Delphinid brain development from neonate to adulthood with comparisons to other cetaceans and artiodactyls

Abstract

Why do neonatal and adult delphinids have much larger brains than artiodactyls when they have common ancestors? We explore relationships between neonatal brain size, gestation duration, maternal body mass, and body growth. Cetacean brains grow fast in the womb and longer gestation results in a larger brain. Allometry shows that the larger the mother's body mass, the larger the neonatal brain. After birth, delphinid bodies grow much faster than brains, and the index of encephalization decreases even as brains grow beyond maturity. Delphinids’ larger brain growth during life at sea may be explained by at least three differences from artiodactyls’ life on land. First, the sea offers high calorie prey to support growth of a large brain. Second, life in water offers relief from gravity, allowing for a large head to contain a large brain. Third, sound in water may pass through an immersed body. This allows sounds from the water to reach the fetus, driving early development of delphinoid auditory brain parts. As an example of this, the dolphin ear bone is very large at birth. Furthermore, the auditory nervous system appears mature well before birth. Compared with artiodactyls, these three differences likely result in a larger delphinid brain.