Pick from the Past
Natural History, December 1947

Payday for Primates

When scientists taught monkeys the value of money, and gave them a Chimp-O-Mat in
which to spend it, significant changes took place in their behavior toward one another,
and new light was shed on the capacity of lower animals for symbolic thought.


AVE you ever heard of an animal that would work for wages—not for food but for actual wages that could be used to pay for food, water, or even a ride home? Most animals that work for man do so because they have very little choice in the matter, but there have been a few who learned to work hard and willingly for payment in poker chips. Of course, the chips had to be worth something, or the animals wouldn’t have worked to earn them; but that’s getting ahead of the story.

“Trader,” the Bartering Monkey

Buried in the pages of the exceedingly technical Journal of Genetic Psychology is an erudite scientific article bearing the imposing title, “Notes on Symbolic Behavior in a Cebus Monkey.” Although it pretends to be a scholarly treatise, it is in actuality the charming story of Trader, the bartering monkey.

Trader lived at the Zoological Garden in San Diego. Soon after his arrival, he became a star attraction because of his ingenious method of obtaining treats from indulgent visitors. When a visitor strolled by his cage carrying a bag of peanuts or a bar of candy, Trader would scurry around until he found a bit of wood, a stone, or a wisp of straw. Then, stretching his skinny arm through the bars, he would chatter and gesticulate to indicate his willingness to trade.

The visiting public delighted in Traderís antics, and it seems likely that he would eventually have died from overeating if fate had not intervened.
If his first offer was accepted, the monkey would hunt for something else to use as barter and then do his best to promote a second exchange.

The visiting public delighted in Trader’s antics, and it seems likely that he would eventually have died from overeating if fate had not intervened. One day, a psychologist saw Trader in action and was so impressed that he purchased the animal and took him home in order to learn more about his abilities.

Trader seemed perfectly happy to exercise his huckstering habits under experimental conditions. The experimenter sat on a chair in the center of the room and extended his hand, saying, “Give me something, Trader.” Thereupon Trader proceeded to collect a handful of sawdust, a scrap of paper, a bit of trash—in fact, anything available—and placed it in the psychologist’s hand. If the first offering was refused, the animal continued his hunt and turned up with something else, which he seemed to hope the experimenter would regard as acceptable. Sometimes when an item of barter had been deposited in the scientist’s palm, the monkey would extend his own paw, palm upward, politely but firmly requesting the peanut which was his usual reward.

Ordinarily Trader was not averse to making one or two advance payments before he received any return, but he made it quite plain that he could not be exploited by any psychologist! When the experimenter accepted one offering after another without delivering any peanuts, the monkey became highly indignant. Screaming at the top of his lungs, he threw himself on the floor, beat the rug with his heels, and behaved in general like a child in a temper tantrum.

Poker Chips for Money

Boxes were placed in the corners of the experimental room, and each one was filled with poker chips of a certain color. Each time that Trader paid in a red chip he was given a piece of orange. A blue chip was worth one shelled peanut. A white one purchased a slice of banana, and a green one bought a slice of bread. It didn’t take the monkey long to master this coinage system, and he indicated plainly that the most valuable item in stock was the banana. White chips were turned in more frequently than any other color. Second came the blue chips for peanuts. Least popular of all were the green. Trader was not at all fond of bread.

One thing about the monkey’s behavior puzzled the psychologist. Although Trader did not like bread and would not eat it, he nevertheless continued to bring in green chips occasionally. Furthermore, he sometimes proffered yellow chips, which were worth nothing at all. This usually happened just after he had eaten a lot of his favorite food, and he seemed to deliberately choose the “valueless” chips.

It seems to me that only one explanation fits the facts. In Trader’s veins there must have coursed a bit of Yankee blood. Even when he made no profit on the transaction, he tried to promote a trade just for the sake of making a deal.

High Finance at Yale

The psychologist who studied Trader had taken his inspiration from experiments conducted at the Yale Laboratories of Primate Biology. A young experimenter, Dr. John Wolfe, trained six young chimpanzees to operate a specially built slot-machine, which automatically delivered one ripe grape every time a poker chip was inserted in the slot.

To begin with, the apes had to learn how to make the Chimp-O-Mat pay off, and some of them caught on very quickly. Moos, a six-year-old male, watched the experimenter insert just one chip in the slot and then saw a grape roll into the food cup. With no hesitation, Moos picked another chip from the floor, pushed it into the machine with only a little fumbling, then put his hand in the food cup and waited for the grape to drop.

At the beginning of the experiment, the animals held white poker chips in low esteem. After all, they weren’t nearly so interesting as toys or other playthings. However, as soon as the apes had learned to buy grapes at the vending machine,

Within just a few days, even the dumbest chimpanzee could differentiate between real money and “counterfeit” coinage.
the little plastic disks were transformed into extremely desirable objects—prizes worth striving for.

