Chimps can tell the difference between different volumes of liquid, even when the bartender hides his actions.
Psychologists at Georgia State University wanted to see whether chimpanzees could understand different volumes of liquid as well as numbers of items.
They found that the chimps chose the fuller container every time - even when the liquid was being poured into an opaque cup, so that they couldn't see which contained more. The canny primates were watching to see how long the juice took to pour.
Dr Michael Beran and his team then wondered if pouring from a greater height would fool the animals into thinking there was more juice. So they played barista, varying the height from which the liquid was poured - to no avail. The chimps weren't fooled.
"Chimpanzees could compare visible sets to nonvisible sets, and they were not distracted by perceptual illusions created through various presentation styles that were not relevant to the actual amount of juice dispensed," say the researcher.
"This performance demonstrated a similarity in the quantitative discrimination skills of chimpanzees for continuous quantities that matches that previously shown for discrete quantities."