Monkeys keep track of small numbers

Without any training, rhesus monkeys recognize when the number of other monkeys’ voices that they hear corresponds to the number of monkeys’ faces that they see. This new finding indicates that monkeys accurately count small quantities that are relevant in their social worlds, say psychologist Kerry E. Jordan of Duke University in Durham, N.C., and her colleagues.

Evidence of counting by monkeys also supports the idea that basic number skills don’t require an understanding of language (SN: 2/19/05, p. 117: Available to subscribers at Math minus Grammar: Number skills survive language losses).

Jordan’s group tested 20 adult male monkeys housed in a colony at a research institute. Each animal sat in front of two video monitors. One monitor showed two monkeys’ faces as they vocalized for 1 minute, and the other displayed three monkeys’ faces as they vocalized for 1 minute. While watching these scenes, the animals being tested heard recordings of either two or three monkeys making loud, cooing noises.

The monkeys looked substantially longer at the monitor with the number of faces that matched the number of voices that they heard, the scientists report in the June 7 Current Biology. In their view, such responses reflect recognition of the difference between two and three units in each of the two categories—faces and voices.