Picture two female chimpanzees hanging out under a tree. One grooms the other, systematically working long fingers through fur, picking out bugs and bits of leaves. The recipient sprawls sleepily on the ground, looking as relaxed as someone enjoying a spa day. A subsequent surreptitious measurement of her levels of oxytocin, a hormone associated with bonding and pleasure, would confirm that she is pretty happy.
And why not? Grooming appears to be a pleasurable way to spend time. Many species of apes and monkeys devote long chunks of the day to it. Among other things, grooming can curry favor and strengthen alliances so it is likely that of these two chimps, the female being primped is of equal or greater rank in the troop than the one doing the work.