Abstract
AbstractWhy 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.
Highlights
Around fifty million years ago, some ancestors of artiodactyls began to leave the land for a life in water (Gingerich et al 2001, Berta et al 2005) and became the animals we know as cetaceans
Body growth curves have been presented for some cetaceans involved in commercial whaling or fishery bycatch (Laws 1959, Lockyer 2007 and references therein), very little data are available on growth of the brain in most cetacean species
We investigated EQ as a function of maturity across six odontocetes, for which we had robust data (Fig. 4), and found that EQ appears to decrease with increasing body length in O. orca, T. truncatus, S. coeruleoalba, P. macrocephalus, P. dalli, and K. breviceps
Summary
Around fifty million years ago, some ancestors of artiodactyls began to leave the land for a life in water (Gingerich et al 2001, Berta et al 2005) and became the animals we know as cetaceans (whales, dolphins, and porpoises). An erroneous mass from an alcohol-dehydrated blue whale (Balaenoptera musculus) brain represented the species in the literature for over 100 yr (Ridgway and Van Alstyne 2017). Another problem arose from the use of endocasts as a proxy for brain mass (Jerison 1973; Marino et al 2000, 2004; Montgomery et al 2013). The cranial vault of cetaceans, especially in larger whales, contains large vascular networks and tentorium cerebelli that may take up to 65% of the cranial vault volume (see table 3 in Ridgway et al 2017) Without taking this nonbrain tissue into consideration, using endocasts alone may lead to overestimation of brain size.
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