We found that the spirit-preserved hearts of 34 species of Australian bat, representing 6 families, weighed from 0.3 to 0.65% of bat mass (m_bat), a variation factor of two. The average mass of the heart specimens of the 34 species was 0.501% of bat mass and this did not vary with bat mass. This value was much lower than the average of the available published data, 0.991%. Insectivorous bats that forage or fly in and near three-dimensional clutter have heart mass fractions ~0.04% larger than average, whereas insectivorous bats that forage around and above the canopy in clear air have fractions ~0.16% smaller than average. Insectivorous bats that are obligate deep-cave roosters have significantly smaller fractions, 0.18% smaller than average, whereas those that hover have fractions ~0.08% larger than average. Available published data, although based on freshly sacrificed animals, show the same trends in relation to heart mass fraction and the same scatter and body-mass relationships. However, the magnitude of the fractions differs by a factor of two and may relate to our removal of all tissue except the musculature and walls of the four cardiac chambers.