At least some species of monitors are known to be able to count; studies feeding rock monitors varying numbers of snails showed that they can distinguish numbers up to six. Nile monitors have been observed to cooperate when foraging; one animal lures the female crocodile away from her nest, while the other opens the nest to feed on the eggs. The decoy then returns to also feed on the eggs.[39][40] Komodo dragons at the National Zoo in Washington, DC, recognize their keepers and seem to have distinct personalities.[40] Blue and green tree monitors in British zoos have been observed shredding leaves, apparently as a form of play.[41]