Brain size and the phylogenetic scale

People often characterize animals of different species as more or less intelligent, and television programs delight in questions such as "What is the most intelligent species of animal?" Scientists have looked to brain size as a way to predict intelligence across species, but absolute brain size does not work, as body sizes vary so much across animal species. The encephalization quotient (EQ) was developed as a measure of relative brain size. The EQ is a calculation based on the size of a species' brain compared to the expected size based on body size. An EQ of 1.0 indicates that the brain size is the size expected for the species body size whereas an EQ over 1.0 indicates a brain that is larger than would be expected and an EQ less than 1.0 indicates a smaller brain than expected. Humans have the largest EQ (7.0), with monkeys and apes (1.5-3.0) and dolphins (up to 4.5) also high on the EQ scale of living species. Although the EQ (as well as other measures of brain complexity) is consistent with expectations of relative cognitive complexity, it indicates nothing about the types of cognitive abilities available to animals of different species.

Historically, the study of animal cognition began with attempts to arrange species along a phylogenetic scale in order of intelligence. This approach was guided by Aristotle's proposal that organisms can be arranged along a "ladder of life" with humans at the top and other animals at different rungs, or levels, down this ladder. This concept, the scala naturae, is no longer accepted in comparative studies of animal behavior or intelligence. Rather, evolution is perceived more as a tree-like structure with individual species or groups of species branching off as they evolve adaptations that distinguish them from ancestral forms. From this perspective, animals are studied in the context of their ecological niche, the specific environment in which the organism evolved, and comparisons are made across species that share evolutionary ancestors. Each species of animal has evolved sensory capacities and a behavioral repertoire that allow success in that species' particular ecological niche. What is "intelligent" behavior for one ani

Barbary macaque (Macaca sylvanus) adults grooming, with baby. (Photo by Animals Animals ©J. & P. Wegner. Reproduced by permission.)

mal might not be so for an animal living in a different habitat. In this sense of intelligence, each species has its own type of intelligence. Investigators interested in animal intelligence have turned from global measures of intelligence and have focused research on various cognitive capacities such as basic processes like learning and memory, complex concepts like number, cognitive flexibility shown by tool use and construction, social cognition, and symbolic processing. The focus here will be on those cognitive processes.

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