Abstract
All bats experience daily and seasonal fluctuation in body mass. An increase in mass requires changes in flight kinematics to produce the extra lift necessary to compensate for increased weight. How bats modify their kinematics to increase lift, however, is not well understood. In this study, we investigated the effect of a 20% increase in mass on flight kinematics for Cynopterus brachyotis, the lesser dog-faced fruit bat. We reconstructed the 3D wing kinematics and how they changed with the additional mass. Bats showed a marked change in wing kinematics in response to loading, but changes varied among individuals. Each bat adjusted a different combination of kinematic parameters to increase lift, indicating that aerodynamic force generation can be modulated in multiple ways. Two main kinematic strategies were distinguished: bats either changed the motion of the wings by primarily increasing wingbeat frequency, or changed the configuration of the wings by increasing wing area and camber. The complex, individual-dependent response to increased loading in our bats points to an underappreciated aspect of locomotor control, in which the inherent complexity of the biomechanical system allows for kinematic plasticity. The kinematic plasticity and functional redundancy observed in bat flight can have evolutionary consequences, such as an increase potential for morphological and kinematic diversification due to weakened locomotor trade-offs.
Highlights
IntroductionDuring pregnancy, a female bat’s body mass can be up to 40% higher than during nonreproductive periods [1,2], and during lactation, body mass may be even higher [3]
Bats, like all mammals, experience both seasonal and daily changes in body mass
With loads smaller than 15% body mass, cockatiels decreased their flight speed with no changes in wingbeat frequency, but at higher loads (i.e., 20% body mass), they increased both flight speed and wingbeat frequency [18]. These results suggest that the kinematic response to loading may not be straightforward, and that an individual may be able to select among multiple strategies for accommodating increased loading, depending on the magnitude of load and others factors, such as flight speed
Summary
During pregnancy, a female bat’s body mass can be up to 40% higher than during nonreproductive periods [1,2], and during lactation, body mass may be even higher [3] Both males and females of hibernating bat species experience changes in body mass as large as those observed in pregnant females [4,5,6]. Frugivorous bats often carry fruits as large as 40% of body mass to feeding roosts [10]. How these large changes in body mass affect kinematics and flight performance, is still poorly understood
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