What Factors Influence Patience In Primates?

By Shweta Iyer on May 15, 2014 10:43 AM EDT

primates
Patience is acquired not just by practice, but through genes during evolution. (Photo: Photo courtesy of Shutterstock)

Christian Calhoun once said, "Patience is not only a virtue, but an acquired trait." And acquired not just by practice, according to researchers at the University of Nebraska-Lincoln, but through genes during evolution. A paper published in the scientific journal Proceedings of the Royal Society B examines the evolutionary process that makes some primates more patient than others, according to a press release Tuesday.

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"Natural selection has shaped levels of patience to deal with the types of problems that animals face in the wild," said author Jeffrey R. Stevens, a comparative psychologist at the University of Nebraska-Lincoln and the study's lead author."Those problems are species-specific, so levels of patience are also species-specific."

We often have to choose between sooner, smaller reward and later, larger reward. This is known as "intertemporal choice" a scientific jargon for patience, self-control, or delayed gratification and it exists among all primates. Though for humans it may be for various issues like "Do I settle for immediate cash or do I wait 3 months for more?" or "Do I go unprepared for tomorrow's test or do I study for two months and give it later?" in most other primates it centers around food. For instance, deciding when and where to forage.

Stevens studied 13 primate species ranging from gorillas to marmosets and compared their species' characteristics with their capacity for intertemporal choice. He found that patience depends on body mass, brain size, lifespan, and home ranges and increases as we go higher up the primate classification order. For example, chimpanzees, which typically weigh about 85 pounds, live nearly 60 years and range about 35 square miles, waited for a reward for about two minutes, the longest of any of the primate species studied. Cotton-top tamarins, which weigh less than a pound and live about 23 years, waited about eight seconds before opting for a smaller, immediate reward.

Stevens used his own research and also relied on several previous experiments to gather data on intertemporal choice in primates.As a part of his experiment, he conducted experiments on lemurs, marmosets, tamarins, chimpanzees, and bonobos at Harvard's Department of Psychology and at the Berlin and Leipzig zoos in Germany where individual animals had to choose between a tray containing two grapes they could eat immediately and a tray containing six grapes they could eat after waiting. The wait times were gradually increased until the animal reached an "indifference point" when it opted for the smaller, immediate reward instead of waiting.

Previous studies have listed two other parameters for patience: cognitive ability and social complexity. But Stevens negates both hypotheses.

"In humans, the ability to wait for delayed rewards correlates with higher performance in cognitive measures such as IQ, academic success, standardized test scores and working memory capacity," he wrote. "The cognitive ability hypothesis predicts that species with higher levels of cognition should wait longer than those with lower levels."

But Stevens found no correlation between patience levels and an animal's relative brain size compared to its body size, the measure he used to quantify cognitive ability. Previous researches have also shown that social structure greatly influences patience and animals in complex social groups tend to wait longer. But Stevens did not find correlations between species' social group sizes and their patience levels. According to Stevens, metabolic rates is the connecting factor between patience, body mass, and other characteristics. Smaller animals tend to have higher metabolic rates.

"You need fuel and you need it at a certain rate," he said. "The faster you need it, the shorter time you will wait."

Metabolic rates may also be behind humans' increased level of patience over a wide range of issues but the cognitive decisions behind the choices need to be explored according to Stevens. "To me, this offers us interesting avenues to start thinking about what factors might influence human patience," he said. "What does natural selection tell us about decision making? That applies to humans as well as to other animals."

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