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High background noise shapes cricket hearing.pdf

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University of Toronto Scarborough
Horatio Morgan

1754The Journal of Experimental Biology 214 17541762 2011 Published by The Company of Biologists Ltddoi101242jeb053819RESEARCH ARTICLEHigh background noise shapes selective auditory filters in a tropical cricket121Arne K D Schmidt Klaus Riedeand Heiner Rmer12Zoology KarlFranzensUniversity Universittsplatz 2 8010 Graz Austria and Zoologisches Forschungsmuseum AlexanderKoenig Adenauerallee 160 53113 Bonn GermanyAuthor for correspondence arneschmidtunigrazatAccepted 11 February 2011SUMMARYBecause of call frequency overlap and masking interference the airborne sound channel represents a limited resource forcommunication in a speciesrich cricket community like the tropical rainforest Here we studied the frequency tuning of anauditory neuron mediating phonotaxis in the rainforest cricket Paroecanthus podagrosus suffering from strong competition incomparison with the same homologous neuron in two species of European field crickets where such competition does not existAs predicted the rainforest species exhibited a more selective tuning compared with the European counterparts The filterreduced background nocturnal noise levels by 26dB compared with only 16 and 10dB in the two European species We alsoquantified the performance of the sensory filter under the different filter regimes by examining the representation of the speciesspecific amplitude modulation of the male calling song when embedded in background noise Again the filter of the rainforestcricket performed significantly better in terms of representing this important signal parameter The neuronal representation of thecalling song pattern within receivers was maintained for a wide range of signaltonoise ratios because of the more sharply tunedsensory system and selective attention mechanisms Finally the rainforest cricket also showed an almost perfect match betweenthe filter for sensitivity and the peripheral filter for directional hearing in contrast to its European counterparts We discuss theconsequences of these adaptations for intraspecific acoustic communication and reproductive isolation between speciesKey words acoustic communication cricket competition matched filters directional hearingINTRODUCTIONantiphonal signaling alternation and a switch to alternative mateThe role of competition for limited resources in natural selectionacquisition tactics Gerhardt and Huber 2002has been recognized since the classical work of the Russian ecologistHabitats differ strongly in species richness and the degree ofG F Gause on different species of ParameciumGause 1934acoustic competition and background noise varies accordinglyEcologists soon recognized that competition for resources betweenTropical rainforests are among those habitats with highest speciesspecies is an important factor affecting the abundance anddiversity Erwin 1982 Acoustic noise measurements in adistribution of species Connell 1983 Competition for aneotropical rainforest of Panama at night have demonstrated soundcommunication channel has however only rarely been consideredpressure levels SPLs as high as 70dB Lang et al 2005 andin this context but see Greenfield Greenfield 1983 and Greenfieldspectral analysis of sounds from distinct rainforest sites revealedand Karandinos Greenfield and Karandinos 1979 for chemicalthat a great proportion is due to the signaling activity of insectscommunication and Nelson and Marler Nelson and Marler 1990Ellinger and Hdl 2003 Lang et al 2005 Diwakar andfor a discussion of birdsong signal space although it should beBalakrishnan 2006 Of these the calling activity of cricketsevident that as the number of species using the same channel in anconstitutes the main audio frequency band between approximatelyecosystem increases the chances of successful communication will2 and 9kHz where most acoustic energy is concentrated thusdecreaseindirectly confirming the high species richness of crickets in tropicalThis is also true for the airborne sound channel as for everyrainforests Riede 1993 These studies together with our ownadditional species that vocalizes at the same time and location thepreliminary measurements see below indicate that calling songbackground noise level increases the signaltonoise level decreasescarrier frequencies of sympatric species sometimes do not differ byand signal detection andor discrimination is severely impairedmore than 200Hz Current models of peripheral and central nervousImpressive examples are mixedspecies choruses of birds Catchpoleprocessing in crickets cannot explain how signal detectionand Slater 1995 anurans Narins and Zelick 1988 Narins 1992discrimination and localization is achieved under these maskingand insects Gogala and Riede 1995 Sueur 2002 Even when onlynoise conditions However the fact that so many species stilltwo species utilize spectrally similar signals this can result incommunicate under these conditions means that they can deal withcomplete suppression of calling activity of one species by the otherthese environmental conditions to an extent that the use of acousticor a shift in the diurnal calling activity of one species Schwarz andsignals in fitnessrelated tasks is still a likely evolutionary outcomeWells 1983 Greenfield 1988 Rmer et al 1989 Wong et alof natural selection Brumm and Slabberkoorn 2005 We consider2009 To reduce acoustic competition and the resulting maskinga sharpening of peripheral filters as an important sensory adaptationinterference species employ a wide variety of behavioralto improve signaltonoise SN ratio in a complex acousticadaptations including temporal and spatial partitioning of habitatsenvironment Evidence for such a scenario comes from work on theTHE JOURNAL OF EXPERIMENTAL BIOLOGY
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