Rapping, a female receptive call, initiates male-female duets in the South African clawed frog

Abstract

Finding a sexually receptive partner of the opposite sex is a challenge; one solution is to advertise. That advertising is usually the province of males has shaped scenarios for sexual selection, especially the ardent active male courting the passive but choosy female. Herein we consider an unusual case in which constraints on reproduction may have led to fertility advertisement by female frogs. When oviposition is imminent, female South African clawed frogs swim to an advertising male and produce an aphrodisiac call, rapping, that stimulates both male vocalization and approach. Males respond to rapping with a distinctive answer call. The rapping-answer interaction thus forms a duet between partners of a receptive pair.

Figure 1

Rapping and effects on male calls. (A) Oscillograph (amplitude envelope) of female rapping. After the fifth rap, the male is beginning to answer; male clicks are indicated. Note that male and female clicks are readily distinguishable by duration. (B) Oscillograph of a male advertisement call. Compared with the advertisement call, the male answer call (C, see box D) has a longer fast trill, a shorter slow trill, and increased amplitude modulation. (C) Oscillograph of a receptive duet; raps are indicated by circles. At the start of the trace the male is advertisement calling, as soon as the female raps, his vocalization changes to an answer call. (D) A portion of the receptive duet (C, box D) at a faster sweep speed illustrating raps and trill durations. [Bar = 500 ms (B and C) and 250 ms (A and D)]. Oscillographs illustrate amplitude (dB at 1 V/μPa) by time. Recordings were obtained in the artificial pond.

Figure 2

Rapping alters male calling. The male advertisement call (A) is altered in response to a rapping female (B) and in response to a tape of rapping (C). Frequency histograms for fast and slow trill durations and percent amplitude modulation are shown for all male calls (A, n = 422; B, n = 729; C, n = 270).

Figure 3

Male vocal responses to rapping and to ticking. (A) Oscillographs of rapping and the male’s response. At the onset of rapping (○), the male switches to the answer call (upper trace). Rapid bouts of rapping maintain answer calling. When raps decrease in frequency (lower trace), the male reverts to advertisement calling. (B) Oscillographs of ticking and the male’s response. The male continues advertisement calling during the first several seconds of ticking (○) but then falls silent (lower trace). The female continues ticking after male calling stops.

Figure 4

Acoustic features of rapping and ticking. The distribution of interclick intervals (A) in milliseconds and amplitudes (B) in decibels (dB at 1 V/μPa) for each call are illustrated for 804 raps from eight females (96–109 samples per animal) and 208 ticks from three females (99–236 samples per animal). (C) Fast Fourier transform analyses of three representative clicks (one per animal) from rapping and ticking; frequency is 0–3 kHz for each graph and relative amplitudes (y axis) are measured in millivolts. Recordings were obtained in the artificial pond.