Mark A. Bee

Neighbour–stranger discrimination by territorial male bullfrogs (Rana catesbeiana): II. Perceptual basis

Some territorial animals discriminate among neighbours and strangers based on individual differences in acoustic signals. Male North American bullfrogs, Rana catesbeiana, display this form of discrimination based on individual variation in advertisement calls. In this study, we investigated the acoustic basis of neighbour–stranger discrimination to determine how individual identity might be encoded by particular properties of bullfrog advertisement calls. We analysed patterns of within-male and between-male variability in 1078 bullfrog advertisement calls recorded from 27 territorial males. All call properties that we examined varied significantly among males. However, fundamental frequency and dominant frequency showed the lowest within-male variation and the highest repeatability between two recording sessions, and both properties were highly correlated with the first canonical root from discriminant function analyses, which typically accounted for 70–80% of the variability between males. We suggest that neighbour–stranger discrimination in bullfrogs is partially mediated by between-male differences in the spectral or fine temporal properties of advertisement calls.

Size assessment in simulated territorial encounters between male green frogs (Rana clamitans)

Abstract We examined the ability of male green frogs to assess the size of an opponent based on the dominant frequency of their advertisement call, which is negatively correlated with size, using synthetic stimuli to simulate intruders of different sizes. In one field playback experiment, we broadcast a pair of stimuli representing a small and a large male; in a second experiment, we broadcast calls of a medium and a large male. In both experiments, males produced calls with significantly lower dominant frequencies in response to each stimulus. Contrast analyses revealed that males lowered the dominant frequency of their calls more in response to the large-male stimulus than in responses to the small- and medium-male stimuli. In the second experiment, males also responded to the large-male stimulus by calling at higher rates. There were no differences in mean note duration or the number of moves made toward or around the playback speaker in response to any stimulus. Thus, the frequency of an opponents calls elicits a differential modification of calling behavior, primarily in the form of differential dominant frequency alteration, suggesting that males use dominant frequency to assess the size of opponents during aggressive encounters.

Call matching in the quacking frog (Crinia georgiana)

Abstract Males of the quacking frog Crinia georgiana produce calls consisting of 1–11 notes. Playback experiments using synthetic calls showed that males tend to match the number of notes in 2-note and 4-note stimuli; however, males tended to produce more than 1 note in response to a 1-note stimulus and fewer than 8 notes in response to an 8-note stimulus. Successive playbacks of two, 4-note calls from separate speakers indicate that males are likely to match the combined number of notes in the calls of two neighbors, even if they are not equidistant from the focal male. The results are compared with the few other studies of matching in anurans, and interpreted in terms of hypotheses developed to explain matching in songbirds. One attractive and testable hypothesis for call matching in C. georgiana is that males are attempting to produce calls that are at least as attractive to females as those of rivals, without wasting energy.

Plasticity of aggressive signalling and its evolution in male spring peepers, Pseudacris crucifer

Abstract Acoustic communication is important for determining and maintaining intermale spacing in breeding aggregations of anurans and insects. Because the number and proximity of signalling males can show extensive temporal and spatial variation, we should expect to find mechanisms that permit males to modify their signalling behaviour in ways that balance the needs to attract females and defend their calling space. We conducted two field playback experiments to investigate the role of plasticity in male aggressive signalling in intermale spacing of spring peepers (Anura, Hylidae). In the first experiment, we found a positive correlation between the amplitude of the advertisement calls of a males nearest neighbour and the stimulus amplitude at which the male first produced aggressive calls. In the second experiment, repeated presentations of advertisement calls above a males aggressive threshold resulted in rapid decreases in aggressive signalling and significant, but temporary, elevations of aggressive thresholds. We suggest that short-term habituation to a neighbours calls could function as a proximate mechanism for plasticity in aggression that would allow males to accommodate nearby callers while also tracking fluctuations in the local density of calling individuals. In a third experiment, we examined female choice as an ultimate-level explanation for plasticity in male aggression. Females preferred advertisement calls to aggressive calls, but this preference was weak and was abolished by a 6-dB reduction in the amplitude of the advertisement call. We suggest that female preferences probably function as only one possible source of selection on plasticity in male aggressive signalling and propose energetic limitations as an additional source of selection. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.

Habituation and sensitization of aggression in bullfrogs (Rana catesbeiana): testing the dual-process theory of habituation.

The aggressive response of male bullfrogs (Rana catesbeiana) habituates with repeated broadcasts of acoustic stimuli simulating a new territorial neighbor. The effects of stimulus repetition rate and stimulus intensity on bullfrog aggressive responses were tested in a field experiment designed to test the assumptions of a dual-process theory of habituation. Synthetic advertisement calls were broadcast at 2 repetition rates and 2 intensities in a factorial design. Bullfrogs were more aggressive at the higher stimulus intensity at both repetition rates. Aggressive responses habituated more slowly at the higher stimulus intensity and slower repetition rate compared with other treatments. Several biotic and abiotic factors had small or negligible effects on aggressive responses. Although consistent with the operation of 2 opposing processes, habituation and sensitization, the data provide only partial support for the assumptions of dual-process theory.