Pre-existing sensory biases in the spectral domain in frogs: empirical results and methodological considerations
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ORIGINAL PAPER
Pre-existing sensory biases in the spectral domain in frogs: empirical results and methodological considerations H. C. Gerhardt • Sarah C. Humfeld
Received: 20 August 2012 / Revised: 31 October 2012 / Accepted: 2 November 2012 / Published online: 18 November 2012 Ó Springer-Verlag Berlin Heidelberg 2012
Abstract In many species of anurans, advertisement calls excite only one of the two inner-ear organs. One prediction of the pre-existing bias hypothesis is that signal innovations that additionally excite the ‘‘untapped’’ organ will be more behaviorally effective than normal calls. However, recent studies have shown that females of three species with single-peaked calls that stimulate only the basilar papilla (BP) preferred single-peaked synthetic calls with a frequency typical of conspecific calls to two-peaked calls that also stimulated the amphibian papilla (AP). We report that in spring peepers (Pseudacris crucifer) that also produce single-peaked calls, females did not show a preference in choices between single-peaked and two-peaked synthetic calls. Thus, the addition of energy exciting the AP had a neutral effect on signal attractiveness. Together, these results are unsupportive of the pre-existing bias hypothesis. An alternative hypothesis is that positive fitness consequences of responding to sounds providing extraordinary spectral stimulation are required for a novel call to become established as a mate-attracting signal. Testing these ideas requires a taxonomically broader examination of responses to sounds with novel spectral complexity, and attention to some methodological details will improve the comparability of such studies. Keywords Pre-existing sensory bias Sensory exploitation Amphibian papilla Basilar papilla Pseudacris crucifer
H. C. Gerhardt (&) S. C. Humfeld Division of Biological Sciences, University of Missouri, Columbia, MO 65211, USA e-mail: [email protected]
Introduction Many animal species are especially sensitive to sounds with frequencies typical of conspecific signals. Their hearing range is nevertheless much wider than the range of sound frequencies typical of conspecific acoustic signals. Frogs and toads exemplify both of these generalizations. These animals have two auditory inner-ear organs, the basilar papilla (BP) and amphibian papilla (AP), each most sensitive to a different frequency range (review: Simmons et al. 2007). Reasonably good matches usually exist between the emphasized frequencies in conspecific signals and the frequency sensitivity of one or both of the auditory organs (Gerhardt and Schwartz 2001). In many species that produce a broadband, mateattracting advertisement call, the spectrum is bimodal, with the lower-frequency peak falling into the range of the AP and the other, higher-frequency peak, into the range of the BP (reviews: Gerhardt and Schwartz 2001; Richards 2006). In most species with such calls that have been tested, females were more attracted to synthetic calls that stimulated both organs than to calls that had only a
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