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.
Highlights
The ability to localize the direction and distance of vocalizing conspecifics is generally advantageous for animals that use 114 Page 2 of 17Behav Ecol Sociobiol (2017) 71: 114 sound to communicate as it allows early decisions to be made before direct contact occurs and reduces the risk of unnecessary aggressive responses (Erulkar 1972; McGregor 1994; Bradbury and Vehrencamp 2011; Hardy and Briffa 2013; Bee et al 2016)
The present study shows for the first time that 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 be used for orientation
Together with individual identification and eavesdropping, acoustic distance assessment plays a crucial role in territorial social systems of vocal species (McGregor 1993) and commonly mediates inter-individual spacing by informing territory holders about the proximity and the threat potential of nearby callers (Brown and Orians 1970; Robertson 1984; Naguib et al 2008, 2011)
Summary
The ability to localize the direction and distance of vocalizing conspecifics is generally advantageous for animals that use 114 Page 2 of 17Behav Ecol Sociobiol (2017) 71: 114 sound to communicate as it allows early decisions to be made before direct contact occurs and reduces the risk of unnecessary aggressive responses (Erulkar 1972; McGregor 1994; Bradbury and Vehrencamp 2011; Hardy and Briffa 2013; Bee et al 2016). Acoustic territory advertisement and ranging allow animals to avoid more costly physical contests and fights over territories (Whitney and Krebs 1975; Richards 1981; Morton 1986; Bee et al 2016). Acoustic signal amplitude corresponds to sound pressure, which is the local pressure deviation from the ambient atmospheric pressure caused by a sound event. Atmospheric spreading, sound pressure follows the inverse distance law by 1/r with distance r from the sound source, resulting in an SPL drop of −6 dB per doubling of the distance r (Rossing 2007; Bradbury and Vehrencamp 2011). SPL alone is not a reliable cue for distance assessment, as a caller could be closer to or further away from the receiver, but could actively vary call amplitude (Richards 1981; Morton 1982). The signal could undergo unpredictable excess attenuation during transmission, caused by vegetation or ground structures, or distortion by wind and temperature gradients (Erulkar 1972; Morton 1975, 1986; Naguib 1997b; Ellinger and Hödl 2003; KreutzErdtmann and Lima Pimentel 2013)
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