to be positively selected for. A 2011 study suggests that the practical functions that some pets provide, such as assisting hunting or removing pests, could have resulted in enough evolutionary advantage to allow for the persistence of this behaviour in humans and outweigh the economic burden held by pets kept as playthings for immediate emotional rewards. Two other studies suggest that the behaviour constitutes an error, side effect or misapplication of the evolved mechanisms responsible for human empathy and theory of mind to cover non-human animals which has not sufficiently impacted its evolutionary advantage in the long run. Animals in captivity, with the help of caretakers, have been considered to have owned "pets". Examples of this include Koko the gorilla who had several pet cats, Tonda the orangutan and a pet cat and Tarra the elephant and a dog named Bella. Pets in art See also Alternative pets: Digital pet Robotic pet Animal captivity Animal hoarding Anthrozoology Classroom pet Pet adoption Pet loss Zoonosis References Further reading External links Companion Animal Demographics in the United States: A Historical Perspective from The State of the Animals II: 2003 Animals and humans