A chimp throwing stones at zoo visitors

A chimp that deliberately fashions discs of concrete to later hurl at zoo visitors is being hailed as definitive proof that the apes plan for future events.

Although similar claims have previously been made about chimps using tools to collect food, what sets Santino – a 30-year-old chimp from Furuvik zoo in Sweden – apart, is that his behaviour, and therefore his apparent state of mind, when collecting the ammunition seems markedly different from when he launches his attacks.

"The chimp has without exception been calm during gathering or manufacture of the ammunition, in contrast to the typically aroused state [when he throws the rocks]," says Mathias Osvath of the University of Lund, also in Sweden.

Unlike previous claims of pre-planning in apes, Santino's planning doesn't seem to be driven by a current emotional or physical drive like hunger or anger, but in anticipation of an event later in the day.

"Nothing like it has as yet been reported from the wild, nor from any captive chimpanzees," says Thomas Suddendorf of the University of Queensland in Australia. "Controlled experiments are now required to determine the nature of the cognitive processes involved."

Ape arsenal

Josep Call of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, agrees. "It is the first report on tool making - that is, the concrete disks – to achieve a future goal," he says. "Future planning may turn out to be more widespread than initially thought."

Santino originally started collecting and throwing rocks shortly after becoming the dominant male in his group at the age of 16. He typically collects rocks from the bottom of the moat that surrounds his enclosure before the zoo opens, and stores them in piles on the side of the island that faces the zoo's visitors. He also hacks pieces of concrete from the artificial rocks at the centre of his enclosure and adds them to the piles.

However, Santino's rock throwing is confined the summer period, when the zoo is open to visitors, and the desire to throw his discs seems to wear off after about six weeks. The chimp is also more inclined to hurl his missiles in the morning, rather than in the afternoon.

'Mental simulations'

"These observations convincingly show that our fellow apes do consider the future in a very complex way," says Osvath. "It implies they have a highly developed consciousness, including life-like mental simulations of days to come. I would guess that they plan much of their everyday behaviour."

However, Nicholas Newton-Fisher of the University of Kent in Canterbury, UK, cautions that with observations of a single individual it is difficult to generalise.

"A sceptical reader might question whether there is a causal link between the caching and the throwing. The location of the caches may simply be a function of retrieving them from the water." He adds that Japanese macaques are also known to cache stones – although they don't generally throw them at passers-by.

Osvath says that zoo caretakers try to stop Santino's attacks. "Sometimes they will keep him in during the morning, and only let him out once the visitors have arrived. It's very hard to stop him because he can always find new stones, and if he can't find them he manufactures them. It's an ongoing cold war." He adds that since chimps don't have a good aim, and throw underarm, there haven't been any serious injuries.

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