Capuchin monkeys (Cebus apella) modulate their use of an uncertainty response depending on risk

Abstract

Metacognition refers to thinking about thinking, and there has been a great deal of interest in how this ability manifests across primates. Based on much of the work to date, a tentative division has been drawn with New World monkeys on 1 side and Old World monkeys and apes on the other. Specifically, Old World monkeys, apes, and humans often show patterns reflecting metacognition, but New World monkeys typically do not, or show less convincing behavioral patterns. However, recent data suggest that this difference may relate to other aspects of some experimental tasks. For example, 1 possibility is that risk tolerance affects how capuchin monkeys, a New World primate species, tend to perform. Specifically, it has recently been argued that on tasks in which there are 2 or 3 options, the "risk" of guessing is tolerable for capuchins because there is a high probability of being correct even if they "know they do not know" or feel something akin to uncertainty. The current study investigated this possibility by manipulating the degree of risk (2-choices vs. 6-choices) and found that capuchin monkeys used the uncertainty response more on 6-choice trials than on 2-choice trials. We also found that rate of reward does not appear to underlie these patterns of performance, and propose that the degree of risk is modulating capuchin monkeys' use of the uncertainty response. Thus, the apparent differences between New and Old World monkeys in metacognition may reflect differences in risk tolerance rather than access to metacognitive states.

Figure 1

A size-discrimination trial illustrated. The squares were presented in an orange color, and the largest had to be selected for food reward. Here, the largest square is to the bottom left. The ? is the uncertainty response. The small circle in the center of the screen is the cursor controlled by the monkey using a joystick. It was red in color.

Figure 2

Monkeys’ performance in Exp. 1 as a function of trial level and condition (2-choice – left column, 6-choice – right column) for each of the five alternating blocks of trials (earliest at top and latest at bottom). Error bars show 95% confidence intervals.

Figure 3

Performances of each monkey in the final alternating block of trials in Exp 1. For each monkey, there are two graphs. The one with the monkey name is performance in the 2-choice condition and the graph to the right of that is performance in the 6-choice condition. The monkeys at left showed varying degrees of uncertainty responding in the 6-choice condition whereas monkeys at left did not.

Figure 4

Performance by the monkeys in the two alternating blocks of trials (first alternating block is the top row, second alternating block is the bottom row) with the 2-choice and the 6-choice conditions when there were no longer any forced UR trials (Phase 2, Experiment 1). Error bars show 95% confidence intervals.

Figure 5

The performance patterns and patterns of UR use for each condition In Experiment 2 as a function of level (1–20 for the 6-choice condition and 1–3 for the 2-choice condition). Error bars show 95% confidence intervals.

Figure 6

The overall performance pattern and pattern of UR use in Experiment 3. Error bars show 95% confidence intervals.