Conservation of brain connectivity and wiring across the mammalian class

Over 100 years ago, Ramon y Cajal hypothesized that two forces played a role in the evolution of mammalian brain connectivity: minimizing wiring costs and maximizing conductivity speed. Using diffusion MRI, we reconstructed the brain connectomes of 123 mammalian species. Network analysis revealed that both connectivity and the wiring cost are conserved across mammals. We describe a conservation principle that maintains the overall connectivity: species with fewer interhemispheric connections exhibit better intrahemispheric connectivity. Brain connectivity and normalized wiring costs are conserved across mammals independently of brain size according to a common principle: species with fewer interhemispheric connections exhibit better intrahemispheric connectivity.