We recorded the vocalization patterns of eight male frogs in the
field under three experimental conditions for a period of three
minutes each: (1) an NS period, during which no sound was
presented, (2) a US period, during which we presented the ultrasonic
components of a previously-recorded conspecific vocal signal at
,77 dB sound pressure level (SPL) (root mean squared or r.m.s.
reading), a sound level that is behaviourally relevant, and (3) an AUD
period, during which we presented the audible components
(,20 kHz) of the same vocal signal at a similar sound level. For
five frogs (asterisks in Fig. 1a), the male’s calling rate was markedly
increased during the AUD and/or US period, compared to spontaneous
calling rates during the NS period. Three frogs (601-4, 602-1,
602-2) showed no overt evoked vocal responses to any playback
stimulus. The stimulatory effect of the US components was most
robust for frogs 531-1 and 601-2. Frog 531-1 did not produce any
calls during the NS period, but emitted 11 calls during the US period.
Frog 601-2 produced six calls during the NS period, and emitted 18
calls during the US period, including four antiphonal responses that
were precisely time-locked (within 30 ms of the stimulus offset) to
the US stimulus (Fig. 1b)—the probability of all four occurrences
by chance is 7.4 £ 1027 (binomial probability). These results show
that males of A. tormotus detect and respond to ultrasound