Annika Felton of the Australian National University and colleagues spent a year in the Bolivian rainforest observing the monkeys' feeding habits.
Felton said tight regulation of daily protein intake is known to play a role in the development of obesity in humans, and the findings from her research suggest the evolutionary origins of such eating patterns in humans might be far older than suspected.
"We found that the pattern of nutrient intake by wild spider monkeys, which are primarily fruit eaters, was almost identical to humans, which are omnivores," said Felton. "What spider monkeys and humans have in common is that they tightly regulate their daily protein intake, i.e. they appear to aim for a target amount of protein each day, regardless of whether they only ate ripe fruit or mixed in other vegetable matter as well.
"Finding this result in spider monkeys was unexpected because, previously, ripe fruit specialists were thought to be energy maximizers," she said. "In other words, they would aim to maximize their daily energy intake. Our findings show this is not the case."
The research was published online last week in the journal Behavioral Ecology.