Noisy Neighbors: Acoustic Interference and Vocal Interactions between Two Syntopic Species of Ranid Frogs, Rana clamitans and Rana catesbeiana
American Bullfrogs (Rana catesbeiana) and Green Frogs (Rana clamitans) share ranges and breeding seasons, are ecologically similar, and often occupy the same breeding ponds throughout the summer. Males of both species use vocalizations to defend territories and attract females; however, Bullfrogs have longer calls and call much more frequently than Green Frogs. The calls of the two species overlap in frequency; thus, Bullfrog calls are a likely source of acoustic interference for Green Frogs that could affect their ability to attract females. Nevertheless, in natural settings where these species co-occur, Green Frogs reproduce successfully. This suggests that Green Frogs respond to the calling patterns of Bullfrogs in ways that maximize Green Frog signal-to-noise ratio. We used long-term pond environment recordings and bioacoustics analyzing software to explore the influence of Bullfrog calling patterns on the vocal activity of syntopic Green Frogs. We found both species call most actively within the same seasonal and diel periods. Our results show that Green Frogs avoid overlapping their calls with Bullfrogs more often than expected by chance. Therefore, to avoid Bullfrog call overlap, Green Frogs use the fine-scale behavioral response of placing their calls in silent gaps between the calls of Bullfrogs. This pattern was even more pronounced in interactions between nearest neighbors in which there was no overlap observed between Bullfrogs and their nearest neighbor Green Frog.Abstract
Waveforms of single-note Rana clamitans and Rana catesbeiana advertisement calls. Each call was recorded from a single male at Caleb's Pond in Lebanon, Connecticut. Recordings made in May 2006. Each typical advertisement note for Rana clamitans is ∼0.07 to 0.43 sec long with a frequency range of about 250–3,200 Hz. Rana catesbeiana advertisement notes are ∼0.3 to 0.6 sec long with frequencies ranging between 200 and 4,000 Hz.
Example of a typical Bullfrog/Green Frog chorusing event over an 8-sec period with few gaps in noise. Two Bullfrogs and one Green Frog participated in this example chorus. Chorus recorded in June 2006 on Caleb's Pond, Lebanon, Connecticut.
Rana catesbeiana and Rana clamitans call activity each week of the 2006 and 2007 breeding seasons. Activity is defined as the average daily number of calls for each species per week normalized to the number of hours recorded each day. Only days with at least 20 h of continuous recordings (54 days in 2006; 23 days in 2007) were used for this plot. Bullfrogs indicated with circles; Green Frogs indicated with triangles. Average daily water temperatures and egg masses per week are below each year's call activity.
Hourly calling rate for the 2006 and 2007 breeding seasons, scaled per species. Each arm is marked for a 24-h clock with the center of clock indicating zero calls and the outer edges of the distributions representing relative number of calls per hour. Only days with at least 20 h of continuous recordings (54 days in 2006; 23 days in 2007) were used for this plot. Bullfrogs indicated with circles; Green Frogs indicated with triangles.
(A) Ninety-second example of Bullfrog chorusing event showing Green Frog calls inserted without overlapping Bullfrogs. Arrows not labeled indicate single-note Green Frog calls. Arrows labeled GM indicate multinote Green Frog calls. (B) Sample 30-sec period of Rana clamitans avoiding overlapping calls with Rana catesbeiana (on 27 May 2007). Call onset time and duration for each species over the period were separated by species. Each 100-msec interval was scored as “true” if a call was heard during that interval or “false” if none was calling. When the scores were then compared, call overlap avoidance on the event scale becomes clear. In this figure, Green Frogs overlap a Bullfrog call only once, and it appears the masked call was emitted at the same time as the Bullfrog call. All other Green Frog calls fall within gaps of Bullfrog noise.
Results of 2,500 randomizations of Green Frog versus Bullfrog calling activity. Each distribution curve represents the percent of time that Green Frogs would be expected to overlap Bullfrog calls if overlaps occur randomly. The arrows represent the observed percent of time Green Frogs overlapped Bullfrogs in 30-min periods over 10 different days (180,010 calls). Stars indicate statistical significance via randomization test.
Histogram showing results of 10,000 randomizations of Green Frog versus Bullfrog focal pair (N = 16 pairs) calling activity. Distribution represents the percent of Green Frog calls that overlap its near neighbor Bullfrog calls if overlaps occur randomly. The empirical observation for each of the 16 pairs is 0% overlap. Dashed line indicates one-tailed 95% confidence level (= 2.0%).
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