Nutritional constraints on brain evolution: sodium and nitrogen limit brain size

Nutrition has been hypothesized as an important constraint on brain
evolution. However, it is unclear whether the availability of specific
nutrients or the difficulty of locating high quality diets limits brain
evolution, especially over long periods of time. We show that dietary
nutrient content predicted brain size across 42 species of butterflies.
Brain size, relative to body size, was associated with the sodium and
nitrogen content of a species’ diet. There was no evidence that host plant
apparency (measured by plant height) was related to brain evolution. The
timing of diet shifts varied from 3.5 to 90 million years ago, but
nutritional constraints did not lessen over time as species adapted to a
diet. While nutrition was linked to overall brain volume, there was no
evidence that nutrition was related to the relative size of individual
brain regions. Lab rearing experiments confirmed the underlying assumption
of most comparative studies that the majority of interspecific trait
variation stems from species differences rather than an individual’s
current developmental environment. This study highlights a novel role of
sodium and nitrogen in brain evolution, which is additionally interesting
given current anthropogenic change in the availability of these nutrients.