EAVESDROPPINGAND DEFENSIVEAUDITORY MASKING IN ANAUSTRALIANBUSHCRICKET , (:: )

by

THOMAS J.HAMMOND and WINSTON J.BAILEY 1,2) (Departmentof Zoology,University of WesternAustralia, 35 Stirling Hwy, Crawley, WesternAustralia, 6009)

(Acc.5-XII-2002)

Summary Weinvestigatedthe potential for a novelalternative male tactic in Caedicia,anAustralian genusof duetting phaneropterine bushcrickets (Orthoptera: Tettigoniidae: Phaneropterinae). Maleslisten to andtrack females as they reply to the calls of othermales, using both the male calland the female reply to initiate their own searching behaviour. W edescribethis behaviour aseavesdropping. Calling males produce a highintensity chirp following their calling song, whichhas no apparenteffect on femaleresponsiveness. We test the possibility that this loud chirpacts to defend the temporary pair bond, established between the calling male and the duettingfemale, by preventing other males from hearing the female reply. W esuggestmeans bywhich the calling male may be ableto hearthe female reply whilst producing the masking chirp.This behaviour may be considereda formof pre-copulatory acoustic mate guarding.

Keywords:acousticbehaviour, duet, alternative mating tactics, satellite behaviour, mate guarding,Tettigoniidae.

Introduction Signallingbehaviour in the subfamily Phaneropterinae(Orthoptera: Tettigo- niidae) commonlytakes the formof amale-female duet(Robinson, 1990).

1) Correspondingauthor’ s e-mailaddress: [email protected] 2) WethankDale Roberts and Leigh Simmons for discussion on earlierdrafts of thispaper. Wealso extend our thanks to an anonymous reviewer whose comments greatly enhanced thispaper. TJH wassupported by a postgraduatescholarship from the University of Western Australia. © KoninklijkeBrill NV ,Leiden,2003 Behaviour140, 79-95 Alsoavailable online - 80 HAMMOND&BAILEY

Males call spontaneously,andfemales announcetheir availability andwill- ingness to mate byproducing a numberof short clicks within aprescribed time windowfollowing the male call (Heller &Helversen,1986; Zimmer- mann et al.,1989).This duetproduces a short-lived pair bond,during which the pair will attempt tolocate eachother before mating. T ypically the male searches forthe female, althoughin several species the female searches for the male orsearching duties maybe shared (Robinson, 1990; Zhantiev & Korsunovskaya,1990). Alternative matingtactics (see review: Gross,1996) are commonamong andthe relative success ofthese tactics maylead tothe evolutionof defensive,or mate guarding,behaviour (Thornhill & Alcock,1983). Alterna- tive male tactics canbe determined by differences in male size andthe tactics employedby each morph can include a large Žghtingclass, anda smaller sneakeror satellite class (Alcock,1997; Emlen, 1997). Males mayadopt suchstrategies bychoosing between two tactics suchas calling anddefend- inga territory, orremaining silent nearbya calling male andintercepting the searchingfemale. Examplesof such satellite strategies exist amonganurans (Arak,1988), and the orthopteranfamilies Gryllidae (Cade,1979; Rowell & Cade,1993) and (Otte, 1972;GreenŽ eld &Shelly,1985). In these examples,a male mayswitch tactics dueto a changein his owncondition, oras aresponseto external variables, suchas the density ofcalling males. While the silent male tactics ofcrickets andfrogs merely requirethe satel- lite to wait bya calling male andintercept the searchingfemale, Otte (1972) describes silent male Syrbula (Orthoptera;Acrididae) locating females as theymake an acoustic responseto the calls ofsignalling males. This behav- iourcan best bedescribedas eavesdropping,which is deŽned by Doutrelant andMcGregor (2000) as ‘: : :areceiver extracting informationfrom an inter- action inwhich it is takingno part’(see also McGregor,1993). As this is a behaviouralresponse to the complete acoustic interaction betweenthe sexes, wewouldexpect such behaviour to be commonamong duetting bushcrick- ets. Populationsof the phaneropterinegenus Caedicia,whichis the subject ofthis paper,form loose aggregations,where the majority ofcalling males is conŽned to anarea less thanone hectare. If amale wereto track females answeringother males withinthis aggregationand were successful inacquir- inga mate, thenwe wouldexpect that calling males shoulddevelop defen- sive tactics that preventtakeover by satellites. Defensivesignalling tactics,