Even well-fed domestic cats hunt and kill birds, mice, rats, scorpions, cockroaches, grasshoppers, and other small animals in the vicinity. They often present such trophies to their owner. The motivation is not entirely clear, but friendly bonding behaviors are often associated with such an action. It is probable that cats in this situation expect to be praised for their symbolic contribution to the group. Some theories suggest that cats see their owners gone for long times of the day and assume they are out hunting, as they always have plenty of food available. It is thought that a cat presenting its owner with a dead animal thinks it's 'helping out' by bringing home the kill. Ethologist Paul Leyhausen, in an extensive study of social and predatory behavior in domestic cats (documented in his book Cat Behavior), proposed a mechanism which explains this presenting behavior. In simple terms, cats adopt humans into their social group, and share excess kill with others in the group according to the local pecking order, in which humans place at or near the top.