Sexual selection and adaptive function of gibbon vocalizations: Do testosterone-driven males sing their own praises, but pay a cost in immuno competence?
Final Report Abstract
Elaborate vocalizations are common among birds but rare among mammals, except for the agile apes of South-East Asia: the gibbons. Male gibbons sing highly complex and elaborate solos the function of which is enigmatic. Therefore, using wild white-handed gibbons (Hylobates lar) from the Khao Yai National Park, Thailand, our overall aim was to investigate the proximate function of this characteristic vocal behaviour and by doing so to help clarify whether elaborate primate male vocalizations are an indicator of male quality. Through a combination of behavioural observations, song recordings and non-invasive endocrine, parasitological and genetic analysis we have assessed to what extent male vocalizations vary between males and whether they may function as a sexual signal. Specifically, using faecal androgen measurements, we examined whether male endocrine status may drive variation in the acoustic structure of male songs. By measuring gibbons' endoparasite load, we assessed the animal's physical condition and its relation to socio-demographic variables as well as their hormonal status. Finally, we determined genetic paternity to elucidate whether male song characteristic is related to male reproductive success. The study population is unique in representing both pair-living (monogamous) and multimale (polyandrous) groups, which make this population an interesting model for studying potential implications of sexual selection in the evolution of male vocalizations. Data collection in the field was highly successful with the data generated representing the most comprehensive and complex set of biological data for any wild living gibbon species. Due to the complexity of the data, however, unfortunately we have not yet been able to complete all analyses required to provide final answers to our main research questions. However, our analyses so far already provide many novel and interesting results. For example, concerning male endocrine status we found higher androgen levels in males living in pairs compared to those living in multi-male groups and in males living with dependent offspring, suggesting that those males may be more at risk from being challenged and to loose infants to aggressive attacks from other males. Higher androgen levels may help them to prepare physiologically for such challenges as known from macaques and baboons. The first data on gastro-intestinal parasitism in wild gibbons indicated a low diversity of parasites compared to other ape species but showed a high prevalence for two potentially pathogenic nematodes and one protozoan known to cause severe pathologies in primates. However, none of the animals showed any symptoms of impaired health, suggesting a low pathogenic effect of these parasites in gibbons. Parasite load and variability were not related to social organization and male social status and were also unaffected by the male's androgen levels. Our data thus indicate a low influence of demographic, social and endocrine factors on parasitism in gibbons and suggest that ecological factors (e.g. climate) are more important in determining parasite load in this species. Analyses of the song recordings also revealed interesting findings. By identifying acoustic parameters reflecting song property and relating these to socio-demographic variables (i.e. age, social organization and social status) and male androgen levels we found that i) songs were highly personalized and greatly differ in acoustic structure (e.g. frequency, element duration, total call duration) between individual males, ii) song structure was influenced by male age, social status and social organization and iii) certain acoustic parameters were related to male androgen levels. Concerning the latter, gibbons seem to sing louder and with a higher degree of song stability when holding higher levels of androgens. However, since the various factors are partly interconnected (i.e. not independent from each other), disentangling the exact contribution of the various factors to song variation is extremely difficult to achieve and more fine-tuned analysis is needed to examine this further. The findings nevertheless strongly suggest that solo songs indeed have the potenfial to function as indicator of male quality and thus may represent a sexual signal that may influence mating opportunities. The latter is particulariy interesting to examine further given the findings of our genetic paternity analysis which, for the first time, provide strong evidence that extra-group paternities make up a substantial proportion (ca. 15%) of infants sired. What drives extra-pair copulations in female gibbons is not clear but it is plausible that females seek for mating opportunities with "higher quality" males assessed through, potentially, their song production. In our future analyses we will examine whether the acoustic characteristics of the elaborate male songs are linked to male reproductive output. If so, this would lend further support to the hypothesis that solo songs in gibbons represent an honest indicator of male quality which may act as a sexual signal in inter-sexual (and possibly tntra-sexual) communication.
