12 2.33 -t 0.03 2.15 2.22 2 0.25 2

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Second response Call Dominant Pulses /call duration frequency

20 25 16 3 10 26 34 33 18 8 23

13.723.6 18.1i2.1 22.4 2 3.5 1 5 . 7 2 1.5 13.2 2 5.2 18.623.1 16.lr1.9 18.123.2 12.3 2 4.1 9 . 3 2 4.2 1 5 . 4 i 3.6

2.03rC-0.08 1.7720.02 1.92+0.08 2.00 2 0 2.11 rC- 0.04 1.9620.14 2.04rC-0.11 2.02-tO.07 2.08 k 0.06 2.12 2 0.03 2.01 2 0.09

15.8 2 3.5 200 2 52 2.01 2 0.10

0.18

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183248 255236 264241 256 2 15 130 2 39 220222 175+19 254k28 191 2 52 1 4 2 2 64 135 2 53

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Fzg.7: Aggressive call response of five males to playbacks of advertisement calls (ADV) and aggressive calls (AGG). A stronger response was elicited by playbacks of aggressive calls than by the previous advertisement-call playback in both the number of pulses per call and the dominant frequency (Wilcoxon test, p < 0.05) but not in call duration (Wilcoxon test, p > 0.05)

frequency (two-tailed Wilcoxon test, p = 0.04 for both tests), but not in call duration (two-tailed Wilcoxon test, p > 0.05; Fig. 7). The number of pulses per call and call duration were highly correlated in 16 out of 20 males for which aggressive calls were recorded (p-values adjusted using a sequential Bonferroni). For the majority of males, no relationship was found between the number of pulses per call and the dominant frequency of aggressive calls. However, the dominant frequency of aggressive calls was significantly lower than that of advertisement calls (two-tailed Wilcoxon test, p < 0.01, n = 20). Likewise, the peak amplitude of aggressive calls was significantly lower than that of advertisement calls by, on average, 42 -C 14 % (two-tailed Wilcoxon test, p < 0.05, n = 10). No significant correlation was found between snout-vent length and the

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dominant frequency of either advertisement or aggressive calls (r, = - 0.16 and 0.35, respectively, p > 0.05 in both cases). Furthermore, the drop in dominant frequency from advertisement to aggressive calls was not related to body size (r, = - 0.32, p > 0.05). Female-discrimination Trials Females discriminated significantly between the standard advertisement call and the aggressive call (two-tailed binomial test; p < 0.001, n = 20) with only one female choosing the aggressive call.

Discussion Advertisement and aggressive calls of H . m. broadleyi are graded signals in which the number of pulses per call, the call duration, and the dominant frequency change in a continuous fashion. Playback experiments demonstrated that males show a graded response to increases in stimulus intensity both in the proportion of aggressive calls given and in the nature of calls. Overall, males significantly increased the number of pulses per call and call duration and significantly decreased dominant frequency during their second aggressive response. However, variation between males was dramatic. Changes ranged between 1 and 191 YOin the number of pulses per call, between - 8 and 103 % in call duration, and between - 1 1 and 14 % in dominant frequency. Despite this variation in response among males, the change in both temporal and spectral characteristics of calls demonstrates that males respond in a graded fashion to playback intensity and, by inference, to neighbor proximity. It is well established that male anurans use call intensity as a measure of neighbor proximity (FELLERS 1979; ARAK1983 b; BRENOWITZ et al. 1984; TELFORD 1985; SCHWARTZ1989; STEWART & BISHOP1994). TELFORD (1985) showed that H . m . marmoratus females prefer calls broadcast from widely spaced speakers, indicating that spacing increases a male’s mating success. In the case of H . m . broadfeyi, males with larger sound fields are predicted to have higher mating success because females are more likely to be attracted into their sphere of influence. Physical combat is pronounced in both subspecies of the painted reed frog (TELFORD1985; DYSON& PASSMORE 1992; pers. obs.), supporting the importance of spacing for mate attraction, since no other resources are known to be involved. The results of the aggressive call playbacks showed that the aggressive call is a stronger agonistic signal than the advertisement call. However, these results do not address whether males can discriminate between aggressive calls, o r how fine their discriminatory ability is. The use of graded signals does not necessarily mean that they are perceived in a continuous fashion (GREEN1975; CHENEY & SEYFARTH 1982). Even though male H . m. broadleyi showed a stronger response to aggressive calls with 20 pulses than to a standard advertisement call with two pulses, and several individuals only responded with aggressive calls to playbacks of aggressive


