Acoustic ranging in poison frogs—it is not about signal amplitude alone

Abstract

Acoustic ranging allows identifying the distance of a sound source and mediates inter-individual spacing and aggression in territorial species. Birds and mammals are known to use more complex cues than only sound pressure level (SPL), which can be influenced by the signaller and signal transmission in non-predictable ways and thus is not reliable by itself. For frogs, only SPL is currently known to mediate inter-individual distances, but we hypothesise that the strong territoriality of Dendrobatids could make the use of complex cues for ranging highly beneficial for this family. Therefore, we tested the ranging abilities of territorial males of Allobates femoralis (Dendrobatidae, Aromobatinae) in playback trials, using amplitude-normalized signals that were naturally degraded over distance, and synthetic signals that were masked with different levels of noise. Frogs responded significantly less to signals recorded from larger distances, regardless of SPL and signal-to-noise ratio (SNR), but showed no differential response to natural minimum and maximum SNRs across the typical communication range in wild populations. This indicates that frogs used signal amplitude and SNR only as ancillary cues when assessing the distance of sound sources and relied instead mainly on more complex cues, such as spectral degradation or reverberation. We suggest that this ability mediates territorial spacing and mate choice in A. femoralis. Good ranging abilities might also play a role in the remarkable orientation performance of this species, probably by enabling the establishment of a mental acoustic map of the habitat.Significance statementAcoustic ranging allows the distance of vocalizing competitors and mates to be identified. While birds and mammals are known to use complex cues such as temporal degradation, frequency-dependent attenuation and reverberation for ranging, previous research indicated that frogs rely only on signal amplitude (sound pressure level) to assess the distance of other callers. The present study shows for the first time that also poison frogs can make use of more complex cues, an ability which is likely to be highly beneficial in their territorial social organization and probably can also be used for orientation.Electronic supplementary materialThe online version of this article (doi:10.1007/s00265-017-2340-2) contains supplementary material, which is available to authorized users.

Listening in the bog: I. Acoustic interactions and spacing between males of Sphagniana sphagnorum

“…The ranging hypothesis that was originally developed for distance estimation in birds states that the signal degradation that occurs during transmission, due to factors such as frequency-specific excess attenuation or degradation in the time domain, may represent an effective sensory cue for receivers (Morton 1986 ; Naguib and Wiley 2001 ; Morris et al 2016 ). Even frogs appear to rely mainly on more complex acoustic cues rather than amplitude alone when assessing the distance to sound sources, such as spectral degradation or reverberation (Ringler et al 2017 ). Maximum hearing distances of about 50 m have been reported for katydids, and these are distances over which acoustic signals will have degraded to varying degrees, depending on the physical properties of the transmission channel (Michelsen and Larsen 1983 ; Römer and Lewald 1992 ; Römer 1998 ).…”

Soil and forest structure predicts large-scale patterns of occurrence and local abundance of a widespread Amazonian frog

“…The ephemeral occurrence of suitable bodies of water also sometimes forces male A. femoralis to deposit tadpoles more than 180 m away from their territories ( Ringler et al, 2013 ), to which they reliably return ( Pašukonis et al, 2013 , 2014 ). Over the last two decades, A. femoralis has been used as a model species to address questions on diversification (e.g., Lougheed et al, 1999 ; Simões et al, 2008 ; Amézquita et al, 2009 ), sexual selection and parental care ( Ringler et al, 2015 , 2016 , 2017a ; Ursprung et al, 2011 ; Pašukonis et al, 2016 , 2017 ), movement ecology and spatial cognition ( Pašukonis et al, 2016 ; Beck et al, 2017 ), and communication ( Hödl, Amézquita & Narins, 2004 ; Amézquita, Castellanos & Hödl, 2005 ; Amézquita et al, 2006 ; Narins et al, 2005 ; Betancourth-Cundar et al, 2016 ; Ringler et al, 2017b ).…”

“…To determine if A. femoralis was present, we recorded data on calling males following the audio strip transect method outlined by Zimmerman (1994) . The A. femoralis advertisement call is one of the best studied anuran vocalizations ( Narins, Hödl & Grabul, 2003 ; Hödl, Amézquita & Narins, 2004 ; Amézquita, Castellanos & Hödl, 2005 ; Amézquita et al, 2006 , 2009 ; Göd, Franz & Hödl, 2007 ; Simões et al, 2008 ; Ringler et al, 2017b ), and in the study area is a trill composed of four whistle-like notes with ascending frequency modulation. Calling males of A. femoralis could be heard from a distance of 30 m, and are easily recognized.…”

Experience shapes accuracy in territorial decision-making in a poison frog

“…Preliminary tests with larger separation distances (50 cm) resulted in very few attacks on the FM and were thus excluded from the present experiment. For the acoustic stimulus, we used the artificial 'standard call' sensu Ursprung et al [20], which is based on recordings by Gasser et al [25]; for a detailed description see [21]. We calibrated the speaker every day to broadcast the playback with the same volume (75 dB measured at 2 m distance) in all trials.…”