The ability of captive spider monkeys, Ateles geoffroyi, to visually discriminate between different sizes of food and of non-edible objects.

Field studies suggest that the size of potential food items plays an important role in the food selection behavior of nonhuman primates. However, there is only limited knowledge about how good primates are at visually discriminating between the size of three-dimensional objects of the same kind and shape. We therefore conducted two experiments on ten adult spider monkeys: a two-choice test based on spontaneous preferences with differently-sized pieces of food of the same kind and shape, and a two-choice test based on an operant conditioning procedure in which the animals were trained to choose the larger one of two cube-shaped wooden blocks. We found that the spider monkeys displayed a robust and spontaneous preference for the larger one of two simultaneously presented pieces of food of the same kind when the size difference was 11% or larger. This was true with cube-shaped food pieces as well as with ball-shaped and hemisphere-shaped food pieces. Considering that the normal size variation among fully ripe fruits of a given plant species is at least 10% this suggests that spider monkeys are as picky as necessary to make size-based food choices in line with optimal foraging theory. We also found that the spider monkeys successfully discriminated between two simultaneously presented wooden cubes when their edge length differed by only 2 mm which was the smallest size difference tested. Thus, they displayed a well-developed ability to visually discriminate between three-dimensional non-edible objects of the same kind and shape which is at least as good as that of other nonhuman primate species tested previously on similar tasks. Our findings support the notion that frugivory may have favored the evolution of cognitive abilities in the physical domain which may, possibly, include the ability to perform fine visual size discriminations.