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Eisenberg and Wilson (Evolution, 32:740-751, 1978) have documented an interesting relationship between the relative brain size of bats and the complexity of the habitat in which they forage. They found that bats that fly and forage through foliage have larger brains relative to their body size than those that forage in open air. Their explanation was that bats in the complex habitat must process more complex sonic information to navigate through the foliage. In order to do this a larger brain is required.
The Peromyscus of North America may offer a similar paradigm as far as habitat complexity is concerned. Species of this genus can be found associated with a variety of habitats: forest, brushlands, rock slides, and grasslands. It seems reasonable that these structurally different habitats may require different climbing abilities by the Peromyscus inhabitants. If functioning in a structurally complex habitat requires a relatively larger brain to process information than is required in a structurally simple habitat, then, following the logic of Eisenberg and Wilson (1978), those species that are good climbers will have relatively larger brains than species that are poor climbers. If this hypothesis is correct, such a pattern of morphological variation in brain size may be expected intraspecifically in P. maniculatus and P. leucopus, two species that occur in a wide varietv of habitats throughout their range.