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calls, this only suggests that males can distinguish between advertisement and aggressive calls, which is not surprising. Neurophysiological work on other anurans has indicated that neurons sensitive to different rates of amplitude modulation exist in the midbrain (ROSE& CAPRANICA 1983, 1984; WALKOWIAK 1984), suggesting that if the graded temporal variability found in H . m. broadleyi calls is encoded, this will take place in higher centers of the auditory pathway, not via the peripheral auditory system. Clearly, further behavioral investigations are necessary to determine whether the described variation in aggressive response is functionally important. The best evidence from studies of anuran communication that shows that signal variants are perceived in a graded fashion comes from work on Hyla 1989), both members ebraccata (WELLS1989) and Pseudacris cvucijer (SCHWARTZ of the family Hylidae. Males of both species elevate their aggressive response as a result of increases in the call duration of playbacks. However, whether males signal likelihood of attack is not known. The functional significance of graded calls in H. m. broadleyi has yet to be elucidated, however, some inferences can be drawn from patterns of nightly aggressive calling. Aggressive calls were given predominantly during the early evening when calling sites were being established, as has been shown for other anurans (review: WELLS1988). However, agonistic interactions between males also occur at a time when females are in the chorus searching for mates. Previous work has shown that female H . m. broadleyi, given the choice between a standard advertisement call with two pulses and calls with 4-1 0 pulses discriminate against calls with a higher number of pulses (GRAFE1995). Discrimination against long aggressive calls with 20 pulses, demonstrated in this study, was even more pronounced. These results suggest that graded calls allow males to gradually increase the aggressive content of a signal without rendering calls completely unattractive to females. A tradeoff of this sort has also been suggested in the neotropical frog Hyla ebraccuta, in which the multinote advertisement call changes into an aggressive call as the number and duration of secondary click notes decrease and the duration of the introductory note increases (WELLS& SCHWARTZ1984b; WELLS1989). Females discriminate in favor of the less-aggressive call variants (WELLS& BARD 1987), while males tend to give calls early in the evening (WELLS& BARD1987) and have been shown to minimize the costs of aggressive calling by gradually increasing the aggressive content of calls in response to playbacks of introductory notes with increasing duration (WELLS1989). Graded aggressive signals and female preference for advertisement-like calls over aggressive calls in H . cinerea (OLDHAM & GERHARDT 1975; GERHARDT 1978) and H. microcephala (SCHWARTZ & WELLS 1985; SCHWARTZ1987) suggest a similar tradeoff between attracting females and repelling males in these species. Such a tradeoff also seems likely in H. m. mamzoratus, in which it has been shown that physical encounters between males are short-lived (mode of 1 min) and none of the males fight on the night they mate (PASSMORE et al. 1992). The observed increase in aggressive calling at higher densities and the often

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immediate aggressive response of male H. m. broudleyi to playbacks of aggressive calls indicate that males tend to give aggressive calls preferentially when other males are also engaged in aggressive calling. In doing so, males seem to maintain the attractiveness of their calls relative to those of their neighbors. It is interesting to note that responsiveness to playbacks was greater under low background-noise levels. Most of the males that did not respond to playbacks of advertisement calls, even at the highest intensity, were calling in dense aggregations where presumably high levels of background noise led to higher thresholds for aggressive calling. Such socially mediated behavioral flexibility has been well documented in H. m. marmoratus (TELFORD1985; DYSON & PASSMORE 1992) and other anurans (WELLS & SCHWARTZ 1984a, b; BACKWELL 1988; LOPEZet al. 1988; NARINS & ZELICK 1988; PALLETT & PASSMORE 1988; WAGNER1989b,c; BRENOWITZ & ROSE 1994). There was no significant correlation between the dominant frequency of either advertisement calls or aggressive calls and snout-vent length in the sample of males in this study. However, analysis of a larger sample from recordings of H. m. broadleyi males unrelated to this study (T. U. GRAFEunpubl. data) showed a significant negative correlation between advertisement-call dominant frequency and snout-vent length (r, = -0.61, df = 26, p < 0.001). This suggests that the advertisement call at least can be used to assess the body size of rival males during encounters. The dominant frequency of advertisement calls is used in assessment of fighting ability in other anurans (DAVIES & HALLIDAY 1978; RAMER et al. 1983; ARAK1983 a; WAGNER 1989a). I n H . m. broadleyi, neither the dominant frequency of aggressive calls nor the magnitude of decrease in dominant frequency from advertisement to aggressive calls was a reliable indicator of body size and, thus, fighting ability. This contrasts with the results of DYSON& PASSMORE (1992), who found a significant negative correlation between the mid-frequency of aggressive calls and the snout-vent length of H. m. marmorutus males, and a good match between the size differential in the body sizes of opponents and the probability of moving away, falling silent, and being displaced in an aggressive interaction. Similarly, in cricket frogs (Acrzs crepituns, family Hylidae), males can assess the size of an opponent based on dominant frequency, and the extent to which males lower the dominant frequency is correlated with their probability of attacking (WAGNER1992). Unfortunately, the significance of increasing call duration and pulse number during agonistic vocal interactions in H. m. broadleyi is unclear. It seems possible that longer pulsatile calls with numerous very-short rise times allow males to locate each other more easily prior to fights (cf. KRAHE& RONACHER1993). Although the significance of changes in call features during agonistic interactions between male H . m. broadleyi and how males perceive graded signals are not understood, the graded signalling system provides males with the option of conveying information on motivation and /or fighting ability. Both theoretical and empirical studies indicate that body size is important in determining the outcome of agonistic encounters. However, the effect of body size is often confounded by other variables such as resource ownership. Anurans with graded aggressive calls that do not defend resources - such as Hperolius marmorutus -


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provide an opportunity to analyse strategic decisions independently of the influence of resource ownership. Acknowledgements Field work was conducted with the permission of the Research Council of Zimbabwe. Special and Philip SHAWfor providing accommodation, and thanks go to Hamish ARMOWR,Kim DAMSTRA to Kenneth and Susan WORSELY for logistical support. I am especially grateful to Kraig ADLERfor his support and advice. Carl GERHARDT provided generous help in the production of synthetic signals. I thank Reginald COCROFT, Stephen EMLEN,Kentwood WELLSand an anonymous reviewer for critically reviewing an earlier version of the manuscript. Financial support was received from a Cornell University Graduate School Travel Grant, a Sigma Xi Grant-in-Aid of Research, the German National Scholarship Foundation, and through USDA grant 191-6404 to Kraig ADLER.

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WILKINSON, L. 1992: SYSTAT: the System for Statistics. SYSTAT Inc., Evanston. Received: July 25, 1994

Accepted: December 14, 1994 (J.Brockmann)