Does physical or acoustical disturbance cause male pickerel frogs, Rana palustris, to vocalize underwater?

Male pickerel frogs (Rana palustris) vocalize both above water and underwater. Males have a complex vocal repertoire consisting of an advertisement call and two additional call types (i.e., aggressive calls) that can be elicited by the playback of a conspecific advertisement call. Additionally, in a previous study, males in these playbacks often dove and vocalized underwater for a few minutes before returning to the surface. The purpose of this study was to investigate whether physical or acoustical disturbance cause male pickerel frogs to call underwater. Experimental manipulations were performed with resident males that, initially, were calling above water. In the first, 16 males were physically disturbed with the intent to make them dive underwater. Eleven of these began to call underwater within 10 to 95 seconds and continued for up to four minutes before returning to calling above water. The second experiment involved playbacks in which males were disturbed acoustically by two minutes of continuous advertisement calling from either one Bufo americanus or Pseudacris crucifer male. Only one of 15 males dove and then called underwater in response to B. americanus calls even though most subjects emitted aggressive calls during the test period, and no male called underwater in response to P. crucifer calls (n = 13). In conclusion, male pickerel frogs will shift to underwater calling in response to a physical disturbance, but do not shift in response to the sounds of one heterospecific caller.