Parental investment and body temperature explain encephalization in vertebrates

The massive increase in relative brain size during vertebrate evolution remains poorly understood. Given the high energetic costs of brain development, we predicted that encephalization (major evolutionary brain size expansion) is only possible in species capable of greater parental investment per offspring. Comparative analyses across all major vertebrate classes (N=2600 species) revealed that protecting or provisioning eggs or embryos is associated with larger newborns. Subsequent analyses confirmed that newborn size and adult brain size underwent correlated evolution in birds, mammals, and cartilaginous fishes, but not in other fishes, amphibians, and reptiles, even if the latter had large offspring. Thus, greater pre-hatching investment is necessary but not sufficient for encephalization in vertebrates. A second prediction from the high costs of brain tissue is that encephalization is impeded in species with low or fluctuating body temperatures. We found a positive relationship (albeit sometimes insignificant) between mean body temperature and brain size within all classes. A combined analysis across all vertebrates revealed significant and synergistic effects of body temperature and newborn size on brain size. In conclusion, encephalization became most pronounced in vertebrate lineages that can both produce large offspring, mostly reflecting internal fertilization, and sustain high body temperature, partly linked to endothermy.