“The main criticism we received before publication was, ‘Well, this activity with the pebbles may have been just accidental — you saw him when coincidentally he had a pebble in his mouth,’” said Amalia P.M. Bastos, an animal cognition researcher at the University of Auckland and the study’s lead author. “But no. This was repeated many times. He drops the pebble, he goes and picks it up. He wants that pebble. If he’s not preening, he doesn’t pick up a pebble for anything else.”
Dorothy M. Fragaszy, an emerita professor of psychology at the University of Georgia who has published widely on animal behavior but was unacquainted with Bruce’s exploits, praised the study as a model of how to study tool use in animals.
“The careful analyses of the behavior in this report allow strong conclusions that the behavior is flexible, deliberate and an independent discovery by this individual,” she said.
The researchers set themselves careful rules.
First, they established that Bruce was not randomly playing with pebbles: When he picked up a pebble, he used it for preening nine times out of 10. When he dropped a pebble, 95 percent of the time he either retrieved it or picked up another one and then continued preening. He consistently picked up pebbles of the same size, rather than sampling pebbles at random.