The authors examined quantity-based judgments for up to 10 items for simultaneous and sequential
whole sets as well as for sequentially dropped items in chimpanzees (Pan troglodytes), gorillas (Gorilla
gorilla), bonobos (Pan paniscus), and orangutans (Pongo pygmaeus). In Experiment 1, subjects had to
choose the larger of 2 quantities presented in 2 separate dishes either simultaneously or 1 dish after the
other. Representatives of all species were capable of selecting the larger of 2 quantities in both conditions,
even when the quantities were large and the numerical distance between them was small. In Experiment
2, subjects had to select between the same food quantities sequentially dropped into 2 opaque cups so that
none of the quantities were ever viewed as a whole. The authors found some evidence (albeit weaker)
that subjects were able to select the larger quantity of items. Furthermore, the authors found no
performance breakdown with the inclusion of certain quantities. Instead, the ratio between quantities was
the best performance predictor. The authors conclude that quantity-based judgments rely on an analogical
system, not a discrete object file model or perceptual estimation mechanism, such as subitizing.