Abstract
Flight is one of the energetically most costly activities in the animal kingdom, suggesting that natural selection should work to optimize flight performance. The similar size and flight speed of birds and bats may therefore suggest convergent aerodynamic performance; alternatively, flight performance could be restricted by phylogenetic constraints. We test which of these scenarios fit to two measures of aerodynamic flight efficiency in two passerine bird species and two New World leaf-nosed bat species. Using time-resolved particle image velocimetry measurements of the wake of the animals flying in a wind tunnel, we derived the span efficiency, a metric for the efficiency of generating lift, and the lift-to-drag ratio, a metric for mechanical energetic flight efficiency. We show that the birds significantly outperform the bats in both metrics, which we ascribe to variation in aerodynamic function of body and wing upstroke: Bird bodies generated relatively more lift than bat bodies, resulting in a more uniform spanwise lift distribution and higher span efficiency. A likely explanation would be that the bat ears and nose leaf, associated with echolocation, disturb the flow over the body. During the upstroke, the birds retract their wings to make them aerodynamically inactive, while the membranous bat wings generate thrust and negative lift. Despite the differences in performance, the wake morphology of both birds and bats resemble the optimal wake for their respective lift-to-drag ratio regimes. This suggests that evolution has optimized performance relative to the respective conditions of birds and bats, but that maximum performance is possibly limited by phylogenetic constraints. Although ecological differences between birds and bats are subjected to many conspiring variables, the different aerodynamic flight efficiency for the bird and bat species studied here may help explain why birds typically fly faster, migrate more frequently and migrate longer distances than bats.
Highlights
The independent evolution of powered flight in birds and bats begs the question of whether the apparent convergence in size, shape and flight style has resulted in the same overall flight performance, or if they differ in any aspect
Most birds and bats operate in the same Reynolds number regime (Re = Uc/n,104, where U is the flight speed, c is the wing chord length and n is the kinematic viscosity of air) [1,2], which indicates an overall fluid dynamic similarity [3]
We studied the effect of phylogenetic origin of flight on performance by comparing the aerodynamic flight performance for two passerine bird species and two New World leaf-nosed bat species, flying across a range of flight speeds under similar conditions in a wind tunnel [10,11]
Summary
The independent evolution of powered flight in birds and bats begs the question of whether the apparent convergence in size, shape and flight style has resulted in the same overall flight performance, or if they differ in any aspect. We studied the effect of phylogenetic origin of flight on performance by comparing the aerodynamic flight performance for two passerine bird species and two New World leaf-nosed bat species, flying across a range of flight speeds under similar conditions in a wind tunnel [10,11]. Flight performance was measured by studying the aerodynamic wake produced by the flying animals using time-resolved particle image velocimetry (PIV) [11,12,13] Since these types of studies are relatively time consuming and labor intensive, the number of species that can realistically be studied is limited. The results were compared among the species, and differences in performance between the birds and bats were related to differences in morphology, kinematics, ecology and fluid dynamics, to identify the effect of phylogenetic origin of flight on the performance of vertebrate flight
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