Table 1. The experimental conditions.

In each condition, the chimpanzees were given a food reward for each completed exchange. Depending on the condition these rewards were either of equal or different value to that offered to their test partner. In the Unavailable condition only, prior to making an exchange, the chimpanzees were shown a high-value reward, but upon completion of their exchange, they were given a low-value reward. In the Test Phase, if the chimpanzees exchanged a token through the picture frame then they would receive a high-value reward (a grape), irrespective of the condition. Thus, the subjects had the ability, in the Test Phase, to gain a more preferred rewards in the Different, Unavailable and Low-value Same conditions. In the High-value Same condition, because the chimpanzees were already receiving the high-value reward, exchanging tokens through the picture frame would not impact the reward they would be given.

Condition	Subject		Partner
	Shown	Given	Shown	Given
Different	—	Celery	—	GRAPE
Unavailable	GRAPE	Celery	GRAPE	Celery
Low-value Same	—	Celery	—	Celery
High-value Same	—	GRAPE	—	GRAPE