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

Area of Science:

• Evolutionary biology
• Comparative anatomy
• Neuroscience

Background:

• Brain size evolution is debated, with larger brains being metabolically costly.
• The expensive-tissue hypothesis proposes a trade-off between brain size and other costly tissues, like the digestive tract.
• Previous support for this hypothesis is inconclusive.

Purpose of the Study:

• To investigate the relationship between brain size, digestive tract size, and winter climate severity in black-capped chickadees.
• To test the expensive-tissue hypothesis in a wild bird population.
• To explore potential dietary factors influencing these trade-offs.

Main Methods:

• Compared mean brain mass, digestive tract mass (stomach and gut), and heart mass across 9 black-capped chickadee populations.
• Analyzed data along a gradient of winter climate severity.
• Examined brain mass and telencephalon volume in relation to body mass, digestive tract mass, and heart mass.

Main Results:

• Larger brain mass and telencephalon volume were associated with harsher winter conditions.
• Brain mass and telencephalon volume showed a negative correlation with stomach and gut mass.
• Smaller body mass in harsher environments correlated with larger brains and smaller digestive tracts.
• Heart mass did not show significant associations with brain mass or climate severity.

Conclusions:

• Results support the expensive-tissue hypothesis, suggesting harsher climates favor larger brains.
• Larger brains may be accommodated by a reduction in digestive tract size, maintaining energy balance.
• Dietary shifts, possibly involving less perishable invertebrate caches in harsher climates, could drive these adaptations.