In a classic episode of the Simpsons, Homer's brain explains to him that "money can be exchanged for goods and services". That's obviously true for humans (even cartoon ones) but monkeys use an altogether different form of payment - grooming. It's as close to a currency as monkeys have and it can be redeemed against a wide range of goods and services including more grooming, a free pass from aggression, permission to handle babies, back-up in fights and even sex.
The purposes of these exchanges go well beyond cleaning. Grooming, it seems, is also an enjoyable activity that releases brain signalling chemicals involved in pleasure and rewarding feelings. It's a social bonding activity, the monkey equivalent of a human hug. Grooming does have costs though, despite its appearance as a leisurely activity. For a wild monkey, time spent cleaning a peer is time that's not spent foraging yourself or watching ...
