Monkeys can keep strings of information in order by using a simple kind of logical thought.

Rhesus macaque monkeys learned the order of items in a list with repeated exposure to pairs of items plucked from the list, say psychologist Greg Jensen of Columbia University and colleagues. The animals drew basic logical conclusions about pairs of listed items, akin to assuming that if A comes before B and B comes before C, then A comes before C, the scientists conclude July 30 in Science Advances.

Importantly, the size of rewards given to monkeys for correctly identifying the higher-ranking item in a pair didn’t always provide reliable guidance to the animals about the item’s ranking on the list, Jensen’s group says.

Previous studies have suggested that a variety of animals, including monkeys, apes, pigeons, rats and crows, can discern the order of a list of items (SN: 7/5/08, p. 13). But debate persists about whether nonhuman creatures can actually develop an internal knowledge about what items come before others in a list.

Jensen’s group designed experimental sessions in which four monkeys completed as many as 600 trials to determine the order of seven images in a list. Images included a hot air balloon, an ear of corn and a zebra. Each item came with a different-sized reward. In some sessions, the size of the reward for a correct answer was proportional to an image’s rank on the list, with a high-ranking image netting a bigger reward. In other sessions, high-ranking images came with the smallest rewards. Rewards consisted of larger or smaller gulps of water delivered through tubes to the moderately thirsty primates.

Monkeys consistently learned list orders in both reward conditions, making relatively few errors by the end of the sessions. The monkeys learned lists slightly faster when given rewards proportional in size to an item’s ranking.

Jensen’s study adds to evidence suggesting that, like humans, monkeys can mentally link together pairs of items into lists that guide later choices, says psychologist Regina Paxton Gazes of Bucknell University in Lewisburg, Pa.

That’s probably a valuable ability in the wild, she says, because many animals need to monitor where group mates stand in the social pecking order. “An ability to construct, retain, manipulate and reference ordered information may be an evolutionarily ancient, efficient [mental] mechanism for keeping track of relationships between individuals,” she says.