Captive Chimps Learn A Digging Behavior Once Thought Unique To Humans And Other Hominins

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Chimpanzees have been caught getting boozy on wine and initiating crop raids on nearby farms. More recently, a particularly nefarious group of chimps was observed harassing a leopard so that they could swoop in and steal its kill.

If we need even more evidence of the ingenuity of chimps, recent research published in PLOS ONE shows that a group of captive individuals were able to figure out how to utilize tools to “excavate” food buried in the ground. This is an interesting development because up until quite recently, it was believed to be a skill reserved for humans and our (now-extinct) hominin relatives. Thus, it was believed that this practice separated us (hominins) from them (other primates).

We now know this isn't true. 

Recent research has shown that chimps are able to use spears to hunt and rods to fish. We also know that wild chimps (and bearded capuchins) have learned to use tools to unearth tasty treats like plant roots and tubers. So, to try to understand how these behaviors developed, researchers performed two experiments examining tool use in captive chimps. 

Not one of the 10 chimps involved in the study had been seen using tools to dig up food beforehand. Eight of the 10 had been born in captivity.

In the first experiment, the researchers put fruit in five small holes dug in the chimp enclosure at Kristiansand Zoo, Norway. Initially, the holes were left open so that the chimps could see the fruit. Later, they were closed. The researchers also left "ready-made" tools (aka tree sticks and bark shards) in the enclosure for the chimps to use if they felt like it.

The second experiment was near on identical but without the addition of ready-made tools.

The team observed six "tool use behaviors" over the course of the experiments, digging being the most frequent followed by probing, pounding, perforating, shoveling, and enlarging. What's more, nine of the 10 chimps managed to excavate the buried fruit at least once, eight of whom used tools to do so. (Digging by hand, however, was still the preferred method.) When tools were not readily provided (experiment two), many chimps went out and sourced their own from natural vegetation in the enclosure.

The study authors also note that chimps were spotted reusing certain tools and would often pick longer tools over shorter ones.

The six tool uses spotted in chimps: A) Probe, B) Perforate, C) Pound, D) Dig, E) Shovel, and F) Enlarge. Alba Motes-Rodrigo et al. (2019), PLOS ONE

The researchers say that while there are problems with comparing an experiment in a captive setting to the real-life development of foraging behaviors in the wild, they believe that our hominin ancestors would have learned to excavate with tools in a similar way to the chimps in the study.

Endearingly, in addition to the above behaviors, the researchers also reported chimps taking turns to dig holes and sharing successfully excavated fruit with others in the group.

Not to spoil their Machiavellian reputation too quickly, one was also caught stealing fruit from a fellow chimp – and running off with its prize.

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