The mentality of crows: Convergent evolution of intelligence in corvids and apes

Discussions of the evolution of intelligence have focused on monkeys and apes because of their close evolutionary relationship to humans. Other large-brained social animals, such as corvids, also understand their physical and social worlds. Here we review recent studies of tool manufacture, mental time travel, and social cognition in corvids, and suggest that complex cognition depends on a ‘‘tool kit’ ’ consisting of causal reasoning, flexibility, imagination, and prospection. Because corvids and apes share these cognitive tools, we argue that complex cognitive abilities evolved multiple times in distantly related species with vastly different brain structures in order to solve similar socioecological problems. I n Aesop_s fable, a thirsty crow spied a pitcher containing a small amount of water. Unfortunately, the water was out of reach of the crow_s bill, but next to the pitcher was a pile of stones. The crow began placing the stones in the pitcher, thereby raising the water until it could drink. Did the crow understand that its actions would increase the water level? Throughout folklore, the corvids (crows, jays, ravens, and jackdaws) (Fig. 1) have been credited with intelligence. Recent experiments investigating the cognitive abil-ities of corvids have begun to reveal that this reputation has a factual basis. These studies have found that some corvids are not only superior in intelligence to birds of other avian species (perhaps with the exception of some parrots), but also rival many nonhuman primates. Traditionally, studies of complex cognition have focused on monkeys and apes (1). However, there is no reason to assume that complex cognition is restricted only to the primates (2). Indeed, the social intelli-gence hypothesis (3) states that intelligence evolved not to solve physical problems, but to process and use social information, such as who is allied with whom and who is related to whom, and to use this information for deception (4). There is evidence that some other large-brained social animals, such as cetaceans, demonstrate similar levels of in-telligence as primates (5). Corvids also appear to meet many of the criteria for the use of social knowledge in their interactions with conspecifics (6)