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Inside Dr. Pepperberg’s Lab: Alex’s Numerical Abilities — Part II

African grey parrot perched near table of objects
Figure 1. Alex being shown a tray of objects for a comprehension task; the question would be, e.g., “What color 5?”

So, if you’ve read our June article, you know that Alex understood how to vocally produce the appropriate number label when questioned “How many X?” about a particular set of objects — even when they were a subset of a huge collection. His use of symbols definitely gave him an advantage over other nonhumans who lacked such symbols. But true understanding of number — and of counting — is a much more complex behavior, as we shall see!

Specifically, although it might seem from the previous studies that Alex truly understood the meaning of his symbolic number labels, we couldn’t be sure that such was the case. Previous studies with apes and their use of symbolic labels demonstrated that a separation could exist between production and comprehension (see Savage-Rumbaugh et al., 1980).

Number Comprehension In Children

Correct production of number labels for a given set certainly did not ensure comprehension for children: Researchers working with young children (e.g., Wynn, 1990) had initially thought that production and comprehension of number labels were acquired simultaneously — until they tested children who succeeded on number production on a task that involved comprehension instead of production. Here children were given a big bowl of items (like marbles) and told, “Give me X,” where X was a number from 1 to 6. Many children failed here — for numbers greater than 1 or 2, they would just grab a handful and give that to the experimenter! So, we had to test Alex on the same type of task.

“Simple Task” Not So Simple

Because Alex couldn’t pick up sets of objects the same way that children could, we gave him a variation of the production task involving collections of subsets. Here he saw a collection of three separate sets of items, and we asked about the color or material of a specific set (see Fig. 1. Note that usually the objects were of different sizes so that he couldn’t use mass or contour to make his discrimination). We would show him three sets of the same object in different colors (or three sets of different objects in the same color) and, for example, ask “What color/object (is the set of) six?”

This task may seem simple, but it isn’t: The procedure required that he comprehend the auditorially presented symbolic numeral label (e.g., “6”) and use its meaning to direct a search for the exact amount specified by that label (e.g., six things). This tested whether he knew exactly what a set of “X” individual items was, even when intermixed with other items representing different numerical sets. To be correct, he couldn’t make approximations. For example, he couldn’t label a set of five objects “six” or “four.”

Unlike the young children, Alex succeeded: He was correct 88% of the time over 66 trials, and made no errors on his first 10 trials. His performance suggested that he was not learning the task, and he immediately understood what to do. His later errors may have occurred because he was bored and had stopped fully attending to the task.

Remember, the objects were his first reward, and these were old toys that probably were no longer particularly interesting. Too, most of his errors were not number-related, but involved colors that he might visually confuse (e.g., orange and red, because parrots see in the UV), or labels that sounded alike (e.g., “wool” and “wood”). So, we knew that he really did understand the meaning of his number labels. Note, however, that we had to answer many more questions before we could argue that he could actually count. I’ll explain this in a future article.

An Unexpected Finding

This comprehension study was even more interesting than we had planned. It led to an unexpected, additional finding. The behavior occurred on the 10th trial within the first dozen. Alex was asked “What color 3?” to a set of two, three, and six objects. He replied “five.” The questioner asked twice more; each time he replied “five.” The questioner didn’t quite know what to make of Alex’s response, as there was clearly no set of five items on the tray. She finally said “OK, Alex, tell me, what color 5?” Alex immediately responded “none.”

He had learned to state “none” if no category (color, shape, or material) was same or different when queried about similarity or difference for two objects (Pepperberg, 1988). He spontaneously transferred this response to “What color bigger?” for two objects of identical size in a study of relative size (Pepperberg & Brezinsky 1991). Alex had never been taught the concept of absence of quantity nor to respond to absence of an exemplar — only to absence of a feature. Note that he not only provided the correct response, but also set up the situation himself! Had he, on his own, devised a zero-like concept? And had he figured out how to manipulate his trainer so that he could be asked a question that he wanted to answer?

Testing Alex For Zero-Like Concept

We tested the first possibility by randomly inserting trials with respect to the absence of each possible number to ensure that this situation was not an odd happenstance. He was correct on 5/6 such trials, and his one error was to give a color that was not on the tray (sort of correct, in that the color was absent, but…). Clearly, his first response was not mere chance. He understood something about the concept.

We really couldn’t figure out how to test the second possibility. Were it true, it would imply some level of advanced consciousness — and that is something researchers don’t entirely understand even with respect to humans!

Nevertheless, Alex’s use of “none” for the purpose of a zero-like concept was unexpected and impressive for at least four reasons (Pepperberg & Gordon, 2005).

First, labeling what is called a “null” set (one with nothing in it), whether by “zero” or “none”, is a fairly recent human development (i.e., around 1500s; Bialystok & Codd, 2000). That Alex, with a walnut-sized brain whose ancestral evolutionary history with humans likely dates from the dinosaurs, represented zero, even if not in a manner identical to that of humans, is striking.

Second, the notion of none is abstract and relies on the violation of expectation of presence. You can’t acknowledge that something is absent unless you expect it to exist — and that again demonstrates quite an advanced level of cognition. Too, even though Alex already associated “none” with absence of similarity and difference and lack of a size difference, the ability to transfer the notion across domains to quantity, without training or prompting, is quite amazing. No other nonhuman had done so spontaneously.

Third, if parrots represent quantity as do children, then his use of zero/none should not yet have appeared (Wellman & Miller, 1986). Children may have a none/nothing concept before learning that this quantity has a special label, “zero”, but do not use existing labels to express it. Unlike the apes in earlier studies (see Biro & Matsuzawa, 2001), Alex was not taught “zero” but, rather, deliberately — on his own — used “none” in a number comprehension task.

Finally, and likely most importantly, he initiated the topic. He repeatedly stated “five” when asked about “three.” When asked about the nonexistent “five,” he responded appropriately. But how did he figure this out? To this day, we still do not know.

Thus, as usual, Alex’s abilities raised more questions than they did answers. Nevertheless, even if we couldn’t figure out how he made the cognitive leap, we could learn more about his understanding of the concept. Specifically, how closely did his notion of “none” match children’s and animals’ understanding of zero? Stand by for the next installment!


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Biro, D., & Matsuzawa, T. (2001). Use of numerical symbols by the chimpanzee (Pan troglodytes): Cardinals, ordinals, and the introduction of zero. Animal Cognition, 4, 193–199.
Pepperberg, I.M. (1988). Comprehension of ‘absence’ by an African Grey parrot: learning with respect to questions of same/different. Journal of the Experimental Analysis of Behavior, 50, 553–564.
Pepperberg, I.M., & Brezinsky, M.V. (1991). Acquisition of a relative class concept by an African Grey Parrot (Psittacus erithacus): Discriminations based on relative size. Journal of Comparative Psychology, 286–294.
Pepperberg, I.M., & Gordon, J.D. (2005). Number comprehension by a Grey parrot (Psittacus erithacus), including a zero-like concept. Journal of Comparative Psychology, 119, 197–209.
Savage-Rumbaugh, E.S., Rumbaugh, D.M., & Boysen, S. (1980). Do apes use language? American Scientist, 68, 49–61.
Wellman, H.M., & Miller, K.F. (1986). Thinking about nothing: development of concepts of zero. British Journal of Developmental Psychology, 4, 31–42.
Wynn, K. (1990). Children’s understanding of counting. Cognition, 36, 155–193.


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