But Alex had never been taught to use "none" to indicate an absence of a quantitythat idea he apparently came up with by himself.
"That Alex transferred the notion from other domains to quantity, without training or prompting by humans, was unexpected," Pepperberg said.
She believes that, after several weeks in which Alex had seemed disinterested in the experiments, the development was possibly an "attempt to make the procedure more challenging."
The findings indicate that Alex may understand the concept of zero, but far more rigorous testing would be required to prove it, Sally Boysen said. Boysen is an animal-cognition expert at Ohio State University's Chimpanzee Center in Columbus, Ohio.
Boysen's work in 1989 showed that chimpanzees could understand the idea of zero and use it when adding quantities of objects.
Boysen argued that Alex's use of "none" could just be a default response, which he uses when he's unable to identify the number of objects in front of him. He may not understand how to add or subtract zero as a quantity, she added.
Pepperberg agreed that more work is required to prove that African gray parrots can be taught to understand abstract numerical notions in the same way as people.
"[Alex's 'none'] isn't the same concept of zero that you and I have," Pepperberg said. "That's why I call it a 'zero-like' concept. We are working to refine our knowledge of what this concept actually is."
In the wild, African gray parrots would require at least a basic understanding of more-versus-less when foraging in their jungle habitat, Pepperberg said. Counting might also be useful when deciphering vocalizations or tracking the number of parrots in a flock.
"That zero was represented in some way by a parrot, with a walnut-sized brain is striking," Pepperberg said. "Evolutionarily the connection [of birds] to humans is ancient yet their brains seem to have homologues [structural similarities] to ours."
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