Harlow emphasized the "strength and persistence" of the monkeys' drive to complete the puzzles.
But the only other known drive also failed to explain the monkeys' peculiar behavior.
No matter how tightly they clung to the surrogate mothers, however, the monkeys remained psychologically abnormal.
Harlow wondered whether what the monkeys were missing in their Isolettes was a mother.
When they returned to Earth, the monkeys were given a general anaesthetic on the first day.
When the coast was clear, I would throw my contraband peanuts to the monkeys.
They were given cucumbers as a reward for executing the task, and the monkeys obliged.
All of the monkeys, between the ages of five and 20, were photographed, face forward.
After training the monkeys to trade pebbles for slices of cucumber, the capuchin happily played the game.
The team then considered the overall health of the monkeys during their first four years of life.
Chen's next round of experiments may introduce the monkeys to yet another irrational human habit: price inflation.
As the monkeys' forest habitat disappears, they are becoming easier to capture and to put to work.
Yet when Harlow tested that approach, the monkeys actually made more errors and solved the puzzles less frequently.
In the 1950s, University of Wisconsin at Madison professor Harry Harlow was raising the monkeys in his lab.
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Like children, the monkeys feel they "need to get the same thing as somebody else, " de Waal said.
The monkeys grew up sturdy, disease-free, and larger than those from the wild.
The monkeys were allowed to observe how the actors interacted with each other.
The monkeys preferred the first researcher, who delivered a sure "gain" of one.
The monkeys are then taught at least six different commands and how to pick coconuts according to how ripe they are.
But both studies reported the same beneficial effects on health - all the monkeys were much healthier and suffered from fewer diseases.
Certain kinds of oil were much better for the monkeys than others.
After 1, 000 tries, with ever smaller cups, the monkeys mastered the challenge.
The results of these studies have led researchers to believe that the monkeys have a preference for symmetry just as humans do.
To eat the wheat, the monkeys would have to pick out the grains one by one from thousands of particles of sand.
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Ms. WORMSER: When the volunteers opened the cargo door to go into care for the monkeys, these are the sounds that we hear.
The monkeys came from three different groups (it was difficult to find one large group) and had been raised in some degree of captivity.
Ms. WORMSER: The sanctuary is a non-profit refuge for small, rainforest monkeys and the monkeys have wound up in laboratories and in the pet trade.
The training lasts from two weeks to a few months, and can be harsh: the monkeys learn by being punished for bad behaviour and rewarded for good.
When facing a "loss" either way, the monkeys preferred the chance of getting three half of the time--even though the expected payoff in all cases was the same.
Supporters tout the valuable research and conservation efforts of zoos, which work with endangered species and serve as more than just a place to see the monkeys on a Sunday afternoon.
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