Sexually selected size differences and conserved sexual monomorphism of genital cortex.

The mammalian somatosensory cortex shows marked species-specific differences. How evolution in general and sexual selection in particular shape the somatosensory cortical body representation has not been delineated, however. Here we address this issue by a comparative analysis of genital cortex. Genitals are unique body parts in that they show sexual dimorphism, major changes in puberty and typically more pronounced species differences than other body parts (Hosken & Stockley, 2004). To study the evolution of genital cortex we flattened cortical hemispheres and assembled 104 complete body maps, revealed by cytochrome-oxidase activity in layer 4 of 8 rodent and 1 lagomorph species. In two species, we also performed antibody stainings against vesicular glutamate transporter-2, which suggested that cytochrome-oxidase maps closely mirror thalamic innervation. We consistently observed a protrusion between hindlimb and forelimb representation, which in rats (Lenschow et al., 2016) corresponds to the penis representation in males and the clitoris representation in females. Consistent with the idea that this protrusion corresponds to genital cortex, we observed a size increase of this protrusion during puberty. Species differed in external genital sexual dimorphism, but we observed a sexual monomorphism of the putative genital protrusion in all species, similar to previous observations in rats. The relative size of the putative genital protrusion varied more than 3-fold between species ranging from 0.5% of somatosensory cortex area in chipmunks to 1.7% in rats. This relative size of the genital protrusion co-varied with relative testicle size, an indicator of sperm competition and sexual selection.