Introduction

Eye-covering play, deliberately closing or covering one's eyes during a play sequence, has been documented in various nonhuman primates. Some observers have suggested that eye-covering play involves pretending – acting as if one can't see, doesn't exist, or exists in some altered form (Cunningham, 1921; Hahn, 1982; de Waal, 1986b, 1989). Only one systematic study has been made of eye-covering play in nonhuman primates (Thierry, 1984) and no systematic attempts have been made to investigate the cognitive processes involved. We systematically studied eye-covering play in captive orangutans (Pongo pygmaeus) and Japanese macaques (Macaca fuscata) with the aim of assessing its cognitive implications in relation to pretending and imagination.

Cunningham (1921) was perhaps the first to describe eye-covering play in a nonhuman primate – in an immature lowland gorilla (Gorilla gorilla) who would shut his eyes tightly then run around knocking into furniture, which was interpreted as pretending to be blind. Eye-covering play has since been reported in all the great apes. Their play often takes the form of stumbling or groping about while “blind” (Hoyt, 1941; Harrisson, 1961, 1962; Lang, 1963; Gautier-Hion, 1971b; Goodall, 1971; Rensch, 1972; de Waal, 1986b, 1989). Occasionally they use objects to cover their eyes (Harrisson, 1962; Lang, 1963; Goodall, 1971; Rensch, 1972; de Waal, 1986b, 1989) and eye-covering play sometimes occurs as a social game (Hoyt, 1941; Harrisson, 1962).