Hovering flight mechanics of neotropical flower bats (Phyllostomidae: Glossophaginae) in normodense and hypodense gas mixtures
1 Section of Integrative Biology, University of Texas at Austin, Austin, TX
78712, USA
2 Smithsonian Tropical Research Institute, PO Box 2072, Balboa, Republic of
Panama
3 Department Biologie, Universität München, Luisenstrasse 14,
80333 München, Germany
* Author for correspondence (e-mail: r_dudley{at}utxvms.cc.utexas.edu)
Accepted 11 September 2002
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Summary |
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Key words: aerodynamics, bat, density, flight, glossophagine, hovering, hyperoxia, Leptonycteris curasoae, power
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Introduction |
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To date, the kinematics and mechanics of hovering flight have been examined
for only one species of flower bat, Glossophaga soricina Pallas
(von Helversen, 1986;
Norberg et al., 1993
). The
latter study estimated rather high values for mechanical power expenditure in
hovering that substantially exceeded comparable estimates for forward flight
at 4.2 m s-1. By contrast, metabolic rates of hovering G.
soricina exceed those in slow forward flight only by a factor of 20%
(Winter, 1998
;
Winter et al., 1998
;
Winter and von Helversen,
1998
), indicating either that muscle efficiency differs
considerably between the two locomotor modes or that expenditure of mechanical
power is inadequately understood for this group. Substantial variation in
muscle efficiency would be unlikely given the broadly similar wing motions in
hovering and forward flight (see Norberg
et al., 1993
), and the present study thus seeks to investigate
further the kinematics and associated mechanical costs of hovering flight in
glossophagines.
Also of interest are the allometric limits to hovering flight capacity in
vertebrates. Compared with the interspecific range of body sizes in
hummingbirds (mass 2-22 g), glossophagine bats tend to be substantially larger
(interspecific range 7-32 g; see Dobat and
Peikert-Holle, 1985). The only glossophagine studied thus far in
hovering flight, G. soricina, is a small species (body mass 9-11 g)
on the lower end of the size range exhibited within the subfamily. Given
adverse allometric effects on animal flight performance (see
Ellington, 1991
;
Dudley, 2000
), we decided to
examine both normal hovering and maximum flight capacity for a large
glossophagine species (Leptonycteris curasoae, the lesser long-nosed
bat; mass 23-30 g) that approaches the maximum size within the subfamily.
Knowledge of constraints on hovering in this species may thus inform us as to
the general nature of body-mass limits on vertebrate flight performance.
To determine limits to hovering flight, we replaced atmospheric nitrogen
with helium to create low-density but normoxic flight media that impose novel
aerodynamic challenges on volant animals (see
Dudley, 1995;
Dudley and Chai, 1996
).
Studies of ruby-throated hummingbirds (Archilochus colubris) flying
in such hypodense gas mixtures have revealed unexpectedly high reserves of
mechanical power, as well as geometrical constraints on stroke amplitude that
limit hovering capacity (Chai and Dudley,
1995
,
1999
). The potential generality
of anatomically determined limits to wingbeat kinematics remains to be
demonstrated, however, and study of further taxa is warranted. Also, increased
oxygen availability does not alter maximum mechanical power output of
hummingbirds under hypodense conditions
(Chai et al., 1996
), and an
additional goal of the present research was to assess the effects of
supplemental oxygen on hovering flight by glossophagines. We accordingly
carried out density-reduction trials under both normoxic and hyperoxic
conditions to determine mechanical responses to low and ultimately
failure-inducing air densities.
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Materials and methods |
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Analysis of hover-feeding behavior
Hypodense gas manipulations were carried out inside a large airtight
plexiglas cube (1.2 mx1.2 mx1.2 m). A perch within the cube was
used by the bats when not flying. Bats fed inside the flight chamber from an
artificial nectar feeder supplied externally with a 17% w/w sugar solution.
Insertion of the bat's head into the feeder interrupted an infrared light beam
and electronically triggered a nectar reward (see
Winter et al., 1998). This
signal was also used to initiate video recording of hover-feeding sequences
(Kappa CF100 camera; 50 fields s-1). The camera was positioned such
that the optical axis was orthogonal to one cube face and at the same height
as the artificial feeder. The feeder was suspended centrally approximately 50
cm from the ceiling of the cube and was oriented such that bats faced the
camera when hover-feeding. A mirror positioned 45° above the experimental
chamber was simultaneously filmed by the video camera to record a horizontal
projection of wing motions. Partial ambient illumination during experiments
was provided by a dim overhead light, whereas video recording was enabled by
custom-built stroboscopic infrared flashes positioned outside the cube and
driven at repetition rates of 300 Hz. Attached to the feeder at a position
just anterior to its opening was an infrared LED, with a corresponding
receiver positioned outside of the experimental cube. Wingbeat frequencies of
hover-feeding bats were determined from recorded rates of interruption of this
infrared light beam by the moving wings, with interruptions occurring once
(and sometimes twice) near the bottom of the downstroke. The total duration of
each hover-feeding bout was similarly determined from these recordings.
