“The main criticism we received prior to publication was, ‘Well, that activity with the pebbles might just have been a fluke — you saw him having a pebble in his mouth by chance,'” said Amalia PM Bastos, an animal cognition researcher at the University of Auckland and lead author of the study. “But no. This has been repeated many times. He drops the pebble, he goes and picks it up. He wants that pebble. Unless he’s cleaning himself, he’s not picking up a rock for anything else.”
Dorothy M. Fragaszy, a psychology professor emeritus at the University of Georgia who has published extensively on animal behavior but was unfamiliar with Bruce’s exploits, hailed the study as a model for studying tool use in animals.
“The careful analyzes of the behavior in this report provide strong inferences that the behavior is flexible, intentional, and an independent discovery by that individual,” she said.
The researchers set up careful rules.
First, they discovered that Bruce wasn’t playing with pebbles by accident: when he picked up a pebble, nine times out of ten he used it to clean. If he dropped a pebble, 95 percent of the time he would either retrieve it or pick up another one on top of it and then continue cleaning. He consistently picked up pebbles of the same size rather than randomly sampling pebbles.