Infrasound in the flutter-jump display of Capercaillie (Tetrao urogallus): signal or artefact?

Abstract

The perceptual world of non-human animals has long been recognized as differing markedly from that of humans (von Uexkull 1909). Selection has favoured animal responsiveness to stimuli outside the perceptual range of humans in the mechanical (Sorex palustris, Catania et al. 2008; Schizocosa spp., Uetz et al. 2009), chemical (Felis catus, De Boer 1977), electrical (Scyliorhinus canicula and Raja clavata, Kalmijn 1971; Polyodon spathula and Ornithorhyncus anatinus, Pettigrew and Wilkens 2003), magnetic (birds, Mouritsen and Ritz 2005), thermal (Boidae and Crotalinae, Ebert et al. 2007), and acoustic (echolocation in bats, Simmons 1973; ultrasound in ground squirrels, Wilson and Hare 2004; ultrasound in toothed whales, Wilson et al. 2007) domains. In addition to the utilization of sounds beyond the upper frequency limit of human hearing ([15 kHz: ultrasound), elephants (Elephas maximus, Payne et al. 1986), rhinos (Diceros bicornis spp., Budde and Klump 2003), giraffes (Giraffe camelonardalis reticulata, von Muggenthaler et al. 1999), and whales (Balaenoptera physalus, Payne and Webb 1971) communicate using frequencies below the lower frequency limit of human hearing (\20 Hz: infrasound). Several bird species produce infrasound. Capercaillie (Tetrao urogallus) produce infrasound in their flutter-jump displays (Lieser et al. 2005), while Cassowaries (Casuarius bennetti) produce low-frequency vocalizations (ca. 23 Hz) which may extend into the infrasonic range, though those are yet to be recorded with equipment capable of resolving sounds below 23 Hz (Mack and Jones 2003). Homing pigeons (Columba livia) are known to perceive infrasound, which aids them in orientation during migration (Yodlowski et al. 1977). Given both the capacity to produce and perceive infrasound, avian infrasonic communication represents a largely unexplored possibility. Lieser et al. (2005) suggested that Capercaillie could use infrasound produced in the context of their flutter-jump display in communication. The flutter-jump display is produced by males at lek sites during the breeding season, and thus could serve as an intrasexual signal (advertising to other males) or as an intersexual signal (advertising to or attracting females). Lieser et al. (2005) recorded the calls and flutter jumps of Capercaillie (Tetrao urogallus) at a lek in the Black Forest (Germany). Sound recordings were then manipulated using Audacity (for infrasound) and Matlab scripts (for audible sound), but the software settings used to perform those manipulations were not reported (Lieser et al. 2006). To test whether Capercaillie perceive infrasonic signals, Lieser et al. (2006) played back recordings of both the infrasonic and audible sounds associated with flutter-jump displays to seven tame, captive Capercaillie females. Lieser et al. (2006) detected no behavioural response to their purported playbacks of Capercaillie infrasound, and thus concluded that infrasound was an artefactual byproduct of the flutter-jump display, with no signal function. We suggest, however, that a fundamental deficiency in the apparatus employed in the Lieser et al. (2006) playback study precluded them from reaching any definitive conclusion, and opens the door to further study. Lieser et al. (2006) used a conventional cone-based automobile subwoofer (Infinity Kappa: Harman International Industries, Stamford, CT, USA) in an unspecified enclosure to reproduce infrasound. While the computer they used to reproduce the infrasound might have been appropriate (uncertain in and of itself since the computer Communicated by F. Bairlein.