Density-reduction trials
Individual bats were placed within the experimental cube, the gaseous
contents of which were then altered through gradual filling with heliox (80%
He/20% O2). Atmospheric nitrogen was thus replaced with helium,
while oxygen concentration declined to slightly below the atmospheric value of
20.9%. A small release valve enabled gas replacement during the filling
process. Atmospheric pressure, air temperature and relative humidity inside
the cube were monitored to enable a precise determination of ambient air
density for each trial. At regular intervals, filling with heliox was halted
and gas composition within the cube was determined acoustically (see
Dudley, 1995). Hover-feeding
behavior of the bat was then recorded, an additional density measurement was
made, and filling of the cube with heliox was again resumed. This procedure
was repeated until an air density was reached at which aerodynamic failure was
obtained. Using the same experimental protocol, effects of hyperoxia on
maximum hovering performance were determined using a hypodense but hyperoxic
gas mixture (65% He/35% O2); air density declined whereas oxygen
concentration increased as this mixture replaced unmanipulated air within the
flight chamber.
We defined aerodynamic failure as the inability of the bat to sustain
hovering flight for durations typical of those in normodense air, followed by
a fluttering descent from the feeder to the floor of the experimental chamber
(see also Chai and Dudley,
1995). This latter behavior was never seen in normodense
circumstances and was assumed to indicate the inability of the bat to generate
sufficient vertical forces so as to offset body mass. Air density at failure
was estimated as the average of the last flight-capable air density and the
subsequent air density at which hovering failed. Because air density declined
exponentially during the filling process, density intervals became
progressively smaller as filling proceeded. Wingbeat kinematics and energetic
estimates, however, were determined for the final air density at which
hovering flight was sustained. A complete density-reduction trial to the point
of aerodynamic failure required 2-3 h; bats typically hung from their perch
or, to a much lesser extent, flew around within the cube when not
hover-feeding. Experiments were conducted at an average air temperature of
23.8°C (range 23.1-24.7°C) and at an average relative humidity of 74%
(range 57-89%).
Determination of morphological and kinematic parameters
Morphological parameters measured on each bat included mean body mass
(m) for each experimental trial, wing length (R), total area
of both wings (S, excluding the uropatagium) and various wing-shape
parameters that were determined following Ellington
(1984a). Values for wing
loading, pw (pw=mg/S, where
g is gravitational acceleration), and wing aspect ratio,
AR (AR=4R2/S), were also
calculated.
Wingbeat kinematics were determined from digitization of field-by-field
playbacks of recorded video sequences using NIH Image 1.62. Kinematic
parameters determined for each hovering sequence included wingbeat frequency
(n), stroke plane angle (ß), maximum wing positional angle
(max), minimum wing positional angle (
min),
mean wing positional angle (
), and
stroke amplitude (
). Detailed illustrations of these parameters can be
found in Ellington (1984b
) and
Dudley (2000
). ß was
calculated from the two known camera perspectives and from
max
and
min. For each hovering sequence, values of wingbeat
kinematic parameters were averaged for the right and left wings, whereas,
typically, three seperate hovering sequences per individual were evaluated at
each air density.
Determination of mechanical power expenditure
Kinematic and morphological values were then used in the aerodynamic model
of Ellington (1984c) to
determine the lift and power requirements of hovering flight. Stroke
amplitude, wingbeat frequency, and a number of morphological parameters are
used in these calculations. Both the down- and upstroke were assumed to
contribute equally to vertical force production (see
von Helversen, 1986
). Mean
lift coefficients were calculated separately for the down-
(CL,down) and upstrokes (CL,up;
assuming equal weight support in each half-stroke), whereas a mean Reynolds
number (Re) was determined for the entire wingbeat
(Ellington, 1984c
). Overall
power requirements were determined from individual components of profile
(Ppro), induced (Pind) and inertial
(Pacc) power. Profile power is the power required to
overcome profile drag forces on the wings, whereas induced power is expended
in the generation of a downwards momentum flux so as to offset the body mass.
