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Forward flapping flight in bats

Bats are unique among extant actively flying animals, with wings constituting a thin membrane stretched by elongated arm and hand bones. Compared to birds and insects, bat wings are very compliant and adjustable, implying an ability to adjust wing morphology according to the aerodynamic demands.

Our first study of bat flight focused on the wake properties of a small nectar feeding bat, Glossophaga soricina. The wakes appeared significantly different from those of birds, especially during the wing upstroke. The bats push air upwards during the upstroke, which was never observed in birds. The bat wake also exhibits other novel features, such that each wing generates its own isolated vortex loop, while in birds there seem to be one vortex loop shed from the whole animal. Overall the bats appear to generate a more complex wake than birds. Our studies of near and far wake of G. soricina also showed that studying the far wake, which has been the standard procedure, captures the overall circulation but risks missing the details of the wake.

To obtain a full understanding of the aerodynamics of flight the wake needs to be connected to the kinematics. We studied the kinematics of Glossophaga soricina flying over a range of flight speeds simultaneously with the wake measurements. The wingbeat kinematics varied gradually with flight speed and reflected the changes observed in the wake of bats.Our studies of how the wings are moved show how the bats regulate the shape and speed of the wings to generate the desired forces.

Subsequently we have studied flight of another closely related bat species, which have a very different movement ecology. Glossophaga soricina is a small nectar feeding bat that lives its whole life in a relatively small area within the rain forest, where it roosts in trees and feeds from flowers. The second bat species is Leptonycteris yerbabuenae, it is a migratory nectar feeding bat that also performs long commuting flights every night from its roosting site to different feeding areas. We were able to estimate the relative flight performance for both species and found that the migratory bat (L. yerbabuenae) flies more efficiently at high speeds, while the non-migrant (G. soricina) is more efficient at low flight speeds. This suggests that both species are well adapted to their relative movement ecology.

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Wakes of two species of bats
The wake behind G. soricina (A) and L. yerbabuenae (B).