The male song of the Javan silvery gibbon (Hylobates moloch)

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20 T. Geissmann et al. – The Male Song of the Javan Silvery Gibbonsuggested for gibbon songs. The most commonlyproposed functions include defence of resources(territories, mates), mate attraction, and strengtheningand advertisement of pair bonds (Geissmann,1993, 1995, 1999, 2000; Geissmann and Orgeldinger,2000; Haimoff, 1984; Leighton, 1987).If the song served to strengthen and/or advertisethe pair bond, mated pairs should engage in duetsinging. This is not the case in H. moloch(Kappeler, 1984 a, Geissmann and Nijman, 2001).Instead, the study males appeared to abort theirsong bouts each time when a female started tosing. This occurred not only in three of the songbouts that were analysed for this study, but also inat least three other male song bouts of captivesilvery gibbons and in one male song monitored inCentral Java (Geissmann, unpublished observations).Although male silvery gibbons may occasionallycall together with females in “screambouts” (Geissmann and Nijman, 2001) or “borderconflict call bouts” (Kappeler, 1984 a), these callshave a different organisation and are notrecognised as song bouts.Traditionally, territorial defence is cited as oneof the main functions of male gibbon songs, butother functions have been proposed, such as mateattraction, singing in order to defend mate and offspringagainst other males, and mediating spacingamong individuals (Cowlishaw, 1992; Geissmann,2000; Mitani, 1992). These functions would requiremore frequent vocal activity from malesthan what we find in H. moloch. Males ofH. moloch roughly sing about once a week (Geissmannand Nijman, 2001), which is much less thanother gibbons. For males of other gibbon specieswhich do sing several times each week, nosignificant relationship was found between the frequenciesof male solo song bouts and territorialconflicts; yet, these males are territorial and doprovide physical defence of their territories. Incontrast, territorial conflicts appear to correlatewith the frequency of song bouts of mated females(Cowlishaw, 1992).Cowlishaw (1996) suggested that gibbon males(but not females) use assessment signals toadvertise resource-holding potential. Whitten(1984a, b) suggested that male songs of H. klossii(but not female songs) follow the handicap principle(e.g. Zahavi, 1975; Zahavi and Zahavi,1998) by requiring a lot of energy and stamina ofthe singer. The songs would then be an honestsignal declaring the singer’s fitness and his con-fidence and willingness to defend his territory.This hypothesis is supported by the observationthat male Kloss’s gibbons tend to maximise theirvocal output by singing about twice as often asfemales (about once every 1.7-2.5 days) and byspending over seven times as much time singingas females. In addition, males sing earlier in themorning than females. They produce most of theirsongs before dawn (Tenaza, 1976; Whitten,1984b), at a time when most diurnal mammals tryto reduce their energy expenditure.In H. klossii and other members of the genusHylobates, male solo songs have a longer durationand tend to occur earlier in the morning thanfemale or duet songs (Geissmann, 2000). Theseobservations partially apply to H. moloch, as well.Whereas an average female solo song bout ofH. moloch has a duration of about 7 minutes(Geissmann and Nijman, 2001), the six completemale solo song bouts we analysed had an averageduration of 23 minutes. This average does noteven include the longest recording of our study(40 min 45 s), because that song bout was notcompletely recorded. Recent field observations inCentral Java also document that most male songsstart before dawn when it is still dark, whereasmost female songs start after dawn (Geissmannand Nijman, 2001).The analogy to the Kloss’s gibbon does notextend, however, to the frequency of male singing.The field observations in Central Java (Geissmannand Nijman, 2001), as well as our observations inHowletts, suggest that males sing only about onceper week, whereas females sing about ten times asoften. This appears to be a reversal of the rule that,in the majority of territorial species, it is the malethat calls and defends the territory (Clutton-Brockand Harvey, 1977). It is questionable whether 30minutes of singing once a week represent asuitable handicap by which a male gibbon canadvertise his fitness.None of the functions suggested for gibbonsongs so far appear to explain satisfactorily therarity and variability of the male song ofH. moloch. It is difficult to find a plausibleexplanation for an event which occurs so rarely. Itis also difficult to assess the biological relevanceof the high variability we found in note typestructure and phrase structure of male silverygibbons.Preferences for certain note types and for certainphrase syntax both appear to differ among