Inertial power expenditure varies according to the extent of elastic energy
storage of wing inertial energy, the latter being estimated from the wing's
mass distribution and its maximum velocity in each of the two half-strokes. A
diversity of kinematic and morphological variables thus contributes to these
lift and power calculations, for which the work of Ellington
(1984a
,c
)
provides further background and details.
Following Norberg et al.
(1993), a profile drag
coefficient of 0.02 was first used to estimate wing profile power. This value
derives from steady-state measurements on a hawk wing at Re ranging
from approximately 140 000 to 190 000 (see
Pennycuick et al., 1992
).
Values of the Re for wings of hovering glossophagines are
substantially lower, however (25,000-40,000; see
Table 2), and use of this
profile drag coefficient yielded unreasonably high values for the wing
lift:drag ratio. Also, recent work on the unsteady aerodynamics of animal
wings has revealed higher values for profile drag coefficients than those
predicted by steady-state considerations (see
Dickinson et al., 1999
;
Usherwood and Ellington,
2002a
,b
).
No information is available on the drag characteristics of flapping bat wings,
although Usherwood and Ellington
(2002b
) derived maximum
lift:drag ratios of approximately 2.0 for a continuously revolving quail wing
at an Re of 26 000. Therefore, we also considered a profile drag
coefficient of 0.2 in calculations for euglossines, the value of which, when
combined with estimated mean lift coefficients for hovering glossophagines
(0.6-1.0; see Table 2) yields
lift:drag ratios of 3-5. Calculations of wing profile power were made
separately for profile drag coefficients of 0.02 and 0.2; this range is likely
to encompass actual values.
|
Inertial power during the first half of a half-stroke was estimated from
the moment of inertia of the wings and their maximum angular velocity assuming
simple harmonic motion. Wing mass and the associated moment of inertia were
estimated from values of body mass using the equation of Thollesson and
Norberg (1991). We also
included the wing virtual mass in estimates of moment of inertia
(Ellington, 1984c
; cf.
Norberg et al., 1993
). Because
of partial wing flexion during the upstroke, the total moment of inertia
during this half-stroke was assumed to equal 50% of the downstroke value
(Norberg et al., 1993
). Total
body-mass-specific power requirements were calculated for the two cases of
perfect (Pper) and zero (Pzero)
elastic energy storage of wing inertial energy
(Ellington, 1984c
). In order
to estimate muscle-mass-specific power output, one male bat was euthanized by
cervical dislocation for postmortem measurement of flight muscle mass. For
this individual, the major down- and upstroke muscles represented 26% of total
body mass (27.1 g). This value was used here as representative for all
individuals to provide a first-order estimate of muscle-mass-specific power
output in hovering flight.
Statistical analysis
Two-way analysis of variance (ANOVA) was used to evaluate the effects of
air density and oxygen availability on kinematic, aerodynamic and energetic
variables; air densities were pooled into 0.1 kg m-3 intervals to
yield a categorical density variable for this statistical analysis.
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Results |
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Hovering duration at the air density immediately prior to failure averaged
0.5 s and did not vary significantly with air density (Tables
1,
4). Of measured wingbeat
kinematic parameters, stroke amplitude () and its constituent parameters
of maximum (
max) and minimum (
min) wing
positional angle showed systematic change with reduced air density (Tables
2,
4; see also
Fig. 1). Stroke amplitude
increased on average by 26% at the near-failure density, reflecting average
increases of 20% in
max and of 31% in
min.
Both mean positional angle (
) of
the wings in the stroke plane and the wingbeat frequency (n) showed
slight but non-significant decreases in hypodense air
(Table 4). Although stroke
amplitude, and thus mean wingtip velocity (given constant flapping frequency),
increased following heliox infusion, mean Re of the wings decreased
significantly (P<0.001, N=5) as a consequence of the
relatively larger decline in air density (see Tables
2,
4). Mean lift coefficients
(CL), however, showed no systematic change with air
density (Tables 2,
4).
