IN CHEATING MONKEYS and Citizen Bees, biologist Lee Dugatkin describes the grooming behavior of the impala. (For Americans who think of an impala as gas-guzzling Chevy, the impala, closely related to the gazelle, inhabits the savannah of Kenya and other parts of southern and eastern Africa.) Impala face a problem unfamiliar to humans: They are unable to clean important parts of their body, especially their backs, since they have no arms, and their legs are facing the wrong way. This is a problem particularly with ticks, which are itchy and carry nasty impala diseases. Whereas the rhinoceros solves this problem though his symbiotic friendship with the oxpecker (tick bird), who stands on his back feasting on his ticks, impala are too jumpy for a piggybacking mate. As a result, they usually ask another impala for help. But there are costs in grooming: lapses in vigilance for lions and wild dogs, hairballs, loss of saliva, and so forth. The substantial health benefits of grooming would justify these costs if another impala would reciprocate. Yet what is to keep a “groomee” from bounding off on his own business after the groomer does his work? Here, impala play a Trust game, with a guarantee of reciprocity hard to secure.