Multimodal fertility cues in chimpanzees: how body odours complement sexual swellings.

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.