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.

Evolution and development of the cetacean skull roof: a case study in novelty and homology

“…Alternatively, the rate of brain growth during ossification of the interparietal, rather than the ultimate size of the brain, might be more important. Although delphinids show a positive relationship between gestation duration and neonatal brain size, Pontoporia has one of the shortest gestation times within Cetacea [65], and it is unclear if any of the increased brain size in delphinoids can be attributed to a faster growth rate.…”

Ask an Acoustician: Sam H. Ridgway

“…I also look at brain structure to compare different cetacean species. I want to know how the auditory system scales in different-size brains (Ridgway et al, 2018). I also want to know how sound production is implemented by the two hemispheres of the brain.…”

Co‐evolution of cerebral and cerebellar expansion in cetaceans

“…Cetacean brains are thought to have numerous features that deviate from general mammalian trends, including a thin and highly laminated cortex, extreme gyrification, low neuron density but high synaptic density, unique neuronal cell types, and small hippocampi that lack adult neurogenesis (Breathnach, 1960;Butti et al, 2015;Eriksen & Pakkenberg, 2007;Haug, 1987;Huggenberger, 2008;Marino, 2002Marino, , 2007Morgane, Glezer, & Jacobs, 1990;Oelschläger & Oelschläger, 2009;Patzke et al, 2013;Poth, Fung, Güntürkün, Ridgway, & Oelschläger, 2005). Cetaceans also show a high degree of variation in several neural traits, including cerebellar size and cerebral cytoarchitecture (Hof & Van Der Gucht, 2007;Marino, Rilling, Lin, & Ridgway, 2000;Ridgway, Carlin, & Alstyne, 2018;Ridgway, Carlin, Alstyne, Hanson, & Tarpley, 2017;Ridgway & Hanson, 2014).…”