Maternal investment, ecological lifestyle, and brain evolution in sharks and rays

Across vertebrates increased maternal investment (via increased pre- and postnatal provisioning) is associated with larger relative brain size, yet it remains unclear how brain organization is shaped by life history and ecology. Here, we tested whether maternal investment and ecological lifestyle are related to variation in brain size and organization across 100 chondrichthyans. We hypothesized that brain size and organization would vary with level of maternal investment and habitat depth and complexity. We found that chondrichthyan brain organization varies along four main axes, according to: (1) absolute brain size, (2) relative diencephalon and mesencephalon size, (3) relative telencephalon and medulla size, and (4) relative cerebellum size. Increased maternal investment is associated with larger relative brain size, while ecological lifestyle is informative for variation between relative telencephalon and medulla size, and relative cerebellum size after accounting for the independent effects of reproductive mode. Deep-water chondrichthyans generally provide low levels of yolk-only (lecithotrophic) maternal investment and have relatively small brains, predominantly comprised of medulla (a major portion of the hindbrain). Whereas, matrotrophic chondrichthyans – which provide maternal provisioning beyond the initial yolk-sac – found in coastal, reef, or shallow oceanic habitats have large relative brain sizes, predominantly comprised of telencephalon (a major portion of the forebrain). We demonstrated, for the first time, that both ecological lifestyle and maternal investment are independently associated with brain organization in a lineage with diverse life history strategies and reproductive modes.

Methods
Data on brain size, body size, brain region size, reproductive mode, and ecological lifestyle for 100 species of sharks, skates, rays, and chimaeras.