Rhesus monkeys (Macaca mulatta) spontaneously compute addition operations over large numbers

Area of Science:

• Cognitive science
• Comparative psychology
• Neuroscience

Background:

• Humans possess unique mathematical abilities, but foundational number sense exists in animals and infants.
• Existing research shows precise small number quantification (<4) and approximate large number quantification in non-human animals.
• Previous studies on animal addition confounded numerical quantity with perceptual properties like size or length.

Purpose of the Study:

• To investigate if animals can spontaneously use large number quantification for mathematical operations.
• To determine if rhesus monkeys can perform addition on large numbers independent of continuous perceptual properties.
• To identify the limitations of this numerical computation ability.

Main Methods:

• Experiments designed to present rhesus monkeys with addition tasks involving large quantities.
• Careful control of stimuli to isolate numerical quantity from continuous perceptual variables (e.g., volume, contour length).
• Analysis of performance to determine if monkeys computed based on number or other properties.

Main Results:

• Rhesus monkeys demonstrated spontaneous computation of addition operations over large numbers.
• The monkeys' performance was based on numerical quantity, not correlated continuous perceptual properties.
• The limit for this addition ability was determined by the ratio difference between quantities, not their absolute difference.

Conclusions:

• Rhesus monkeys possess the capacity for abstract numerical computation, specifically addition of large numbers.
• This ability is independent of continuous perceptual features, indicating a true number sense.
• The findings suggest that ratio-based processing is a fundamental aspect of numerical cognition in primates.