Estimation of quantity and/or related phenomena (e.g., estimation of time) have been demonstrated in a variety of species. Numerous hypotheses have been proposed to account for a range of behaviors observed in rats, raccoons, various species of birds, rhesus monkeys, and chimpanzees relative to the emergence of counting skills in children. An understanding of the cognitive processes that underlie numerical skills in nonhuman animals could provide information critical to clarifying the evolution of cognitive capabilities.

Capaldi and Miller (1988) suggested that despite opportunities to employ other strategies for task solution, animals readily use counting cues to solve a variety of instrumental problems. Capaldi and his colleagues further proposed that counting mechanisms may explain a wide range of learning situations in rats and other mammals. In contrast, Davis and Memmott (1982) proposed that animals engage in counting as a last-resort strategy and questioned the evolutionary significance of such capacities, given that these behaviors often require intensive training in a highly structured laboratory situation (e.g., Davis, 1984; Ferster, 1964).

The degree to which counting behaviors in nonhuman species fit the criteria for true counting, as defined for children (Gelman & Gallistel, 1978), is a point of controversy (Davis & Perusse, 1988). Interestingly, discussion of related issues pervades the human developmental literature (e.g., Gelman & Gallistel, 1978; Piaget, 1952), including the age at which children are capable of demonstrating counting, as opposed to subitizing (Beckmann, 1924; Mandler & Shebo, 1981), that is, the proposed ability to estimate small numbers of items in an array through a direct perceptual apprehension mechanism (von Glasersfeld, 1982).