Long-tailed macaques (Macaca fascicularis) understand what conspecifics can see in a competitive situation

Visual perspective taking (VPT), an understanding
of what others can see, is a prerequisite for theory
of mind (ToM). While VPT in apes is proven, its presence
in monkeys is much-debated. Several different paradigms
have been developed to test its existence, but all face
interpretational problems since results can be explained by
simpler cognitive mechanisms than VPT. Therefore, we
adjusted one method where two individuals compete for
access to food, visible or invisible for the dominant competitor,
to preclude cognitively simpler mechanisms. The
subordinate long-tailed macaques tested, selected significantly
more often the food item invisible than the item
visible to the dominant. In most trials, subjects retrieved
only one food item and preferred the invisible food item.
Surprisingly, they occasionally adopted an alternative
strategy to obtain both food items, by first choosing the
visible, most at risk food item. Faster animals adopted this
strategy proportionally more often than slower ones. Contrary
to previous research, our results cannot be explained
by simpler cognitive mechanisms, since behavioural reading
was prevented by a one-way mirror between the
competitor and the food, and accessibility was equal to
both food items. This is the first unequivocal evidence of
VPT in a monkey species, suggesting that this precursor to
ToM is an evolutionarily conserved capacity present in
monkeys, apes and humans.