|
|
|
In normodense air, estimates of profile power (Ppro) ranged from 2.9 W kg-1 body mass to 29.0 W kg-1 body mass, depending on the assumed profile drag coefficient (Table 3). Profile power thus ranged from 14% to 63% of the aerodynamic power expenditure; the sum of the induced and profile powers (see Table 3, Fig. 2). Inertial power expended during the first half of a half-stroke (Pacc) typically exceeded aerodynamic power by a factor of four to five. Assuming perfect elastic storage of wing inertial energy and a profile drag coefficient of 0.02, total power expenditure (Pper) in normodense hovering averaged 20.1 W kg-1 body mass (Fig. 2), yielding a value of 77.3 W kg-1 muscle mass given a relative flight muscle mass of 26%. The corresponding values for a profile drag coefficient of 0.2 were 31.9 W kg-1 body mass and 122.7 W kg-1 muscle mass. At near-failure air densities, induced power expenditure (Pind) was 14% greater than in normodense air, whereas the associated increase in Ppro was approximately 42% (Table 3). Again, assuming perfect elastic energy storage, a profile drag coefficient of either 0.02 or 0.2 and a flight muscle ratio of 26%, the muscle-mass-specific power output at the point of near-failure was either 90.8 W kg-1 or 175.6 W kg-1, an increase of either 17% or 43%, respectively, relative to normodense hovering. At both normal and near-failure air densities, total power requirements assuming zero elastic energy storage of wing inertial energy (Pzero) were substantially greater than the aerodynamic power requirements alone (Table 3).
|
In hyperoxic density-reduction trials, stroke plane angles (ß) increased significantly (P<0.001, N=5) at lower air densities, by approximately 10% on average (Tables 2, 4). No other kinematic or energetic variable changed systematically with increased oxygen availability. Also, none of the potential interaction effects between air density and oxygen treatment were significant. At the point of aerodynamic failure in hypodense hyperoxia, oxygen concentration was estimated to be approximately 33%. Near-failure air densities in hyperoxia did not differ significantly from those obtained for normoxic density reduction (unpaired t-test, t=1.05, P>0.30; see also Table 4).
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Discussion |
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For glossophagines generally, the kinematic and energetic reserves induced
by low-density aerodynamic challenge are unlikely to derive from adaptations
for flight under conditions of natural hypobaria. In contrast to the often
montane and even high-altitude habitats of many hummingbirds, flower bats tend
to be found at low- to mid-elevations, except for some species in the genus
Anoura, which can be found up to 3400 m (F. Matt, personal
communication). L. curasoae is predominantly a lowland species,
although a night roost has been recorded at an elevation of 1240 m
(Herrera Montalvo, 1997; see
also Reid, 1997
). By contrast,
the average air density at failure for L. curasoae corresponds to an
altitude of approximately 3000 m. The capacity of this species to sustain
hovering under hypodense but normobaric conditions probably derives from the
need in normodense air for supplemental power in vertical-ascent, climbing
flight or for translational accelerations and fast forward flight. Female
glossophagines are confronted by the additional need to carry their young both
pre- and postnatally, increasing their effective mass by approximately 30% and
entailing substantial energetic costs (see
Voigt, 2000
).
Some of the aerodynamic assumptions used here should be considered
provisional, although the overall conclusions are probably robust. The extent
to which force partitioning between half-strokes changes with increased
aerodynamic requirements is unclear given the complexities of tip reversal and
variable upstroke geometry in glossophagines. Substantial longitudinal wing
flexion during the upstroke also suggests considerable deviation of wingtip
motions from simple harmonic motion. Following Norberg et al.
(1993), and in the absence of
high-speed kinematic analysis for L. curasoae, we here assumed equal
durations for the down- and upstroke. Aerodynamically, the assumption of a
constant profile drag coefficient is also probably inappropriate for the range
of Reynolds numbers exhibited during density-reduction trials. Our estimates
of aerodynamic power based on two extremes for the profile drag coefficient
probably bracket actual values, although we emphasize the present ignorance of
profile drag data for reciprocating vertebrate appendages. As with all flying
vertebrates, the actual extent of elastic energy storage within the flight
apparatus is unknown, although potential sites for such storage include flight
muscle, tendons, wing bones and even the wing membrane itself. Costs of flight
in the absence of such storage will increase substantially
(Table 3).
Norberg et al. (1993)
estimated induced and profile powers for hovering and slow forward flight (4.2
m s-1) of the glossophagine G. soricina. Calculations of
instantaneous inertial torque were then compared with estimated aerodynamic
torque at different wing positions to determine potential inertial
contributions to aerodynamic work. As noted by Ellington
(1984c
), this approach
presupposes accurate calculation of wing accelerations from wing positional
information, a procedure potentially subject to error. Instead, mean values of
wing inertial energy can be estimated for the first half of a half-stroke, as
was done in the present study. If all such energy is stored elastically, then
total power over the half-stroke is the aerodynamic power alone. In the
absence of such storage, total power equals half the sum of the inertial power
and the aerodynamic power (Ellington,
1984c
). Reanalyzing the power values of Norberg et al.
