Sexual Selection in the Spring Peeper, Hyla Crucifer (Amphibia, Anura): Role of the Advertisement Call

Abstract

1. The advertisement call of the spring peeper exhibits significant variation in each of the four parameters examined: intensity, duration, frequency, repetition rate. 2. In playback experiments involving calls differing in a single parameter, females exhibited preference for loud calls, calls with a rapid repetition rate and low frequency calls. 3. Although we had hypothesized that females would prefer long calls to short calls, they did not. We attribute this lack of preference to unanticipated differences in the sound envelope of the two experimental calls. 4. Non-random choice by females during playback experiments may have been in response to the conspicuousness of the male call, or to encoded clues pertaining to male fitness. 5. Call repetition rate is positively correlated with body size (snout-vent length) while frequency is negatively correlated. 6. Anuran growth is thought to be indeterminate, so that larger males are typically older males. As a consequence of having survived longer, large males may be more fit. 7. Since females exhibited a preference for call parameters which were correlated with body size, we had anticipated that large males would experience differential mating success in nature. A comparison of amplexed and non-amplexed males over a three year period revealed no intergroup size difference. 8. We conclude that while females may select calls indicative of size, and perhaps fitness, during laboratory trials, such choices are likely more difficult in an active breeding congregation where acoustical interference and competition may tend to obscure call variation. 9. Finally we report the presence of small, non-calling satellite males and suggest that their success as sexual parasites may have influenced the size relationship between amplexed and non-amplexed males in our study. 1. The advertisement call of the spring peeper exhibits significant variation in each of the four parameters examined: intensity, duration, frequency, repetition rate. 2. In playback experiments involving calls differing in a single parameter, females exhibited preference for loud calls, calls with a rapid repetition rate and low frequency calls. 3. Although we had hypothesized that females would prefer long calls to short calls, they did not. We attribute this lack of preference to unanticipated differences in the sound envelope of the two experimental calls. 4. Non-random choice by females during playback experiments may have been in response to the conspicuousness of the male call, or to encoded clues pertaining to male fitness. 5. Call repetition rate is positively correlated with body size (snout-vent length) while frequency is negatively correlated. 6. Anuran growth is thought to be indeterminate, so that larger males are typically older males. As a consequence of having survived longer, large males may be more fit. 7. Since females exhibited a preference for call parameters which were correlated with body size, we had anticipated that large males would experience differential mating success in nature. A comparison of amplexed and non-amplexed males over a three year period revealed no intergroup size difference. 8. We conclude that while females may select calls indicative of size, and perhaps fitness, during laboratory trials, such choices are likely more difficult in an active breeding congregation where acoustical interference and competition may tend to obscure call variation. 9. Finally we report the presence of small, non-calling satellite males and suggest that their success as sexual parasites may have influenced the size relationship between amplexed and non-amplexed males in our study.