Numerity of a socially housed hamadryas baboon (Papio hamadryas) and a socially housed squirrel monkey (Saimiri sciureus).

To extend a study conducted by E. M. Brannon and H. S. Terrace (1998, 2000), the authors trained 1 hamadryas baboon (Papio hamadryas) and 1 squirrel monkey (Saimiri sciureus) to respond to stimuli representing the numerosities 1-4 in ascending order. When tested with novel stimuli of the same numerosities, both subjects' performance appeared to be based on the numerical attributes of stimuli. Subjects were then tested on their ability to order pairs of numerosities derived from the values 1-9. Both subjects successfully ordered pairs that included the untrained numerosities 5-9 regardless of the total surface area of numerosities. Accuracy and latency of responding also showed numerical distance and magnitude effects. Numerosity was a salient cue to both subjects, suggesting that these 2 families of primates perceive ordinal relations. The outcome shows that cognitive studies of this type can be effectively conducted with socially housed animals.