Vocal sacs do not act as visual cues in acoustically guided courtship in Cope’s gray treefrog (Hyla chrysoscelis)

Abstract

Components in multiple sensory modalities are present in many animal signals, which provides opportunities for receivers to use them as complementary cues in communication, especially in noisy environments that impose difficulty on signal perception. In frogs, it has been suspected that females use the visual byproduct of call production - the inflation of vocal sacs - as a cue in finding individual calling males in loud choruses. This mate recognition and selection behavior was traditionally considered as acoustically guided but recently there has been rising discussion on whether it was a multimodal process. We investigated whether female Cope’s gray treefrog (Hyla chrysoscelis) use visual cues in the context of sexual communication to find and select males. We performed playback experiments in a field setting under natural light using robotic frog models as visual stimuli and examined females’ responses. Acoustic stimuli were played back in quiet, in noise, and with ambiguous acoustic features. Despite the various acoustic conditions tested in a realistic lighting environment, we did not find any evidence that females use visual cues in the context of sexual communication. We review previous reports on the use of vocal sacs as visual cues in nocturnal anurans and discuss potential reasons for the stark contrast between those reports and this research.