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

Abstract

Atmospheric conditions impact how animals use the aerosphere, and birds and bats should modify their flight to minimize energetic expenditure relative to changing wind conditions. To investigate how free-ranging straw-colored 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: 4.3-23.9 ms −1) and airspeeds of 10.2 ± 2.5 ms −1 , and did not modify their wingbeat frequency with speed. Free-ranging straw-colored 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-colored 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.

Coping with the commute: behavioural responses to wind conditions in a foraging seabird

“…In western sandpipers Calidris mauri and cockatiels Nymphicus hollandicus, wingbeat frequency declined with flight speed in a wind tunnel, while lowest wingbeat frequency was recorded at intermediate speed in teals Anas crecca and thrush nightingales Luscinia luscinia (Pennycuick et al 1996, Hedrick et al 2003, Maggini et al 2017. Outside of avian flight, wingbeat frequency of strawcoloured fruit bats Eidolon helvum, is not modified with changes in speed, again suggesting other wingbeat kinematics may be more important (O'Mara et al 2019). Across a wide variety of birds and bats, flight muscle efficiency decreases with forward speed (Guigueno et al 2019), implying that any change in wingbeat frequency leads to inefficiencies in conversion to mechanical work.…”

Assessing roost disturbance of straw-coloured fruit bats (Eidolon helvum) through tri-axial acceleration

“…Data were retrieved using a base station connected to a directional high-gain antenna. Further details on field procedures and the tracking devices can be found in earlier studies that used data from the same bats [ 25 , 26 , 31 , 37 , 40 ].…”

“…Bats were captured with mist nets set at the level of the canopy, which disturbed animals that were roosting in the nearby trees, and we expected their escape flights to spread throughout the roost affecting bats tagged in previous trapping sessions. We also assumed that solar irradiance, precipitation and wind speed could potentially influence daytime flight behaviour of bats in the roosts based on earlier studies [34][35][36][37]. These variables were obtained from open access weather databases (http://www.sasscalweathernet.org for Zambia roosts and https://globalweather.tamu.edu for all the others) with a temporal resolution of one day.…”

“…The occurrence of daytime roost flights was then modelled against our activity in the roosts during the days of trapping, as a predictor of roost disturbance, and natural stressors that may influence flight probability during the roosting period. We predicted that: (1) our presence in the roosts will lead to higher daytime flight probability; (2) bats will be more prone to fly in days of high solar irradiance to find shaded perches and avoid heat stress [ 34 , 35 ]; (3) bats will fly less during daytime when it rains, as rain increases flight energy costs [ 36 ]; (4) bats will fly less at higher wind speeds, which might increase flight energy costs [ 37 ].…”

Foraging movements are density-independent among straw-coloured fruit bats

“…We used only data from individuals with at least one complete night of tracking, on average: 4.4 ± 1.9 nights (see electronic supplementary material, table S2). We classified the GPS locations into three behaviours (resting, commuting and foraging) before analysing foraging parameters, based on the percentage of flapping flight identified within an accelerometry segment [30] (additional description, see moveACC: https://gitlab.com/anneks/moveACC). The percentage of flapping bouts per GPS fix was used to determine the behavioural classes.…”