Prototype symbolization in hooded crows.

Studies were undertaken to determine whether four crows previously trained to an image-based abstract selection rule could establish a relationship between the number of elements in a group and initially indifferent symbols (arabic numerals) and operate with these symbols, i.e., studies addressed the ability of these birds to symbolize. Unlike other similar studies, there was no use of special development of associative connections between the symbols and the corresponding arrays of elements, but conditions were created in which birds could observe these relationships on the basis of comparison with previously obtained information. Demonstration series were performed for this purpose, in which correct solutions by the crow resulted in receipt of a number of larvae corresponding to the number of elements in the array pictured or numeral imaged on the stimulus selected by the crow. Images belonged to the same category as the corresponding stimulus for selection (the second stimulus was another category): if the image was an array, then the corresponding picture for selection also bore an array, and vice versa. Crows could solve the task successfully by using an image-based selection rule. In test series, the image and both selection stimuli were initially from different categories: if the image was a numeral, then both selection stimuli were arrays, and vice versa. All four crows successfully coped with this task. Despite the absence of any similarity between the image and "correct" stimulus, they selected the array corresponding to the numeral and vice versa. It is suggested that the birds could achieve this result by comparing the information obtained during the presentation series--about the number of reinforcement units corresponding to each stimulus. Similar experiments showed that crows could use numbers to perform operations analogous to arithmetic addition.