By Matt Walker
Editor, Earth News
Panzee goes with the flow
Chimpanzees are intelligent enough to appreciate how big a pint of liquid is, or the volume of any other measure.
That shows they have an ability to gauge the difference between continuous quantities, such as a pint or half pint of non-alcoholic fruit juice.
Previously, apes have only been known to differentiate discrete quantities, such as eight sweets over five.
That means chimps are more intelligent than we thought, and shows they have a basic grasp of the physics of liquids.
Details of the discovery are published in the journal Animal Cognition.
Comparative psychologist Dr Michael Beran of Georgia State University, Atlanta, US has spent over decade researching animal intelligence, in particular the mental abilities of monkeys and apes, including people.
In the past he has shown primates are able to keep track of how many sweets are in a container: by performing simple addition and subtraction calculations they can keep count of how many treats are added or taken away.
Knowing that eight sweets are more than five shows an ability to distinguish between discrete quantities.
However, liquids pose a different challenge. Because a liquid flows, it forms one continuous quantity, that gets larger as more liquid is added.
"So I wanted to know whether they would perform as well when they had to judge two poured amounts of juice," says Dr Beran.
He tested three chimps, a 37-year-old female called Lana, a 21-year-old female called Panzee and a 34-year-old male called Sherman.
In the first experiment, Dr Beran poured quantities of fruit juice from a 600ml syringe into a clear cup and opaque cup.
The chimps watched as he did so, and then choose the larger to drink.
It did not matter if Dr Beran poured 100ml, 200ml, 300ml or so on up to 600ml into either cup (one UK pint = 568ml).
More than three quarters of the time, the chimps would select the larger volume.
Crucially, by pouring the liquid into opaque containers, the chimps could only see how much was being poured, not how much had accumulated in the measuring cup.
That means the chimps could accurately visualise or understand how much liquid was being poured, rather than collected.
"They had to watch juice pour into containers and once the juice was there, it was out of sight. So they had to remember how much juice is there, just from seeing it fall," Dr Beran told the BBC.
Overcoming an illusion
In a second set of experiments, the chimps had to choose between a clear cup already containing a certain volume of juice, and another they couldn't see, but into which was poured a drink.
That meant the chimps could not take the relatively easy option of timing the pouring events, and choose whichever cup had liquid poured into it for longer.
"This is a complicated feat because there are no cues such as duration of pouring or height of the liquid that can be used," explains Dr Beran.
Panzee passes another test with flying colours
"They must represent and compare the poured amount to the visible amount, and estimate which is larger."
Again the chimps easily appreciated the difference.
In a third set of experiments, Dr Beran then varied the height from which the liquids were poured.
That creates a perceptual illusion that might confuse the chimps.
"I wanted to see whether the chimps overestimated the amount of juice if it was poured from higher up," says Dr Beran.
"This is an old favourite of the experienced bartenders of the world, where the patron gets the impression of getting more alcohol than is really true because of varying the height of the pouring."
However, it made little difference to all three chimps, who picked the largest amount over 80% of the time, with Panzee scoring a high of 86%.
"The results support the position that chimpanzees are good mental accountants who judge various forms of quantities," says Dr Beran.
"They can track quantities in ways not previously demonstrated.
"In some sense, this is a kind of folk understanding of the physics of liquids."
The experiments also suggest the chimpanzees use the same mechanism to gauge discreet and continuous amounts.
Dr Beran, whose research is supported by the US National Institutes for Health and National Science Foundation, believes such intelligence could help chimpanzees in their natural environment.
"I have no doubt such skills would prove valuable in the wild," says Dr Beran.
"Chimpanzees make many decisions regarding how to spend their time foraging, and where to forage, and also they must attend to who else is around them in terms of the number of individuals.
"In many of these cases, quantity offers valuable information, and so sensitivity to quantity and the ability to judge quantities and use forms of mental accounting would be adaptive."
Such findings may also force us to think again about how clever animals really are.
"Certainly, these kinds of capacities, like many others that we continue to find in nonhuman animals, require rethinking our positions on animal intelligence.
"The results also support the position that there is psychological as well as biological continuity across species, at least for many cognitive and intellectual abilities," says Dr Beran.