Overall Dynamic Body Acceleration in Straw-Colored Fruit Bats Increases in Headwinds but Not With Airspeed

Anne K. Scharf,Michael Abedi-Lartey,M. Teague O'Mara,Kamran Safi,Jakob Fahr,Martin Wikelski,Dina K. N. Dechmann

doi:10.3389/fevo.2019.00200

Abstract

Atmospheric conditions impact how animals use the aerosphere, and birds and bats should modify their flight to minimise energetic expenditure relative to changing wind conditions. To investigate how free-ranging straw-coloured fruit bats (Eidolon helvum) fly with changing wind support, we use data collected from bats fit with GPS loggers and an integrated triaxial accelerometer and measure flight speeds, wingbeat frequency, and overall dynamic body acceleration (ODBA) as an estimate for energetic expenditure. We predicted that if ODBA reflects energetic expenditure, then we should find a curvilinear relationship between ODBA and airspeed consistent with aerodynamic theory. We expected that bats would lower their airspeed with tailwind support and that ODBA will decrease with increasing tailwinds and increase with wingbeat frequency. We found that wingbeat frequency has the strongest positive relationship with ODBA. There was a small, but negative, relationship between airspeed and ODBA, and bats decreased ODBA with increasing tailwind. Bats flew at ground speeds of 9.6 ± 2.4 ms-1 (mean ± sd, range: 4.3 to 23.9 ms-1) and airspeeds of 10.2 ± 2.5 ms-1, and did not modify their wingbeat frequency with speed. Free-ranging straw-coloured fruit bats therefore exerted more total ODBA in headwinds but not when they changed their airspeed. It is possible that the flexibility in wingbeat kinematics may make flight of free-ranging bats less costly than currently predicted or alternatively that the combination of ODBA and airspeed at our scales of measurement does not reflect this relationship in straw-coloured fruit bats. Further work is needed to understand the full potential of free-ranging bat flight and how well bio-logging techniques reflect the costs of bat flight.

Highlights

Vertebrate flapping flight is an energetically costly, but economical form of locomotion
By integrating GPS and ACC data collected from free flying E. helvum with regional wind models, we show that strawcolored fruit bats increase wingbeat frequency with wind support (Figure 2C), and that flying into headwind increases fruit bat overall dynamic body acceleration (ODBA), while increasing airspeed slightly decreases ODBA (Figures 3A,D, Table 2)
As in previous work (Sapir et al, 2014), bats reduced their airspeed with increasing wind support (Figure 2B) likely to reduce costs of flight, or to maintain ground speed to aid in visual navigation (Chapman et al, 2011; Hedenström and Åkesson, 2017)

Summary

Introduction

Vertebrate flapping flight is an energetically costly, but economical form of locomotion. Flight in Fruit Bats measurement, theoretical relationships built from first principles and wind-tunnel experiments are largely applied to predict the energy requirements of free-flying animals (Pennycuick, 1978; Norberg, 1990; Rayner, 1999; Tobalske et al, 2003b). These relationships are based on the size and shape of a bird, its airspeed, and the resulting power requirements for that flight (Rayner, 1999; Pennycuick, 2008). By applying accelerometer-derived motion estimates for both behavior and energy expenditure we should be able to better understand the ways that bats and birds respond to their environment

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Results

Conclusion