(1993
) in this way yields a
body-mass-specific power expenditure of 14.9 W kg-1 for the case of
perfect elastic energy storage and a profile drag coefficient of 0.02, as used
in the original paper. This value may be compared with a similarly modified
estimate of 8.7 W kg-1 body mass based on data provided in the same
paper for a different individual of G. soricina in slow forward
flight. The original estimates of Norberg et al.
(1993
) are 32.4 W
kg-1 for hovering and 12.3 W kg-1 for forward flight,
the ratio of which quantities is substantially higher than the corresponding
ratio for the above modified estimates. Note, however, that wingbeat
frequencies of the two individual bats used in this comparison were also
substantially different (i.e. 15.2 Hz in hovering versus 11.8 Hz in
forward flight), which would yield a reduced profile power in forward flight
and thus underestimate total costs. Detailed kinematic data for individual
glossophagines in both hovering and in slow forward flight are unfortunately
not available, although existing metabolic data suggest that the costs of
flight are broadly similar for these two modes of locomotion (see
Winter, 1998
;
Winter and von Helversen,
1998
; Winter et al.,
1998
; Voigt and Winter,
1999
). In summary, the previously reported differences between
hovering and forward flight in glossophagines are probably overestimated, but
particularly our understanding of unsteady profile drag force on bat wings
precludes a more detailed energetic analysis at present.
The estimate of maximum power output in L. curasoae varied between
91 W kg-1 muscle and 176 W kg-1 muscle, assuming perfect
elastic energy storage of wing inertial energy and a likely range of profile
drag coefficients. If power output is not equally distributed between down-
and upstrokes, as assumed here, then average power output during the
downstroke may be much higher. By comparison, ruby-throated hummingbirds in
hypodense but normoxic gas mixtures fail to sustain hovering at
muscle-mass-specific power outputs of approximately 140 W kg-1
(Chai and Dudley, 1995,
1996
). This failure is
associated with a geometrical constraint on stroke amplitude and is unaffected
by an increased oxygen partial pressure
(Chai et al., 1996
). In
glossophagines, hyperoxia similarly fails to enhance hovering performance
(ruling out diffusive constraints on power availability), but stroke
amplitudes are well below limiting values for hummingbirds in heliox. Does an
anatomical constraint pertain for L. curasoae or are there additional
kinematic and power reserves that may be exhibited in different behavioral
contexts of flight? Compared with ruby-throated hummingbirds, the large bats
in the present study exhibited a much smaller relative increase in power
output with respect to hovering in normodense air. Maximum stroke amplitude in
this case may simply yield the maximum power output available from the muscle.
An additional approach to this issue for glossophagines would be to carry out
load-lifting experiments (Chai et al.,
1997
) that evaluate the ability to sustain loads vertically during
hovering. For ruby-throated hummingbirds, such experiments have shown
short-duration but high-intensity power outputs (approximately 200 W
kg-1 muscle) that exceed by 50% the maxima found in
density-reduction trials (Chai et al.,
1997
). Stroke amplitudes or power outputs significantly in excess
of those exhibited in hypodense air would further illustrate
context-dependence of the limits to flight performance. Physical manipulation
of glossophagine wings on live animals suggests that maximum positional angles
well in excess of those attained in heliox (i.e. >61°; see
Table 2) can be attained
anatomically but have not been documented for free-flying bats.
Comparison of hovering energetics between hummingbirds and glossophagines
indicates lower flight costs in the latter taxon because of their much higher
wing surface area relative to body mass
(Voigt and Winter, 1999). Wing
loading is accordingly much lower in glossophagines, and induced power costs
are correspondingly reduced. In part, such morphological differences may
explain why large glossophagines substantially exceed the largest hummingbird
species in terms of body mass. A relative reduction in mechanical power
expenditure for the former taxon may simply permit an increased body size up
to the point of a mechanical power limitation. Hovering data for the largest
hummingbird species (Patagona gigas) are not currently available,
but, if power limits generally pertain to vertebrate hovering performance, we
might expect comparable maximum mechanical power output between this
hummingbird and the largest glossophagines.
