Upland Calling Behavior in Crawfish Frogs (Lithobates areolatus) and Calling Triggers Caused by Noise Pollution
The breeding calls of frogs and toads serve to attract females and advertise the fitness attributes of individual males, but male frogs will also call outside the breeding season. Here, we document non-reproductive (upland) calling in Crawfish Frogs (Lithobates areolatus), a species of conservation concern. Adults are obligate crayfish burrow dwellers and generally live in isolation. We document post-breeding upland call characteristics and identify peak upland seasonal calling times. We show that Crawfish Frog upland and breeding call spectral and temporal structures differ, suggesting these calls serve different functions. We also show that the transition between these distinct call types can occur within a relatively short time, 120 min, as Crawfish Frog males leave their burrows to begin breeding migrations. We demonstrate that upland calling rates increase as the summer progresses and that males call in response to specific stimuli—results consistent with a burrow defense hypothesis. Further, 28 (51%) of the 55 upland calling bouts we recorded were associated with human-generated triggers, including the sounds of airplanes and automobiles, a finding confirmed using playback experiments. Calling in response to noise pollution can increase exposure to predators, and we discuss the conservation implications of this action for this imperiled species.
Waveforms, spectrograms, and spectrogram cross sections of a typical Crawfish Frog upland call (left) and a typical breeding call (right). Ambient temperatures for both calls were 16°C.
The relationships between three call characteristics and temperature for the upland and breeding calls of male Crawfish Frogs. Error bars represent standard error of the mean, and the line represents the best fit.
Frog 26’s (106 mm SVL) transition from a pre‐breeding upland‐like call to a pre‐breeding call occurred over the course of two hours on 11 March 2013. This frog had just emerged from its burrow, emitted an upland‐like call, then emitted various forms of intermediate calling before commencing its breeding migration.
Upland calls exhibit seasonality. Average number of post‐breeding upland calls recorded per month by Crawfish Frogs inhabiting primary burrows during the warm seasons of 2009–2011. Error bars represent standard error of the mean, and the line represents the best fit. Shading indicates the range of Crawfish Frog metamorphosis during same time period.
Triggers to post‐breeding upland calling (A) and types of artificial triggers (B) in Crawfish Frogs. Note that upland calling is preceded by a trigger 60% (33 bouts) of the time; 9% (5 bouts) of triggered bouts were natural (e.g., rain, thunder, wind); 51% (28) of triggered bouts were artificial (manmade). Manmade stimuli included aircraft (64%), vehicles (cars or trucks; 18%), radio transmitter receiver noise (static; 14%), and other forms of human disturbance (4%).
Contributor Notes
Associate Editor: J. Kerby.