The first time a handful of white chips and brass slugs was thrown on the floor, the little animals seized both with equal avidity. The slugs would fit the slot in the Chimp-O-Mat but would not cause it to disgorge grapes. Within just a few days, even the dumbest chimpanzee could differentiate between real money and “counterfeit” coinage. When a mixture of white chips and slugs was tossed onto the floor of the cage where Alpha, Bula, and Bimba lived, the three little females scrambled madly for white chips, stealing them from one another whenever possible. They never touched the brass slugs.

Human parents know only too well that when a child receives a regular allowance, he encounters no difficulty in spending it. People can recognize the value of money even if they don’t have to work for it. However, one measure of the importance of money to an individual is the amount of effort required for its acquisition. This line of reasoning applies equally well to the Yale chimpanzees. They had, in effect, been given an allowance, and their behavior clearly showed that money, in the form of white poker chips, had a real value in their scheme of things. The next question was: Would they work to earn it?

In order to discover the answer, Dr. Wolfe introduced his “subjects” to the mysteries of a Work Machine. Each time the handle was lifted, the animal could reach in and pick up one grape. Then the handle returned automatically to the starting position, and when it was raised again another grape was disgorged. When the chimpanzees had mastered the operation of the Work Machine, white poker chips were offered instead of grapes, and the worker was later allowed to buy one grape with each chip. They worked for chips as readily as for grapes and would lift very heavy weights in order to secure their “money.” When the machine was loaded with brass slugs, they performed no labor. As far as the apes were concerned, the entire affair was strictly a business proposition.

Every chimpanzee has a unique personality, and they differ as much as people in their temperamental make-up. Velt proved to be a very common type of laborer. His money burned holes in his pockets. He would work energetically as long as he was permitted to run to the Chimp-O-Mat to spend each chip as soon as it was earned; but he went on strike when forced to wait an hour with wages in hand. Moos and Bimba were the thrifty type. They were quite willing to operate the Work Machine and collect their pay in poker chips, even though it couldn’t be spent until the next day. However, when the next day arrived and opportunity for spending was offered, the animals demanded prompt and efficient service.

Once, the machine was set so that the apes, after inserting a chip, had to wait several minutes before a grape would appear. Moos had participated in the “forced savings plan” without complaint, but he had no patience with this sort of foolishness. He dropped a poker chip in the slot and extended one hand into the food cup. When no grape appeared, Moos grasped the Chimp-O-Mat and shook it vigorously. He looked like a frustrated subway customer who has just wasted a penny in a defective gum machine.

The Idle Rich

By this time Moos and Bimba were definitely money-mad. When permitted to operated the Work Machine as often as they wished, these two apes amassed great piles of poker-chip wealth. In a single ten-minute period,

By this time Moos and Bimba were definitely money-mad . . . . These two apes amassed great piles of poker-chip wealth.
Moos continued to lift the eighteen-pound handle until he had collected one hundred eighty-five counters. Unwilling to waste time, he didn’t even grasp a chip in his fingers but feverishly brushed it to the floor and pumped the handle again. A minute-to-minute record showed that Moos and Bimba worked fastest during the first part of their tests and then slowed down considerably in each succeeding minute. Fatigue didn’t seem to be the answer, and so the psychologist cast about for some other explanation. Finally he thought he had found it. He reasoned that as the pile of earned chips increased, the apes became less and less inclined to work for more.

Once again Moos was permitted to operate the Work Machine as often as he wished for ten minutes. But this time he was given 30 white chips before starting to work. He lifted the handle exactly thirteen times. The tests were repeated several times and always turned out the same. When Moos was absolutely broke, he almost always pumped the handle at least 100 times; but if he was grubstaked before starting, his performance was very inferior. Like many human beings, the chimpanzee’s willingness to exert himself for pay depends in large measure on the current state of his financial reserves.

An Expanded Economy

Little children learn to prize money long before they know the relative values of different coins. Many a four-year-old clamoring for a nickel has been entirely satisfied when given a penny instead. Dr. Wolfe’s chimpanzees had readily learned to choose white poker chips in preference to worthless brass slugs, but was their mentality equal to the task of dealing with “coins” of different values?

To find the answer the psychologist introduced new forms of specie and broadened the market. Brass slugs still bought nothing, and white poker chips were still worth one grape; but when a blue chip was dropped in the Chimp-O-Mat two grapes rolled out. A red chip bought a drink of water. And if a chimpanzee inserted a yellow chip in a slot beside the door of the experimental room, he was allowed to ride “piggy-back” on the scientist’s shoulders for prompt delivery to his home cage in the living quarters.

The young apes seemed to comprehend this new system after they had gained some practical experience with it. When given a choice, they came to spurn the white chips as long as they could get blue ones with twice the buying power. Thirsty animals passed by both white and blue chips to select the red tokens, with which water could be obtained.

The response to the yellow chips was in some ways the most interesting. A few chimpanzees never used them. These were individuals who seemed perfectly content to remain in the experimental room all day long. Other apes were continually bribing the experimenter to carry them home. Bula was standing by the Chimp-O-Mat buying grapes with blue chips when the psychologist suddenly opened a box that contained a white rat. He placed the animal on the floor beside the chimpanzee. For a moment she stared at the rat with obvious trepidation. Then, sidling carefully around the strange beast, she seized a yellow chip from the floor, dashed to the door, inserted the chip in the slot, and leaped onto the experimenter’s back, chattering loudly to be removed from the presence of so menacing a creature.