Alternatively, flight muscle efficiencies may differ substantially between
the two taxa for phylogenetic reasons, although calculations using available
data suggest that this is not the case. For an average L. curasoae
weighing 26 g (Table 1), the
scaling of power input given by Voigt and Winter
(1999) for glossophagines
predicts a body-mass-specific metabolic rate in hovering of 154 W
kg-1. Assuming perfect elastic energy storage, this value can be
combined with the average values determined here for mechanical power output
(20.1 W kg-1 body mass and 31.9 W kg-1 body mass;
Table 3) to yield muscle
efficiencies between 13% and 21%. This value is somewhat higher than that
derived for ruby-throated hummingbirds in normal hovering flight (10-11%; see
Chai and Dudley, 1995
,
1996
), but these latter
estimates were made prior to the aforementioned upwards revisions in unsteady
profile drag coefficients and may thus be substantial underestimates of total
power. Also, maximum metabolic power input at the point of hypodense failure
in ruby-throated hummingbirds averages 307 W kg-1, but muscle
efficiencies are unchanged relative to normodense hovering (Chai and Dudley,
1995
,
1996
). We emphasize, however,
that metabolic rates in hovering L. curasoae have not yet been
measured under either normodense or near-failure hypodense conditions. In
particular, the equation of Voigt and Winter
(1999
) is derived from data
for three glossophagine species, all of which are below 20 g in body mass.
Hovering, and more generally the ability to generate vertical forces during
flight, represents only one component of flight maneuverability, namely force
production along one of the three orthogonal body axes. Other features of
flight performance involving axial force production (e.g. thrust generation
during forward flight), as well as various components of torsional agility
(i.e. the rapidity of body rotations about each of three orthogonal axes), are
equally important features of animal flight (see
Dudley, 2002). Features of
body design conducive to performance in one context may be limiting in other
modes of flight. In ruby-throated hummingbirds, for example, sexually
dimorphic morphological features yield strong differences in maximum hovering
performance, but maximum flight speeds are equivalent between the sexes
(Chai and Dudley, 1999
;
Chai et al., 1999
). Little is
known about forward flight performance in L. curasoae, although the
long-distance commuting flight of this species in nature occurs at airspeeds
close to 8 m s-1 (Sahley et
al., 1993
). Such speeds are likely to exceed the slow forward
flight mentioned previously and may well lie on the ascending portion of the
`U'-shaped power curve proposed by Voigt and Winter
(1999
) as characteristic for
bats weighing >6-7 g.
As with hummingbirds, hovering in glossophagines is of monophyletic origin
and is confined to the New World. Most nectarivorous fruit bats land on rather
than hover at flowers (Dobat and
Peikert-Holle, 1985). However, Gould
(1978
) observed hovering of a
small macroglossine pteropodid in front of flowers prior to landing (see also
Strahan, 1983
). Transient
hovering has been reported for a number of other bat taxa (see
Norberg and Rayner, 1987
), and
Voigt and von Helversen (1999
)
observed a hovering display flight of 17 s duration in the emballonurid
Saccopteryx bilineata. Nonetheless, glossophagines effect stable
hovering with apparently little force asymmetry between the down- and
upstrokes, although quantitative partitioning of weight support between the
two half-strokes remains unknown. Such hovering abilities are even more
remarkable given the limited (but distal) wing reversal described for the
glossophagine upstroke (von Helversen,
1986
). Morphological features of flower bats associated with
hovering include rounded wingtips, relatively long third digits and high
wingtip (chiropatagial) areas (see Smith
and Starrett, 1979
; Norberg
and Rayner, 1987
).
Given that sustained hovering is a behavioral novelty that originated only
twice among volant vertebrates, evolutionary study of the kinematics and
mechanics of hovering behavior among phyllostomids would be of substantial
interest. Recent phylogenetic analyses suggest that predominant nectarivory
arose once within the Phyllostomidae, although occasional nectar-feeding
characterizes a number of other lineages within the family (see
Ferrarezzi and Gimenez, 1996;
Wetterer et al., 2000
). We do
not know, however, which and to what degree flight-related features have
changed in concert with such dietary specialization. Of particular interest
would be a comparison of glossophagine flight performance with that of closely
related phyllostomid taxa such as the phyllonycterines and the brachyphillines
that occasionally feed on nectar. Dedicated nectarivory has also been
associated in phyllostomids with substantial shifts in digestive and renal
function (Schondube et al.,
2001
). As with hummingbirds
(Altshuler and Dudley, 2002
),
biomechanical and physiological origins of the hovering lifestyle in
glossophagines remain obscure. Future studies mapping morphological and
kinematic features of hovering flight onto a well-resolved phyllostomid
phylogeny should resolve such uncertainties.
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Acknowledgments |
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