Chimpanzees use manipulative dexterity to evaluate and select figs, similar to the way humans shop for fruits and vegetables, researchers say.
Figs are a vital resource for chimps when preferred foods are scarce. The study demonstrates the foraging advantages of opposable fingers and careful manual prehension, or the act of grasping an object with precision.
The findings shed new light on the ecological origins of hands with fine motor control, a trait that enabled our early human ancestors to manufacture and use stone tools.
"The supreme dexterity of the human hand is unsurpassed among mammals, a fact that is often linked to early tool use," said Nathaniel J Dominy from Dartmouth College in the US.
For the study, researchers observed the foraging behaviours of chimpanzees, black-and-white colobus monkeys, red colobus monkeys and red-tailed monkeys in Uganda.
The primates depended on figs, and although ripe figs come in a range of colours, many stay green throughout development and every phase can be present on a single tree, making it difficult to discern solely by colour, which figs are ripe.
To determine if the green figs of Ficus sansibarica are edible, chimpanzees ascend trees and make a series of sensory assessments — they may look at the fig's colour, smell the fig, manually palpate or touch each fig (using the volar pad of the thumb and lateral side of the index finger) to assess the fruit's elasticity and/or bite the fig to determine the stiffness of the fruit.
Colobus monkeys do not have thumbs and evaluate the ripeness of figs by using their front teeth.
Researchers examined the spectral, chemical and mechanical properties of figs, which included boring into individual figs to assess the elasticity of the fruit and extracting fig contents to estimate nutritional rewards such as sugar.
They observed the non-selection, rejection and ingestion of individual figs, and collected specimens of figs that were avoided; palpated and rejected; palpated, bitten and rejected; and edible for which less than 50 per cent of the fruit was left. Chimpanzees also use their sense of smell to assess individual figs.
Based on the sensory data obtained, researchers estimated the predictive power that sensory information may have on chimpanzees when estimating the ripeness of figs.
Palpating figs was about four times faster than detaching and then biting the fruit, suggesting that chimpanzees may have a substantial foraging advantage over birds and monkeys, which rely on visual and oral information.
The study provides new insight into how chimpanzees exhibit advanced visuomotor control during the foraging process and more broadly, on the evolution of skilled forelimb movements.
The findings were published in the journal Interface Focus. — PTI
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