Fig. 2.

Experimental evidence for a causal link between contributions to sentinel behavior and receipt of grooming. (A) Illustration of the experimental protocol. Additional sentinel contributions of focal individuals were simulated using playback of their surveillance calls, with close calls of focal individuals played back in control sessions. Focal individuals (red) received a greater proportion of group grooming in terms of (B) grooming duration and (C) number of grooming bouts following surveillance-call playback than close-call playback and compared with control individuals (blue) following focal surveillance-call playback. Shown in all cases are results for each individual separately (dotted lines; n = 12, although data values for some individuals are the same; thus, the number of dotted lines can appear less than 12) and the overall treatment mean (solid squares) ± SE (solid squares overlap in some cases).