Focus on the essential: all great apes know when others are being attentive.

When begging for food, all great ape species are sensitive to a human's attention. However, studies investigating which cues are relevant for chimpanzees to assess the attentional state of others have produced highly inconsistent results. Some have suggested chimpanzees differentiate attention based on the status of the face or even the eyes, while others have indicated that body posture alone is the relevant cue. Kaminski et al. (Anim Cogn 7:216-223, 2004) compared the behaviour of chimpanzees, bonobos and orangutans while begging for food from a human experimenter who systematically varied his face and body orientation. Their results indicated that both factors, face and body orientation, affect apes' begging behaviour. The authors claimed that while body orientation provides information about the experimenter's general disposition to offer food, the visibility of the face provides information about the human's attentional state. In the current study, we tested this hypothesis with all four great apes species. However, unlike Kaminski et al. (Anim Cogn 7:216-223, 2004), the experimenter was able to hand over food regardless of body orientation. The results show that as soon as the offering of the food was no longer restricted, the orientation of the face became the key factor. Therefore, we present the first evidence that all great ape species are able to assess the attentional state of a recipient based on the orientation of the face.