Interspecific variation in sexual dimorphism in brain size in Nearctic ground squirrels (Spermophilus spp.)

A possible relationship between sexual dimorphism in brain size and mating system was investigated in five ground squirrel species: Spermophilus later alis, S. tridecemlineatus, S. richardsonii, S. columbianus, and S. parryii. Relative brain size was measured by determining the endocranial volume of 247 ground squirrel skulls and regressing these values against two measurem ents of body size: mass and length. Analyses of covariation in the brain si ze / body size relationship within the five species revealed that sexual br ain-size dimorphism occurs in three of the five species: S. lateralis, S. r ichardsonii, and S. tridecemlineatus. Application of a reduced major axis r egression model indicated, however, that only S. lateralis and S. richardso nii exhibit significant sexual brain-size dimorphism. These findings sugges t that the degree of sexual brain-size dimorphism is not directly correlate d with variation in mating systems. Spatial abilities may play a role in th e evolution of sexual brain-size dimorphism in ground squirrels, but the sp atial requirements of mating systems appear to be insufficient. The possibi lity of sex differences in cognition, resource competition, and other varia bles as contributory factors to the evolution of sexual brain-size dimorphi sm is offered, but detailed knowledge of sex differences in the behaviour o f ground squirrels is required to provide a definitive answer.