For the first time, chimpanzees have been seen using tools to chop up and reduce food into smaller bite-sized portions.
Chimps in the Nimba Mountains of Guinea, Africa, use both stone and wooden cleavers, as well as stone anvils, to process Treculia fruits.
The apes are not simply cracking into the Treculia to get to otherwise unobtainable food, say researchers.
Instead, they are actively chopping up the food into more manageable portions.
Observations of the behaviour are published in the journal Primates.
PhD student Kathelijne Koops and Professor William McGrew of the Leverhulme Centre for Human Evolutionary Studies, University of Cambridge, UK, studied a group of chimps living wild in the Nimba Mountains.
Ms Koops research is focused on the use by the chimps of elementary technology, such as the use of tools while foraging.
"Chimpanzees across Africa vary greatly in the types of tools they use to obtain food. Some groups use stones as hammers and anvils to crack open nuts, whereas others use twigs to fish for termites," she says.
The apes' use of such tools can be surprisingly sophisticated.
"For example, nut-cracking in the Bossou chimpanzee community in Guinea involves the use of a movable hammer and anvil, and sometimes the additional use of stabilising wedges to make the anvil more level and so more efficient," explains Ms Koops.
"Termite fishing in some chimpanzee communities in the Republic of Congo involves the use of a tool set, i.e. different tool components used sequentially to achieve the same goal.
"These chimpanzees were found to deliberately modify termite fishing probes by creating a brush-end, before using them to fish for termites."
But together with Prof McGrew and Prof Tetsuro Matsuzawa of the Primate Research Institute at Kyoto University in Japan, Ms Koops has discovered another startling use of tools not previously recorded.
During a monthly survey of chimps (Pan troglodytes) living in the mountain forests, she came across stone and rocks that had clearly be used by the apes to process Treculia fruits.
These fruits, which can be the size of a volleyball and weigh up to 8.5kg, are hard and fibrous.
But despite lacking a hard outer shell, they are too big for a chimpanzee to get its jaws around and bite into.
So, instead, the chimps use a range of tools to chop them into smaller pieces.
Ms Koops found stone and wooden cleavers, as well as stone anvils used
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