Contributions to Zoology, 74 (1/2) – 2005 21male silvery gibbons, although it is unknownwhether the gibbons individually recognise eachothers’ songs. Bornean gibbons (H. muelleri) didnot respond differentially to experimental playbacksof their own, neighbours’, and strangers’duets (Mitani, 1985). The author concluded,however, that failure to show vocal discriminationmay be due to factors associated with the experimentalprocedure (e.g. playback periods lastedonly 3 minutes) or may indicate that there is noselective advantage gained by respondingdifferentially under the playback procedure.New functional hypothesesEven if inter-individual variability serves to facilitateindividual recognition, this does not explainthe adaptive value of high intra-individual variability.Why should males produce stereotyped phrasesin some gibbon species and highly variablephrases in other species? Without knowing theadaptive values of variable versus stereotypedmale songs, it is plausible to assume that at leastone of the two strategies provides a selectiveadvantage to the singer. Therefore, two alternateviews can be compared: (1) There may be aselective advantage for gibbon males to producestereotyped phrases, but – for unknown reasons –selection became secondarily relaxed in thesilvery gibbon. (2) The opposite is true: intraindividualvariability has been highly selected forin the songs of silvery gibbons, but at unknowncosts. Of course, more complex intermediatesolutions are also possible, but less easy to test.Accordingly, a high degree of either songvariability or song stereotypy could be used toadvertise the singer’s fitness to conspecifics(potential mates or potential competitors). In orderto qualify as a handicap (Zahavi, 1975; Zahaviand Zahavi, 1997), the feature in question mustinvolve costs, otherwise it could not be used tosignalise fitness. Either song type could qualify asan honest “handicap” signal according to thehandicap principle, because neither variability norstereotypy could be “forged”, provided that theycome with a cost. But which of the two characterstates is the cost-intensive one? As a proximatemechanism, variability could be either result froman easily created randomness or from complexvariability generator. Likewise, stereotypy couldrequire a complex mechanism to countereffectentropy, or else it could represent formulaicrepetition of a simple code.Fitch et al. (2002) recently suggested thatnonlinearities in the peripheral vocal productionsystem of many primates and other mammals“allows individuals to generate highly complexand unpredictable vocalizations without requiringequivalently complex neural control mechanisms.”Although we do not think that theunusually high intra- and inter-individual variabilityof the male song in H. moloch are related tothe non-linear phenomena described by Fitch et al.(2002), their paper is important in the contextdiscussed here because it documents that vocalvariability and unpredictability is not necessarilyan “expensive” character state.Here we present two different hypothesis whichcould explain how either song variability or songstereotypy produce costs to the singer. Bothhypotheses can be used to generate predictionsthat can be tested in order to determine whethersong variability or song stereotypy produce morecosts.Hypothesis 1: Singing may require not onlymuscle activity, but also brain activity, both ofwhich consume energy. In this case, brain activitymay represent a decisive cost-factor differentiatingbetween a variable song and a stereotypedsong. If the energy requirements of singinggibbons were determined (e.g. by measuringoxygen consumption), it could be tested which ofthe two song types uses more energy.Hypothesis 2: It is well known to both gibbonhunters and gibbon observers that approachingwild gibbons is easier when they are singing thanwhen they are silent. The concentration requiredto produce song phrases may represent a decisiveinvestment provided by a gibbon. A more concentratedor focused singer should be morevulnerable because he could be surprised bypredators or competitors more easily because hemay pay less attention to potentially dangeroussounds or movements in his environment. Thequestion is: What requires more attention from asinger, producing stereotyped phrase or producingmore loosely organised variable phrases? The twoalternatives could be tested, for instance, bypresenting increasingly loud or long noise stimulito singing gibbons. The concentration of thesinger could be determined by the minimumstimulus duration or loudness required to benoticed by the gibbon (as estimated by hisresponses such as song interruption or songabortion). This would represent an estimate of thesinger’s concentration.Both hypothesis can, in principle, be tested, and
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The male song of the Javan silvery gibbon (Hylobates moloch)

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