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

Abstract

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 found 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 across species varied from 3.5 to 90 million years ago, but nutritional constraints did not lessen over time as species adapted to a diet. Although nutrition was linked to overall brain volume, there was no evidence that nutrition was related to the relative size of individual brain regions. Laboratory rearing experiments confirmed the underlying assumption of most comparative studies that the majority of interspecific trait variation stems from genetically based differences across species rather than developmental plasticity. 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.

Rapid expansion and visual specialization of learning and memory centers in Heliconiini butterflies

“…Additional results on mushroom body evolutionary rates Our findings of a marked expansion the mushroom body in Heliconius are corroborated by our reanalysis of an independently collected neuroanatomical dataset of 41 species of North American butterflies, including H. charithonia and the Heliconiini Agraulis vanillae(54). Using the R package…”

“…Analyzing sensory brain regions in the same way fails to recapitulate these shifts and bursts in evolutionary rate (Figure 2E-H), demonstrating that increases in mushroom body size are not primarily caused by changes in the sensory periphery. Finally, to contextualize this variation within a broader sample of butterflies, we used the same approach to reanalyze a phylogenetically broad dataset of 41 species of North American butterflies which includes Heliconius charithonia and the non-pollen feeding Heliconiini Agraulis vanillae ( 32 ). Both the BAYOU and rates analysis highlight the Heliconius branch as the sole stand-out lineage, and a remarkably clear outlier in mushroom body evolution across butterflies (Figure S4).…”

Dissections of Larval, Pupal and Adult Butterfly Brains for Immunostaining and Molecular Analysis

“…The growing exploration of the butterflies' neurobiology prompted the development of experimental techniques for producing relevant molecular and neuroanatomical data. Brain dissections are an essential step in the experimental process (e.g., [35][36][37]). Protocols describing the dissection of Drosophila brains are currently available [38][39][40][41][42], but a detailed description of butterfly brain dissections at various developmental stages is still lacking.…”

Mitigating Elemental Imbalance

“…Such an approach has been used with some success to describe macroevolutionary patterns. For example, total brain volume of different butterfly species shows a strong positive correlation with the N and sodium (Na) content of the plant species fed upon as larvae (Snell-Rood et al 2020), suggesting that the availability of elements limits the number of neurons produced for a given body size. The relative abundance of elements has been linked to major evolutionary transitions such as the radiation of metazoan life that occurred during the Cambrian explosion (Elser et al 2006), the evolution of omnivory (Diehl 2003), and contrasting life history strategies (Swanson et al 2016).…”

“…All traits require certain biomolecular compounds that have a set stoichiometric composition, and if the elements necessary for the compounds are not available in the amounts or relative abundances necessary, the expression of that trait is constrained (Sterner & Elser 2002). While several studies have provided correlative evidence suggesting elemental availability is a selective force that actively shapes phenotypes (e.g., Elser et al 2006;Swanson et al 2016;Snell-Rood et al 2020), there have only been a limited number of direct tests of this hypothesis so far (Declerck et al 2015;Turner et al 2017;Archambeault et al 2020).…”