An assessment of memory awareness in tufted capuchin monkeys (Cebus apella)

Abstract

Humans, apes, and rhesus monkeys demonstrate memory awareness by collecting information when ignorant and acting immediately when informed. In this study, five capuchin monkeys searched for food after either watching the experimenter bait one of four opaque tubes (seen trials), or not watching (unseen trials). Monkeys with memory awareness should look into the tubes before making a selection only on unseen trials because on seen trials they already know the location of the food. In Experiment 1, one of the five capuchins looked significantly more often on unseen trials. In Experiment 2, we ensured that the monkeys attended to the baiting by interleaving training and test sessions. Three of the five monkeys looked more often on unseen trials. Because monkeys looked more often than not on both trial types, potentially creating a ceiling effect, we increased the effort required to look in Experiment 3, and predicted a larger difference in the probability of looking between seen and unseen trials. None of the five monkeys looked more often on unseen trials. These findings provide equivocal evidence for memory awareness in capuchin monkeys using tests that have yielded clear evidence in humans, apes, and rhesus monkeys.

Fig. 1

The apparatus as seen from experimenter’s side. A The opaque and transparent screens in the down position. B The opaque tubes on the tray. C The tube latching mechanism

Fig. 2

The percent of trials on which monkeys looked down at least one tube on seen (black) and unseen (white) trials in Experiment 1. ** P<0.01, see main text for full statistics

Fig. 3

The percent of unseen trials on which monkeys terminated looking before having seen the food (early), terminated immediately after having seen the food (appropriate), or continued looking after having found the food (late) in Experiment 1. Black bars are the performance of monkey IK. White bars are the mean (+SEM) of the other four monkeys

Fig. 4

The percent of trials on which monkeys looked down at least one tube on seen (black) and unseen (white) trials in Experiment 2. *P < 0.05, see main text for full statistics

Fig. 5

The percent of unseen trials (+SEM) on which monkeys terminated looking before having seen the food (early), terminated immediately after having seen the food (appropriate), or continued looking after having found the food (late) in Experiment 2

Fig. 6

Top view of a monkey holding up one of the flaps to look down the tube (left) and side view of a monkey selecting a tube by pulling up on it (right) in Experiment 3

Fig. 7

The percent of trials on which monkeys looked down at least one tube on seen (black) and unseen (white) trials in Experiment 3