Supply and demand predict male grooming of swollen females in captive chimpanzees, Pan troglodytes

The notion of a biological marketplace as a framework for explaining cooperative acts among animals is well supported. However, direct evidence for a biological market in a mating context is limited to a handful of studies. We collected data over a 3-year period to test supply/demand predictions of male grooming of swollen females according to a biological market in captive chimpanzees while controlling for other potential interchange. Females received more grooming from and gave less grooming to males when swollen than when not swollen, with parous females receiving more grooming from males than nulliparous females. The central tenet of a biological market that the market value of a commodity should vary according to its availability was supported. Males groomed swollen females less as the availability of swollen females in the group increased. There was also an effect of male dominance rank such that higher ranking males groomed swollen females less than lower ranking males. Taken together, our findings provide evidence for male grooming of swollen females according to a biological market while controlling for potential confounds.