Variation in reptilian brains and cognition.

The class Reptilia is monophyletic, if all synapsid tetrapods are excluded and birds are included. The phylogenetic position of turtles within the reptilian clade is still problematic, but recent microRNA data suggest that turtles are the sister group to lepidosaurians. Brain-body data for approximately 60 reptilian taxa indicate that the relative brain size for a given body weight varies some six-fold among reptiles, with some turtles and lizards having relatively large brains and other turtles and lizards having relatively small brains. Snakes appear to be characterized by relatively small brains, and crocodilians appear to possess the largest brains among living reptiles, with the exception of birds. Data on the relative size of major brain divisions among tetrapods are limited, but the telencephalic and cerebellar hemispheres account for much of the variation. Telencephalic hemispheres in reptiles are approximately twice as large as those in amphibians, and the relative size of the telencephalic hemispheres in monitor lizards and crocodilians approaches that in basal birds and mammals. New data on the relative volumes of telencephalic pallial divisions in tetrapods reveal that the dorsal ventricular ridge, a ventral pallial derivative, accounts for much of the increase in pallial size that characterizes reptiles. Studies of spatial and visual cognition in nonavian reptiles reveal that they learn mazes and make visual discriminations as rapidly as most birds and mammals. Studies of social cognition and novel behavior, including play, reveal levels of complexity not previously believed to exist among nonavian reptiles. Given this level of neural and cognitive complexity, it is possible that consciousness has evolved numerous times, independently, among reptiles.