Josef Brzoska

Acoustic communication in the grass frog (Rana t. temporaria L.): Calls, auditory thresholds and behavioral responses

Summary1.The grass frogs vocal repertoire includes mating and territorial calls, as well as release and warning calls. These may be distinguished by differences in spectral composition, in number of pulses, and in pulse repetition rate.2.Recording of the responses of neurons in the torus semicircularis has revealed no effects of season or sexual state upon auditory ability (Figs. 3, 4, 5). The shape of the auditory thresholds vs. frequency curve is related to body size (Fig. 7).3.Behavioral response thresholds to tones determined by the electrodermal response (i.e., the galvanic skin response; Figs. 8, 9) and the neural audiograms demonstrate that the grass frog is sensitive to its various types of calls.4.The sensitivity of the electrodermal response differs in a characteristic way from the audiograms obtained at the neuronal level (Fig. 10). The largest differences occur at the mating call frequencies. The relevance of the call frequency spectrum in intraspecific communication among grass frogs is discussed.

Modification of prey-catching behavior by learning in the common toad (Bufo b. bufo [L], Anura, Amphibia): Changes in responses to visual objects and effects of auditory stimuli

An attempt to train common toads (Bufo b. bufo) to make the turning movements associated with prey-catching in response to a tone (1000 Hz, 90 dB) was unsuccessful. However, some toads learned to discriminate food that had been made unpalatable from palatable food of identical appearance, when the former was accompanied by the auditory stimulus. By making the prey unpalatable flight behavior could be induced in toads presented with a housefly (Musca domestica). On the other hand, toads could be trained to exhibit prey-catching behavior when shown predator objects 30 cm wide and 60 cm high (at a distance of ca. 50 cm). The toads also learned to snap at motionless, unscented food in certain surroundings.

Acoustic signals influencing the hormone production of the testes in the grass frog

SummaryThe effect of acoustic stimuli on the testis volume and size of the interstitial cells and their nuclei was studied. Two different synthetic calls were used, both with the time pattern of the species-specific mating call: one at the natural frequencies, the other at 1,100 Hz (a major component of the territorial and release call). Exposure to 1,100 Hz resulted in diminished size of the interstitial cells and their nuclei, implying a reduction in androgen production. By contrast, presentation of the synthetic mating call maintained the function of the androgen system, so that the rate of production of androgen hormones and thus the sexual stimulation of the frog remained high. The role of the endocrine system in auditory communication among grass frogs in particular, and anurans in general, is discussed.

Significance of spectral and temporal call parameters in the auditory communication of male grass frogs

SummaryDuring the spawning period, male grass frogs (Rana temporaria L.) frequently produce short and long territorial calls in addition to mating calls. The calls differ in mean pulse number, duration, and pattern of amplitude modulation. Experiments in which recorded natural calls are played back reveal that male grass frogs are capable of discriminating the different conspecific calls. A male frog stimulated by mating calls always responds by producing mating calls in greater numbers (Fig. 3). Territorial calls presented at low intensity also cause an increase in the mating-call rate (Fig. 4), but at high intensity they clicit territorial calls and turning toward the loudspeaker. A combination of short and long territorial calls was especially effective in eliciting the phonotaxis response. As play-back experiments with simulated calls show, the carrier frequency and the pulse repetition rate are particularly important cues for recognition of conspecific calls (Fig. 5). A simulated call with a 400-Hz carrier frequency (the dominant frquency of the mating call) is just as effective as the natural call with the complete frequency spectrum (Fig. 3), whereas a 1100-Hz simulated call is ineffective (Fig. 5). The chief factors in discrimination among the conspecific calls are the call repetition rate and probably the amplitude and frequency modulation. Changes in the duration of the calls had little effect (Fig. 6). The available evidence suggests that the mating call has a reciprocally stimulating action on males in a chorus, whereas the territorial calls experess aggressiveness and give warning to other males.

Vocal response of male European water frogs (Rana Esculenta complex) to mating and territorial calls.

The responses of male European water frogs (the two species Rana lessonae and Rana ridibunda and their hybrid, Rana esculenta) to playback of their mating and territorial calls were studied during the mating season. In order to select biologically relevant intensities for the presentation of the recorded calls, the sound pressure of the calls produced by the frogs themselves was established prior to the experiment. At a distance of 1 m the most intense calls were those of R. ridibunda, with a sound pressure of 110 dB (peak SPL). The smaller males of R. esculenta gave calls about 5 dB lower in intensity. The calls of R. lessonae, the smallest phenotype, were still less intense, 10 dB lower than those of R. ridibunda. The territorial calls of all three phenotypes elicited territorial calls in all of the males tested, as a rule accompanied by approach to the sound source. The sound pressure required to elicit a vocal response was nearly the same for each of the three different territorial calls. Sometimes encounter calls and warning calls were given in addition to territorial calls. When the mating calls were presented at low intensity, in some cases the males responded with their own mating calls. Mating calls at higher intensity elicited the same behavior that appeared following presentation of territorial calls, but significantly higher sound pressures were required to elicit such a response to mating calls than to territorial calls. The males of R. lessonae and R. esculenta did not respond to the mating calls of R. ridibunda, and each of them had significantly lower thresholds to the mating call of its own phenotype than to that of the other. The males of R. ridibunda responded only to conspecific mating calls. The vocal-response thresholds are compared with those of the electrodermal response reacting to the same stimuli. The significance of the different calls of the European water frogs is discussed.

Temperature dependence of visual fusion frequency in Rana lessonae cam., Bufo bufo L. and Bombina bombina (L.) (Amphibia)

The electroretinogram (ERG) was used to measure the flicker-fusion frequencies of Rana lessonae, Bufo bufo and Bombina bombina over the temperature range 5-25°C. In all three species the fusion frequency increased with increasing temperature. In the intermediate range of temperatures, the fusion frequencies of Rana lessonae and Bombina bombina doubled when the temperature was raised by 10°C.

Amplitude, latency, and habituation of the electrodermal response to acoustic stimuli in the frog

The electrodermal response (EDR) of frogs to various acoustic stimuli was measured in the form of the skin potential response (SPR). There was no correlation between the polarity of the SPR and the intensity of the stimuli. When different frequencies were presented at the same intensity, the amplitude of the SPR to each was inversely proportional to the sound pressure at which that frequency just elicited an SPR. The amplitude of the sound-induced SPR increased monotonically with increasing sound pressure. The latency of the SPR decreased with increasing intensity of the acoustic stimulus. Acoustic stimuli repeated at intervals of 1 and 2 min elicited responses with progressively decreased amplitude and increased latency; with 4 min intervals there was little habituation. Fatigue participates to only a very slight extent in reducing the amplitude of the SPR and increasing its latency. The results are compared with the published data on frogs and mammals, including humans.