We knew that primates can learn short time intervals, but there was still very little evidence that they could keep track of intervals of several days,' says Janmaat. During the research, the team introduced two food sources, carrots and grapes, to the group of mandrills at ARTIS. The carrots and grapes were hidden every two and five days, respectively, at fixed locations in the animals’ outdoor enclosure. The researchers then tracked the location choices of each mandrill, every day for a period of 113 days. In other words, whether a carrot or a grape location was chosen. The researchers found that the mandrills had learned the carrots’ two-day interval after about 30 days. They failed to learn the grapes’ five-day interval, appearing to need more time to learn this.

According to the authors, the study provides unique scientific evidence that primates can keep track of how many days have passed since specific events in the past. ‘The fact that we are only now finding this evidence is surprising,' says Janmaat, who is affiliated with the department of Cognitive Psychology (Leiden University) and the Institute for Biodiversity and Ecosystem Dynamics (University of Amsterdam). ‘Because in nature fruit can take from days to weeks to ripen, you would expect it to be advantageous to be able to keep track of how many days have passed. Maybe we wee the first who found this evidence because no one had observed the animals for such a long time before.'

Importance of timing
It was previously assumed that only humans possessed the cognitive mechanisms that enable individuals to learn time intervals of several days. But more and more scientific evidence is now emerging that other species can do the same. 'Until now, research into memorising time intervals has mostly involved animals that hide food for later,' says Kavel Ozturk, who conducted the ARTIS research as part of his Master's degree in Biological Sciences at the UvA. ‘We developed this new study method involving finding and collecting food specifically for primates. This enables researchers to further investigate the evolutionary origins of this capacity in primates.'

'Knowledge of elapsed time could help wild primates decide whether or not to return to a tree that previously contained unripe fruit. Correct timing allows the animals to get ahead of the competition and be the first to arrive. Since ripe fruit contains a lot of energy, this capacity enables the animals to gather more energy than other individuals and thus maintain a larger brain,' says Ozturk. ‘That could explain why primates have relatively larger brains compared to other orders in the animal kingdom.'

Natural behaviour in ARTIS
In order to obtain these unique scientific insights, Ozturk simulated a natural foraging situation that Janmaat had observed in sooty mangabey monkeys. That species is a close relative of the mandrill, which lives in similar rainforests. The mangabeys pull young seedlings out of the ground to get at the underlying seeds. Instead of seedlings, the researchers used Dutch willow branches of different lengths to help the mandrills learn the different food locations.

‘It was great to see that the mandrills use their teeth to pull the branches out of the ground and to see them actively dig for carrots. I saw the same behaviour with closely related monkeys in the rainforest,' says Janmaat.

Publication details
Kavel Ozturk, Martijn Egas, Karline Janmaat: ‘Mandrills learn two-day time intervals in a naturalistic foraging situation’, in Animal Cognition (2020). https://link.springer.com/article/10.1007/s10071-020-01451-7