Habitat-related variations in brain and body size of pocket gophers.

Cranial capacities were measured in two sets of genetically-related pocket gophers, one from desert shrubland, the other from an irrigated alfalfa field. The two sets differed in sex ratio and in age structure, but both showed the strong sexual dimorphism characteristic of pocket gophers. After sorting by trapping site, sex and suture score (age), the covariations between brain weight, body length and cranial basilar length were calculated. The brains of the alfalfa field animals followed the same somatic scaling relations as those of the desert shrubland animals, all the differences in brain weight being explained fully by the sexual dimorphism in the somatic variables. The slopes defined by the means of brain and body weights of females and of males from the two sites were nearly equal, and were the same as the trajectories calculated from a geometric model based on the statistical characteristics of the samples. It is argued that these slopes define how much brain size must increase to serve a body of increased size, with no change in life style. The differences in age structure between males from the two sites provided evidence of a mechanism by which sexual dimorphism might develop and be enhanced in a nutritionally-enriched environment.