A big brain is a resource-hungry organ, demanding large amounts of energy-rich foods to keep it functioning. Understanding how species like humans and chimpanzees evolved large, energy-hungry brains is a difficult task: explanations must account for not only how the large brain provided an evolutionary advantage, but also how its energy demands could have been met.

A recent paper in PNAS suggests that a bigger brain can help with the energy needs. A group of female chimpanzees was found to show an unexpectedly advanced ability to plan their movements around what they intended to eat for breakfast. The chimps’ movements were tracked over three-quarters of a year, and their behaviors helped them get through three periods of food scarcity.

Previously, researchers suggested that having a consistent source of food was vital for animals with bigger brains. After all, that extra brain tissue is only worthwhile if the animal can eat enough calories to support it. Large-brained primates like chimps have a surprisingly consistent calorie intake despite seasonal fluctuations in their food supplies, which suggests that their intelligence plays an important role in finding food during periods of scarcity.

Maintaining a cognitive ‘map’ of where different foods can be found goes some way toward planning around shortages, but it’s also important to know when scarce foods can be found. Until now, it wasn’t clear whether chimps were able to incorporate time into their planning.

There are a number of factors that would need to be taken into account in planning food timetables. Certain foods in tropical rainforests are prized over others because they are more nutritious or have a higher caloric value. Small fruits are fair game for a great number of species because smaller animals can eat them. Fruits that are unprotected and easy to eat, like figs, are also the object of intense competition. Because certain fruits are ripe for only a short period of time, their availability is ephemeral.

The anthropologists who tracked the chimps found that the group would wake up and leave their nests earlier if they were planning to breakfast on more ephemeral fruits. They would leave earlier if their breakfast consisted of small fruit and earliest of all for figs, a highly prized food. The further the figs were from their nests, the earlier they would leave. For bigger and less ephemeral fruits, the chimps did the opposite: they would leave the nests later if the food was farther away. The researchers suggest that this is because being the early ape for big fruits doesn’t have enough advantage to cause the chimps to brave a predator-ridden twilight journey.

Waking up earlier wasn’t the only behavior the chimps altered: they also changed where they built their nests. Nest building is a complex behavior because it depends not just on breakfast plans, but also on finding the ideal tree for a nest—some trees are prized because they have softer leaves or insect repellent properties. After supper, the chimps didn’t travel farther before building their nests in order to be closer to fig sites in the morning. But they did tend to head straight in the direction of the breakfast site when breakfast involved figs; bigger fruits saw them wandering off-path more in order to find a better nest site.

The complexity of the chimps’ behavior is difficult to explain without appealing to forward planning abilities, say the researchers. For less scarce fruits, we might assume that the stronger smell of a meal gets the chimps out of bed earlier. But this can’t explain why they would leave earlier for figs that are farther away. Visual cues, caloric intake, and simple preference are also unable to account for all the timing differences.

The best explanation is that the chimps weigh various pieces of information when deciding their actions. They think about risk of predators, the type of fruit they plan to eat, what time they want to arrive, and distance. All those factors are fed into planning their movements. Chimps have previously been found to plan ahead for tool use and social situations. This observation adds a third context to chimp planning abilities, indicating that chimp planning skills are general, rather than specific to certain domains.

The findings are also important, write the researchers, because they provide the “first clear example of a future-oriented cognitive ability used in a food-scarce period... that could enable large-brained foragers to buffer drastic declines in food supply.”

Large-brained foragers could use this kind of forward planning to reduce the effects of competition from other species and ensure a consistently high supply of energy for their big brains. This provides an important clue as to how flexible planning skills could have played a role in the evolution of the big brains of primates, including humans.

PNAS, 2014. DOI: 10.1073/pnas.1407524111 (About DOIs).