Chickadees with bigger brains have smaller digestive tracts: a multipopulation comparison

The factors leading to the evolution of large brain size remain controversial. Brains are metabolically expensive and larger brains demand higher maintenance costs. The expensive-tissue hypothesis suggests that when selection favors larger brains, evolutionary changes in brain size can occur without an overall increase in energetic costs when brain size represents a trade-off with the size of other expensive tissues, such as the digestive tract. Still, support for this hypothesis is equivocal. We compared mean brain mass, digestive tract mass (stomach and gut) and heart mass in 9 populations of black-capped chickadees along a gradient of winter climate severity. Mean brain mass and telencephalon volume showed significant population variation with larger brains associated with harsher winter conditions. Mean population brain mass and telencephalon volume were also negatively related to both stomach and gut mass. Mean population heart mass, on the other hand, was not significantly associated with either mean brain mass or winter climate severity. Mean brain mass was negatively associated with body mass, with chickadees from harsher environments being smaller but having larger brains and smaller digestive tracts. Our results are consistent with the expensive-tissue hypothesis, and suggest that a harsher winter climate might favor larger brains, which might be associated with a reduction in size of the digestive tract. These findings could potentially be a result of population differences in the winter climate diet related to the perishability of more efficient invertebrate-based food caches.