While decision regarding with whom cooperate is influenced by relatedness (kin selection), spatial constraints (e.g. spatial selection, multi-level selection), or information about potential partners (e.g. reputation), a large remaining puzzle concerns the detection of cooperation in the absence of these mechanisms. One possibility is that people use phenotypic cues to accurately assess the cooperativeness of potential partners. Several studies support this possibility but have mainly focused on facial traits. Yet, evolutionary framework highlights the adaptive functions of multimodal signalling in animal communication. According to the redundant signal hypothesis, individuals use multiple traits because in combination they provide a better estimate than any single trait. It has been suggested that voice and body odours could be used to assess cooperativeness, but no study investigated this possibility so far. To fill this gap, we quantified cooperativeness of 64 native French men through a one-shot public goods game, recorded their spontaneous speech and collected their body odours through cotton wool pads under their armpits. We firstly examined whether individual’s contribution to the public good was associated with four vocal acoustic features (namely mean fundamental frequency, pitch variations, roughness and breathiness). We then used the pads of the highest (n=8 men) and lowest (n=8 men) contributors to the public good to test, in a second experiment, whether body odours influenced partner choice in a modified version of a one-shot public goods game. Participants (n= 58 men and 58 women) had to smell a series of 8 pads (4 pads from the highest cooperative men versus 4 from the lowest cooperative men) and decide whether they wanted or not to interact with the men smelled. Our results found that both fundamental frequency and pitch variations (i.e., intonation) were significantly associated to contributions to the public good: men with a lower-pitched voice and with greater pitch variations (more intonation) were more cooperative. In the second experiment, we found that women (but not men) were significantly more likely to interact with the high cooperative men and more likely to avoid interacting with the low cooperative men, based on body odours only. This is the first evidence of vocal and olfactory cues of cooperativeness. Combined with the literature on face-based cooperation detection, we suggest that several sensory modalities may advertise cooperativeness and could therefore be simultaneously used to assess cooperativeness more accurately.

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