Abstract
Atelopus franciscus is a diurnal bufonid frog that lives in South-American tropical rain forests. As in many other frogs, males produce calls to defend their territories and attract females. However, this species is a so-called “earless” frog lacking an external tympanum and is thus anatomically deaf. Moreover, A. franciscus has no external vocal sac and lives in a sound constraining environment along river banks where it competes with other calling frogs. Despite these constraints, male A. franciscus reply acoustically to the calls of conspecifics in the field. To resolve this apparent paradox, we studied the vocal apparatus and middle-ear, analysed signal content of the calls, examined sound and signal content propagation in its natural habitat, and performed playback experiments. We show that A. franciscus males can produce only low intensity calls that propagate a short distance (<8 m) as a result of the lack of an external vocal sac. The species-specific coding of the signal is based on the pulse duration, providing a simple coding that is efficient as it allows discrimination from calls of sympatric frogs. Moreover, the signal is redundant and consequently adapted to noisy environments. As such a coding system can be efficient only at short-range, territory holders established themselves at short distances from each other. Finally, we show that the middle-ear of A. franciscus does not present any particular adaptations to compensate for the lack of an external tympanum, suggesting the existence of extra-tympanic pathways for sound propagation.
Highlights
Vocalizations play an important role in the mating and territorial behavior of most anurans [1]
This is exactly what we observe for the advertisement call of A. franciscus: it consists of a train of short pulses, each pulse corresponding to a non-frequency-modulated sound with a wide frequency bandwidth
The call of A. franciscus is emitted at a low intensity (72 dB sound pressure levels (SPL)) which is likely due to the lack of an external vocal sac
Summary
Vocalizations play an important role in the mating and territorial behavior of most anurans [1]. And in contrast to other species of the genus, Atelopus franciscus males only use advertisement calls and no visual displays to attract females and defend territories [6]. This is unexpected, given that they lack an external vocal sac, and that they live in a noisy calling environment consisting of river banks in tropical forest. The emission of advertisement calls in an absorbent and noisy environment imposes limits on the ability to communicate as calls suffer attenuation and degradation Communication in this habitat is rendered even more difficult by the presence of numerous other frog species producing high energy calls
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