Developmental lead exposure has mixed effects on butterfly cognitive processes.

While the effects of lead pollution have been well studied in vertebrates, it is unclear to what extent lead may negatively affect insect cognition. Lead pollution in soils can elevate lead in plant tissues, suggesting it could negatively affect neural development of insect herbivores. We used the cabbage white butterfly (Pieris rapae) as a model system to study the effect of lead pollution on insect cognitive processes, which play an important role in how insects locate and handle resources. Cabbage white butterfly larvae were reared on a 4-ppm lead diet, a concentration representative of vegetation in polluted sites; we measured eye size and performance on a foraging assay in adults. Relative to controls, lead-reared butterflies did not differ in time or ability to search for a food reward associated with a less preferred color. Indeed, lead-treated butterflies were more likely to participate in the behavioral assay itself. Lead exposure did not negatively affect survival or body size, and it actually sped up development time. The effects of lead on relative eye size varied with sex: lead tended to reduce eye size in males, but increase eye size in females. These results suggest that low levels of lead pollution may have mixed effects on butterfly vision, but only minimal impacts on performance in foraging tasks, although follow-up work is needed to test whether this result is specific to cabbage whites, which are often associated with disturbed areas.

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 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.