No cost of echolocation for bats in flight

ECHOLOCATION has evolved in relatively few animal species¹. One constraint may be the high cost of producing pulses, the echoes of which can be detected over useful distances². The energy cost of echolocation in a small (6 g) insectivorous bat, when hanging at rest, was recently measured at 0.067 Joules per pulse³, implying a mean cost for echolocation in flight of 9.5 × basal metabolic rate (range 7 to 12×). Because flight is very costly⁴, whether the costs of echolocation and flying are additive is an important question. We measured the energy costs of flight in two species of small echolocating Microchiroptera using a novel combination of respirometry and doubly-labelled water⁵. Flight energy expenditure (adjusted for body mass) was not significantly different between echolocating bats and non-echolocating bats and birds. The low cost of echolocation for flying vertebrates may have been a significant factor favouring its evolution in these groups.