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Abstract
To explain variation in relative brain size among homoiothermic vertebrates, we propose
the Expensive Brain hypothesis as a unifying explanatory framework. It claims that
the costs of a relatively large brain must be met by any combination of increased
total energy turnover or reduced energy allocation to another expensive function such
as digestion, locomotion, or production (growth and reproduction). Focusing on the
energetic costs of brain enlargement, a comparative analysis of the largest mammalian
sample assembled to date shows that an increase in brain size leads to larger neonates
among all mammals and a longer period of immaturity among monotokous precocial species,
but not among the polytokous altricial ones, who instead reduce their litter size.
Relatively large brained mammals, altricial and precocial, also show reduced annual
fertility rates as compared to their smaller brained relatives, but allomaternal energy
inputs allow some cooperatively breeding altricial carnivores to produce even more
offspring in a shorter time despite having a relatively large brain. Thus, the Expensive
Brain framework explains why brain size is linked to life history pace in some, but
not all mammalian lineages. This framework encompasses other hypotheses of energetic
constraints on brain size variation and is also compatible with the Brain Malnutrition
Risk hypothesis, but the absence of a mammal-wide correlation between brain size and
immature period argues against the Needing-to-Learn explanation for slower development
among large brained mammals.