To attract mating partners, female mammals communicate their reproductive status through one or multiple sensory modalities, providing redundant or complementary information. Chimpanzees (Pan troglodytes) are an excellent model for studying multimodal communication. Exaggerated sexual swellings of females serve as visual proxy for ovulation but their temporal imprecision suggests that olfactory cues may exist in addition to swellings to pinpoint fertility more accurately.
Here, we combined gas chromatography–mass spectrometry, hormonal analyses, and two-choice sniffing bioassays to examine (1) whether female anogenital odours change during the fertile period, and (2) whether males are able to detect these changes by odours alone. Our chemical results suggest that complementary changes in chemical profiles linked to menstrual cycle phases exist in addition to prominent changes of anogenital odours associated with swelling stages, even if they are statistically less strongly supported. Male responsiveness in behavioural bioassays was too low to draw conclusions regarding their ability to detect these subtle shifts when presented with anogenital odours only.
Overall, our chemical analyses of female anogenital odours suggest the existence of a multimodal fertility cue in chimpanzees, wherein visual signals seem to be complemented by subtle changes in odours’ chemical profiles associated with menstrual cycle phases. Mating success is a fundamental part of individual fitness. Sexual selection exerts strong pressures on traits that improve inter-sexual communication2.
Such communication may involve a single modality, but highly social, group-living animals often engage in multimodal communication3,4. For example, many species of birds show complex visual displays during singing5,6,7, and some bats combine vocalizations with fanning odours toward potential mates8. Primates are remarkable in the diversity of sensory modalities used to communicate their reproductive status9.
Some indicators of reproductive status may constitute signals, i.e., traits that evolved to communicate fertility-related information to conspecifics, or cues, i.e., traits that did not evolve for communicative purposes, e.g., being by-products of hormonal changes10. Female primates can display visual cues, such as sexual swellings in chimpanzees (Pan troglodytes)11 and olive baboons (Papio anubis)12,13 or sexual skin colour in rhesus macaques (Macaca mulatta)14. Some primates use auditory cues, such as copulation calls in Barbary macaques (Macaca sylvanus)15, or behavioural cues, such as solicitations and other proceptive behaviours in black howler monkeys (Alouatta pigra)16 and long-tailed macaques (Macaca fascicularis)17.
Finally, female primates use olfactory cues, such as scent marks in common marmosets (Callithrix jacchus)18 and ring-tailed lemurs (Lemur catta)19. These cues can be exhibited in various combinations, whereby different components may transmit redundant or complementary information20. Whereas visual and acoustic cues of fertility have been well-studied in non-human primates, recent studies also highlight the importance of smell, a modality which had long been neglected compared to other senses in primate communication studies21,22,23.
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