Apparently humans aren't the only primates that plan outdoor events based on weather.

Gray-cheeked mangabey monkeys rely on recent trends in temperature and solar radiation to forage for figs and insect larvae, report Karline Janmaat and her colleagues of the University of St. Andrews in Scotland. The results support a lesser-studied notion that primate cognition evolved to solve problems rooted in ecology--such as foraging--instead of the more favored viewpoint, that cognition evolved as a way to cope within a complex society.

The findings, published today in Current Biology, come from field studies conducted for 210 days in the Kibale National Park of Uganda, where Janmaat mapped out the locations of 80 fig trees, noting whether the trees contained ripe fruit, unripe fruit or no fruit at all. Next, her team followed a group of mangabey monkeys from dawn to dusk, recording their position every 10 minutes using global positioning system (GPS) satellite technology, and observing whether the animals revisited or bypassed fig trees visited earlier. The researchers also recorded the maximum and minimum daily temperatures, as well as the percentage of high-level solar radiation.

They found that if the weather had been warm and sunny--as opposed to cool and cloudy--for a period of about five days, the monkeys were more likely to revisit a fruiting tree. "During the rainy season, the fruit takes really long to ripen--up to two months before they are finally ripe," Janmaat says. "In some periods when it's sunny, it can be in one week. There are big variations. Maybe it's worthwhile for the monkeys to know that."

Sunny weather also increases the likelihood of finding tasty weevil larvae--which infest unripe fruit and develop faster when temperatures are higher. "The monkeys pick the fruit and then they suck the larvae out," Janmaat explains.

In a place where fruit ripens intermittently and often in widely disparate locations, a strategy for efficient foraging could mean the difference between life and death.