Two unlike patterns of random-ratio responding associated with different eating habits in rhesus monkeys

Four rhesus monkeys were exposed to an identical series of schedules that specified a uniform probability of reinforcement for every response. As probability was lowered slowly in 10 steps of 20 sessions each from 1.0 through 0.01, two distinct patterns of responding emerged. Two subjects showed high, pause-free response rates that increased with each successive reduction in reinforcement probability. The other two showed consistent post-reinforcement pausing at all probabilities, including 1.0, and substantially lower response rates that peaked at the moderate probability values of 0.04 and 0.03. This low-rate pattern was found to be correlated with a pre-experimental preference in the two subjects for mouthing and chewing food pellets one at a time, while the former high-rate, pause-free pattern was linked to a long-standing habit of "pouch feeding" in the other monkeys. These idiosyncratic collateral behaviors that differentiated the schedule performances appeared neither superstitious in origin, nor useful in the case of the low-rate monkeys.