Trill Rate Research Articles

Ecological and Reproductive Differentiation in the Cryptic Species of the Hyla versicolor Complex (Hylidae)

The ecological and reproductive behavior of the cryptic species Hyla chrysoscelis and H. versicolor was examined in two sympatric localities and in one allopatric H. chrysoscelis locality. An analysis of the stomach contents of both species at one locality suggests that proportionately more H. chrysoscelis are eating arboreal insects, and proportionately more H. versicolor are eating terrestrial insects. The two species may be avoiding complete overlap of their niches in this way. There was a significant statistical, although not absolute, difference between the two species in the positions of the males calling around the breeding ponds; H. chrysoscelis tends to call from trees or bushes, while H. versicolor tends to call from the ground. A relationship between the ecological and reproductive behavior is dis- cussed in terms of natural selection working in the same direction for both in each species. Limited relative humidity data, general geographic distribution of the two species, and the ground versus tree differences in food habits and calling position suggest that relative humidity is a key factor involved in these ecological and reproductive differences. It is thought that H. chrysoscelis can tolerate or may prefer lower rela- tive humidities than H. versicolor. Two factors that were thought to affect the trill rates of both species were ana- lyzed. High positive correlations between temperature and trill rate were found in regression analyses for both species. No correlation between body size and trill rate was found for either species. Population differences in the percentage of individuals using either the upper or lower harmonic as the dominant frequency of the mating call may prove to be of significance in the sympatric relations of the two species. Two populations of H. chrysoscelis sympatric with H. versicolor had mean trill rates 7% and 10% faster than the mean trill rate of an alloptric H. chrysoscelis population. It is suggested that reinforcement for differences in trill rate is occurring where the two species are sympatric.

Hearing and Responses To Calls in Anurans

Abstract1. Nerve responses evoked by tone pips of known frequency were recorded from the medulla of Bufo americanus, Rana pipiens, Rana clamitans, Pseudacris triseriata, and Hyla crucifer. 2. Nerve responses were obtained between 20 cps and 4000 cps in Bufo and Rana, and between 100 cps and 6000 cps in Pseudacris and Hyla. 3. In all species, the best frequency for nerve responses was 1000 cps. In Pseudacris and Hyla there was a second low threshold area at 2000-3000 cps. It is suggested that the lower frequency dip in the response curve may represent responses from the pars amphibiorum and that the higher frequency dip may represent responses from the pars basilaris. 4. The threshold at the best frequency in Rana is similar to the sensitivity of human hearing. 5. Three Pseudacris and one Hyla were induced to answer tone pips. 6. These answered pips of frequencies between 700 cps and 4600 cps. Since this conforms to the high frequency dip in the nerve response curve, it is suggested that the Hylids might have been hearing pips with the pars basilaris only. 7. Pseudacris answered pips of a great variety of durations and intervals. 8. The data suggest that responses of Pseudacris males to frog calls may be, to a great extent, non-specific. 9. Alternating of calls in duets is controlled by at least two factors. There is a constant latency between a call and the stimulus eliciting it. A stimulus following a call too closely inhibits calling. 10. The data obtained here and by others suggest that even for location of males by females, the repetitive nature and frequency of calls may be of only minor importance. Rather, trill rate is implicated as an important call parameter.