Three years ago, a stone-throwing chimpanzee named Santino jolted the research community by providing some of the strongest evidence yet that nonhumans could plan ahead. Santino, a resident of the Furuvik Zoo in Gävle, Sweden, calmly gathered stones in the mornings and put them into neat piles, apparently saving them to hurl at visitors  when the zoo opened as part of angry and aggressive "dominance displays."
But some researchers were skeptical that Santino really was planning for a future emotional outburst. Perhaps he was just repeating previously learned responses to the zoo visitors, via a cognitively simpler process called associative learning. And it is normal behavior for dominant male chimps to throw things at visitors, such as sticks, branches, rocks, and even feces. Now Santino is back in the scientific literature, the subject of new claims that he has begun to conceal the stones so he can get a closer aim at his targets—further evidence that he is thinking ahead like humans do.
The debate over Santino is part of a larger controversy over whether some humanlike animal behaviors might have simpler explanations . For example, Sara Shettleworth, a psychologist at the University of Toronto in Canada, argued in a widely cited 2010 article  entitled, "Clever animals and killjoy explanations in comparative psychology," that the zookeepers and researchers who observed Santino's stone-throwing over the course of a decade had not seen him gathering the stones, and thus could not know why he originally starting doing so. Santino, Shettleworth and some others argued, might have had some other reasons for caching the stones, and the stone throwing might have been an afterthought.
In the new study, published online today in PLoS ONE, primatologist Mathias Osvath of Lund University in Sweden—author of the earlier Santino paper—teams up with Lund University primatologist Elin Karvonen to report new observations of Santino's behavior during 2010. Santino's first attempts to throw stones during 2010 came during the May preseason. As a zoo guide led visitors toward Santino's island compound, the chimpanzee began to engage in a typical dominance display: screeching, standing on two feet, and carrying a stone in his hand. The guide and the visitors retreated before Santino began hurling the stones, and then advanced again for a total of three approaches. When the people returned about 3 hours later, Santino advanced toward them, holding two stones, but he did not act aggressively, even picking up an apple from the water surrounding the island and nonchalantly munching on it. But when Santino got within close range, he suddenly threw one of the stones. (It didn't hit anyone.)
The next day, Santino again threatened visitors with stones, but the group again backed away to avoid being hit. Santino was then observed pulling a heap of hay from inside his enclosure and placing it on the island close to where the visitors approached. He put several stones under the hay and waited until the group returned about an hour later. Then, without performing a dominance display, Santino pulled a stone from under the hay and threw it. Later, he pulled a stone that he had apparently hidden behind a log and tried to hit the visitors with that, as well.
Over the course of the summer, Osvath and Karvonen observed repeated episodes of this behavior, and also recovered stones that Santino had hidden under hay or logs, racking up 114 days of observation. They recovered a total of 35 projectiles that Santino had apparently concealed: 15 under hay heaps, 18 behind logs, and two behind a rock structure on the island. The researchers conclude that Santino deliberately engaged in deceptive concealment of the stones, and that this was a new, innovative behavior on his part : Before 2010, Santino had never put stones under hay piles or behind logs.
This innovation, the team argues, is further evidence that Santino plans ahead for how he will react to the visitors' approach to his compound, and that this is inconsistent with interpretations that he cached the stones for some other reason and then just happened to have them at hand when he got mad. By hiding the stones and then trying to deceive zoo visitors into thinking that his intentions were peaceful, Osvath and Karvonen argue, Santino was actually anticipating and planning for a future situation rather than simply responding repetitively to a past one.
And because the team was able to observe this new behavior from its very beginning, Osvath and Karvonen argue, the new study overcomes some of the objections to the earlier report. "No matter what mechanisms lie behind the behavior," Osvath says, Santino is engaging in planning for the future, and "that is not trivial."
But the "killjoys" are still not entirely convinced. Shettleworth calls the study "provocative," but insists that further experiments with more animals are needed before Santino's behavior can be interpreted as advance planning. "Did he bring the first hay pile into the arena with the intent of using it to hide projectiles? We cannot know," Shettleworth says. She says that the authors should have tried additional tests such as putting a hay pile in the compound themselves and "seeing if the animal still persisted in carrying hay," or "putting the hay piles in unfavorable locations for throwing."
Thomas Suddendorf, a cognitive psychologist at the University of Queensland in Brisbane, Australia, adds that while the observations suggest "extraordinary capacities" such as "planning and premeditated deception"—what he calls "rich" interpretations of Santino's behavior—"we cannot rule out leaner interpretations without experimental study."