Read the original here (but you have to pay for it.)
Picture two 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.