The digestive efficiency of insectivorous bats

Abstract

To analyze foraging strategies and energy budgets, a knowledge of the digestive efficiency of animals is essential. There are no such data for insectivorous bats feeding on natural prey. We fed moths of various sizes to bats of three species, Myotis lucifugus, M. evotis, and M. volans and calculated digestive efficiencies using a micro-bomb calorimeter to measure the energy content of prey, uneaten remains and feces. Bats typically did not consume the legs or wings of the moths, and occasionally also rejected the heads. Despite this, all three species had mean digestive efficiencies between 75 and 78%, significantly lower than that of individuals fed mealworm larvae in this and previous studies (88–90%). There was a significant positive correlation between moth size and digestive efficiency, with small moths (20 mg) being digested at under 75% efficiency. Since most natural prey are even smaller than that and are typically consumed whole (including legs and wings), and many are likely less digestible than moths, we predict that actual digestive efficiencies realized by bats in the field will be under 70%. Variation in the efficiency of digestion of different prey may have significant implications for prey selection by bats. In addition, our results suggest that to meet energy demands, bats must consume greater quantities of prey than was previously estimated, and likely consume more than their own body weight per night during times of high energy demand, such as lactation.