The difficulty with correlations: Energy expenditure and brain mass in bats

Brain mass has been suggested to determine a mammal's energy expenditure. This potential dependence is examined in 48 species of bats. A correlation between characters may be direct or derived from shared correlations with intervening factors without a direct interaction. Basal rate of metabolism in these bats increases with brain mass: large brains are more expensive than small brains, and both brain mass and basal rate increase with body mass. Basal rate and brain mass also correlate with food habits in bats. Mass-independent basal rate weakly correlates with mass-independent brain mass, the correlation only accounting for 12% of the variation in basal rate, which disappears when the combined effects of body mass and food habits are deleted. The correlation between basal rate and brain mass seen in this and other studies usually accounts for <10% of the variation in basal rate and often <4%, even when statistically significant, a minimalist explanation for the level the basal rate. This correlation probably reflects the intermediacy of secondary factors, as occurred with food habits in bats. Most biological correlations are complicated and must be examined in detail before assurance can be given as to their bases.