New Study Suggests That Capuchin Monkeys Depend On Growth And Size -- Not Just Learning -- For Successful Foraging

October 02, 1997

ATHENS, Ga. -- Tufted capuchins are gregarious, cat-sized monkeys that South Americans have traditionally kept as pets. With curling tails as long as their bodies, capuchins are agile and intelligent, and can live more than 40 years. Like other monkeys, they must forage for a living, and psychologists have often studied foraging for keys to primate behavior. In capuchins, foraging involves dextrous use of the hands and intensive manipulation of objects.

New research by scientists, including one at the University of Georgia, shows that self-sufficiency in foraging among capuchins arrives long after they have sufficient manual skills to achieve it. One-year-old capuchins use the same foraging actions as adults over the course of weaning, and afterward they continue to forage in the same way. Only when they are of sufficient size and have strong enough teeth do young capuchins manage to feed themselves entirely and complete the weaning process.

"This view doesn't preclude a significant role for experience in fashioning an individual's foraging behavior," said biopsychologist Dorothy Fragazy, "but it does suggest that independent foraging is related at least as much to increasing physical strength or stamina as to improvements in motor skill."

The research was recently published in The Journal of Comparative Psychology. Co-author on the paper was Leah Adams-Curtis of Washington State University. A companion paper in the same issue, authored by Fragazy and two students from the University of Georgia, examined how food is transferred from adult to infant tufted capuchins. (Getting food from others is another way, in addition to nursing, that young capuchins may manage to survive even when they are not efficient at feeding themselves.

Monkeys have been extremely valuable for decades in helping psychologists tease out the roots of social behavior, motor development and problem-solving skills. Capuchins are now recognized as useful for these studies. These New World monkeys have unusually large brains, and parts of their brains are relatively enlarged, just like those in humans and apes. Still, they are skeletally immature at birth in comparison to many other monkeys, and they nurse for a long time, only becoming completely weaned in their second year. Thus, capuchins have a long childhood that makes them ideal for the study of behavioral development.

It has been suggested, for example, that changes in foraging behaviors between infancy and weaning reflect cognitive development.

Capuchins have a foraging style that is extractive -- that is, they go after food that is in some way hidden and then take it out," said Fragazy. "Though they are primarily frugivorous, they also rely on animal protein."

In the wild, they find animal prey in dead wood, palm fronds forest floors or stream beds and break open snails and oysters. They also eat a considerable amount of fruit, but fruit is highly prized by other species, and often capuchins, using their strong hands and powerful jaws are able to eat nuts or husked fruits, hidden invertebrates or even vertebrate prey such as snakes, birds, squirrels or even bats. In South America, capuchins still flourish in the wild, through their habitat is threatened in many places in part because they can eat so many different kinds of foods.

Extractive eating, while effective, demands finely controlled movements and eye-hand coordination. Just how capuchins acquire the skill necessary to forage can be an important indicator of development.

"Becoming efficient at extractive foraging is really demanding for the young capuchin," said Fragazy.

Fragazy admits it is hard to untangle the roles of motor and cognitive development from physical growth, and indeed, probably all these factors (and others) contribute to successful foraging among capuchins. To study just how these monkeys acquire foraging skills, Fragazy and Adams-Curtis studied eight captive capuchins (four male, four female) over their first 25 months of life.

The study focused on behavioral changes across four significant phases of foraging competence, which were divided into six-month blocks. In the first six months (early infancy), the capuchins have limited mobility and ability to feed themselves. In the second six months, the monkeys are mobile but still depend on their mothers for nutrition. In the third block, the infant assumes increasing responsibility for feeding itself, and in the final six months, the infant becomes fully independent nutritionally.

Since the monkeys in the study were not foraging in the wild, the scientists had to closely follow foraging behaviors -- actions well-known from earlier studies, including those of Fragazy.

What they discovered was that even though the monkeys do show foraging behaviors while very young, capuchins don't succeed at the necessary survival skills until they have the size and bite strength to crack open hard foods, such as nuts. They don't change how they forage as they get older and more experienced. Thus, in natural populations, weaning seems to occur at the time when the permanent incisors and the first molars appear and the young monkey is more than half of its adult size. Increasing skeletal and muscular growth add to the monkey's ability to feed itself.

The fact that self-sufficiency occurs long after the capuchins have the motor skills to feed themselves is quite striking, said Fragazy. This pattern is unusual among monkeys, though there are indications a rare primate called the aye-aye may do the same thing.

The aim of Fragazy's research is to develop a better understanding of how motor abilities and problem-solving skills arise. In particular, the primacy of movent in cognition attracts her attention. This problem can be studied in monkeys and in humans.

The research was supported by grants from the National Science Foundation, the National Institute of Mental Health and the National Institute of Child Health and Human Development.


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