Mandrills keep track of how many days have passed to be the first to gather the food. This is shown by a team of researchers from the University of Amsterdam, Leiden University and ARTIS Amsterdam Royal Zoo. The team discovered that mandrills have the cognitive skills to learn time intervals of several days. The results of the research led by ARTIS professor and UvA researcher Karline Janmaat have now been published in the scientific journal Animal Cognition.
‘We know that primates can learn short time intervals, but there was still very little evidence that they could keep track of time intervals of several days,’ says Karline Janmaat. During the research, the team introduced two food sources, carrots and grapes, to the group of mandrills at ARTIS. The food was hidden every two and five days respectively, at fixed locations in their outdoor enclosure. The researchers then tracked the choice 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 two-day time interval of carrots after about 30 days. They failed to learn the five-day interval of the grapes and seemed to need more time to learn this.
According to the authors of the study, there is now 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 Professor Dr. Karline Janmaat, 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 days to weeks to ripen, you expect it to be advantageous to be able to keep track of how many days have passed. Maybe we are the first who found this evidence, because no one observed the animals for such a long time before’.
Previously it was assumed that only humans possess the cognitive mechanisms that enable individuals to learn time intervals of several days. More and more scientific evidence is emerging that other species can do the same. ‘Until now, research into memorizing time intervals is mostly done on animals that hide food for later,’ says Kavel Ozturk, who conducted this research as part of his master’s degree in Biological Sciences at the UvA. ‘Specifically for primates we have developed a new study method aimed at finding and collecting food. 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 Kavel 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, Kavel Ozturk simulated a natural foraging situation that Karline Janmaat had observed in sooty mangabey monkeys. This is a close relative of the mandrill that lives in similar rainforests. These monkeys pull young seedlings out of the ground to dig for the underlying seeds. Instead of young 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 Karline Janmaat.
Kavel Ozturk, Martijn Egas, Karline Janmaat: Mandrills learn two-day time intervals in a naturalistic foraging situation, in Animal Cognition (2020).
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