Wake structure and kinematics in two insectivorous bats

Abstract

One contribution of 17 to a theme issue 'Moving in a moving medium: new perspectives on flight'. We compare kinematics and wake structure over a range of flight speeds (4.0-8.2 m s 21 ) for two bats that pursue insect prey aerially, Tadarida brasiliensis and Myotis velifer. Body mass and wingspan are similar in these species, but M. velifer has broader wings and lower wing loading. By using high-speed videography and particle image velocimetry of steady flight in a wind tunnel, we show that three-dimensional kinematics and wake structure are similar in the two species at the higher speeds studied, but differ at lower speeds. At lower speeds, the two species show significant differences in mean angle of attack, body -wingtip distance and sweep angle. The distinct body vortex seen at low speed in T. brasiliensis and other bats studied to date is considerably weaker or absent in M. velifer. We suggest that this could be influenced by morphology: (i) the narrower thorax in this species probably reduces the body-induced discontinuity in circulation between the two wings and (ii) the wing loading is lower, hence the lift coefficient required for weight support is lower. As a result, in M. velifer, there may be a decreased disruption in the lift generation between the body and the wing, and the strength of the characteristic root vortex is greatly diminished, both suggesting increased flight efficiency. This article is part of the themed issue 'Moving in a moving medium: new perspectives on flight'.

Airplane tracking documents the fastest flight speeds recorded for bats

“…To date, PIV has been employed to investigate flight in seven of the over 1300 currently described species of bats [26]. Four of these, Glossophaga soricina, Leptonycteris yerbabuenae [5,27], Cynopterus brachyotis [28] and Carollia perspicillata [14], are nectar/fruit-eating bats with broad low aspect ratio wings that fly in cluttered habitats and are capable of hovering, and two are small broad-winged insectivores [8,29]. The seventh species is T. brasiliensis .…”

“…The seventh species is T. brasiliensis . In general, the six broad-winged species produced the complex wake structures typically associated with lower lift-to-drag ratios and reduced span efficiency in bats [5,8,27,28,29]. By contrast, T. brasiliensis demonstrated wake architecture and three-dimensional wing kinematics much more similar to swifts [6].…”

Common Noctule Bats Are Sexually Dimorphic in Migratory Behaviour and Body Size but Not Wing Shape

“…Interestingly, the only migratory species in that study ( Eidolon helvum ) shows more variation in wing camber than the non-migratory flying foxes [47]. Furthermore, bats with very different wing shapes can show similar kinematics when flying at higher speeds [48]. When the wingbeat kinematics of similar-sized broad-winged and narrow-winged bats are compared, it seems that the largest difference in flight efficiency is at slow speeds [30, 48].…”

“…Furthermore, bats with very different wing shapes can show similar kinematics when flying at higher speeds [48]. When the wingbeat kinematics of similar-sized broad-winged and narrow-winged bats are compared, it seems that the largest difference in flight efficiency is at slow speeds [30, 48]. Narrow-winged bats may have to make larger adjustments to their wingbeats when flying at slow speeds than do bats with broader wings.…”

Speed-dependent modulation of wing muscle recruitment intensity and kinematics in two bat species

“…The degree of wing folding, measured as the span ratio or ratio between minimum to maximum wingspan during a wingbeat cycle, is pronounced during slow flight in fruit-and nectar-feeding bats (von Busse et al, 2012;Wolf et al, 2010), in contrast to less wing folding observed in fast flight of aerial insectivores (Hubel et al, 2016). Performing upstroke with partially folded wings tends to reduce inertial cost and negative lift .…”