Bula’s closest friend was one of the smaller chimpanzees, who relied heavily upon her for companionship and protection. During the first part of the experiment, Bula was working in the experimental room when her friend began to cry loudly in the living quarters. Bula dropped the task she had been assigned and refused to resume work. She stood by the door, straining at the handle and pleading to be let out. Weeks later, after she had learned the value of the yellow poker chips, Bula again heard her little friend calling from the home cage. This time she made no attempt to open the door but quickly found a yellow chip, put it in the slot by the door, and ran to the experimenter with her arms outstretched, waiting to be picked up and delivered to her lonely companion.

The Root of All Evil

It might have been anticipated that the introduction of money into chimpanzee society was bound to make trouble sooner or later. Perhaps it is too much to say that the profit motive could wreck ape economy. And as far as the scientific report goes, there is no absolute proof that the Yale Colony succumbed to an irreversible process of moral decay as a result of these experiments. Nevertheless, it is just as well that they came to an end when they did, for certain ominous trends were rapidly becoming apparent.

Despotism and cruelty are not unknown among man’s nearest living relatives, but usually they take simple and straightforward forms such as wife-beating or commandeering of the food supply. New and subtle kinds of treachery are possible when wealth is involved.

When Bula and Bimba were living in the same cage, a large supply of white poker chips was offered to the two females. Bula promptly assumed ownership of nearly every one, and Bimba was left with a very small hoard. She protested vocally and with gestures, whining and holding out an empty hand. Like a rich man tossing a coin to a beggar, Bula impatiently selected one chip from her huge pile and dropped it negligently in Bimba’s palm. When the vending machine was wheeled up to their cage, both animals rushed to spend their windfalls. Bula roughly shouldered Bimba away and took complete possession of the machine. The menu that day was slices of unpeeled oranges. Bula calmly bought and devoured one slice after another, and when Bimba started to complain, Bula handed her the peels!

Poor Bimba! Later, when she set up housekeeping with Velt, she found that she had jumped from the frying pan into the fire. Her new lord and master wouldn’t permit her to pick up a single poker chip until he had carefully selected the ones he wanted. Then she was free to take the leavings; but they did her no good. As soon as the Chimp-O-Mat was available, Velt would use up his entire supply of chips and then turn to stare at Bimba. His glance was one that some human husbands would envy, for when he turned it on Bimba, she invariably dropped the few chips that she had been allowed to collect. Needless to say, Velt picked them up and, of course, spent them on himself!

Though he was a lion at home. Velt behaved more like a lamb when he was out with the boys. At first, when he and Moos were together, Velt picked up a few poker chips, but each time he did so, Moos came over and gently but firmly appropriated the prize. After a few such experiences, Velt just gave up. Making no protest, he simply stopped gathering chips while Moos was around. As far as he was concerned, they could stay right there on the floor until Moos was ready to collect them.

So What?

The stories of Trader, Moos, Bimba, Bula, and the others are amusing and appealing, but some readers may say, “So what? What is the value of such experimentation? What does it all mean, if anything?”

Actually, of course, scientists who study animal behavior are not in the least interested in the entertainment value of their experiments. To the psychologist these particular studies are worthy of thought because they show that monkeys and apes are capable of dealing with symbols. The psychological chasm between man and the rest of the animal world yawns widest at just this point. A great deal of human behavior is based upon—in fact, utterly dependent upon—symbolism.

The outstanding example is language. Words are written or spoken symbols representing things or events. They are also something more important. Words are the ground stuff, the indispensable raw material, of thought. Nearly all of our thought consists of language—implicit language that never reaches the tongue. When we “sit and think” we are actually talking to ourselves. Try to do it sometime without using words, and you will discover that even silent thought depends heavily upon language.

Now, as far as we have been able to determine, no other animal has a true language. To be sure, many species are capable of rudimentary communication, but not all communication necessitates symbolic language. For instance, the shrill trumpeting of one frightened elephant may send a whole herd stampeding in panic through the jungle. But this represents the contagious spread of a vague though powerful emotional state rather than the transfer of a specific thought or idea. No symbolism is involved.

Are all animals except man inherently incapable of symbolic behavior? Is this the reason for their lack of language, their relatively limited mental powers? Psychologists believe that man far excels any other animal in his capacity for dealing with symbols, but they are equally certain that at least the higher mammals can learn to use symbols in a limited way.

The classic “poker-chip experiments” with young chimpanzees are partially responsible for this conviction. Moos and his fellows did learn to respond to chips as symbols or temporary substitutes for food and other biologically useful rewards. To be sure, this is a rather elementary form of symbolic behavior, but the fact that it could be established at all is significant.

It is generally conceded that the human body is a product of long, slow evolutionary change. Experiments in symbolic behavior support the gradually growing conviction that the human mind is also an evolutionary product. Such studies in the field of animal psychology reveal to us the rudimentary and primitive beginnings from which our own mentality has sprung.

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