Effects of load type (pollen or nectar) and load mass on hovering metabolic rate and mechanical power output in the honey bee Apis mellifera
1 Department of Biology, Arizona State University, Tempe, AZ 85287-1501,
USA
2 Department of Biology, University of Nevada, Las Vegas, NV 89154-4004,
USA
3 Department of Ecology and Evolutionary Biology, University of Kansas,
Lawrence, KS 66045, USA
* Present address: Department of Integrative Biology, University of California,
Berkeley, 3060 Valley Life Sciences Building #3140, Berkeley, CA 94720-3140,
USA
Author for correspondence (e-mail:
j.harrison{at}asu.edu)
Accepted 28 February 2003
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Summary |
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Key words: flight metabolism, loading, power output, wing kinematics, insect, honey bee, Apis mellifera.
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Introduction |
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The loads carried by honey bee foragers range widely and can approach their
maximal lifting capacity. Honey bees typically carry pollen and nectar loads
representing 20% and 35% of body mass, respectively, maximally reaching 80% of
body mass (Winston, 1987).
This load-lifting capacity is what would be expected from the ratio of thorax
to body mass of 0.40.5 (Marden,
1987
).
Currently, there is a conflict in the literature regarding the effect of
load carriage on flight metabolic rate in honey bees. Wolf et al.
(1989) found that metabolic
rates during hovering and slow forward flight were positively correlated with
nectar load, increasing by approximately 40% when carrying loads equal to 75%
of body mass. In contrast, Balderrama et al.
(1992
) and Moffat
(2000
) found that metabolic
rates of honey bees were unrelated to load size (for loads up to 30% of body
mass), but were linearly related to the reward rate of experimental flowers.
These authors suggested that behavioral activation may be a more important
determinant of flight metabolic rate than increases in power requirements
during loading (Balderrama et al.,
1992
; Moffat,
2000
).
The prior studies of load-lifting in honey bees have all examined nectar
carriage. Flight metabolic rate of honey bees can be affected by the sugar
content of hemolymph and the crop
(Gmeinbauer and Crailsheim,
1993), so it is conceivable that the results of Wolf et al.
(1989
) were confounded by
variation in hemolymph sugar levels. However, the Balderrama et al. (1993) and
Moffat (2000
) studies can be
criticized because metabolic rates were averaged over periods including flight
and non-flight (during which bees sat and fed on nectar). We address this
controversy in two ways. First, we compare loading effects on metabolic rates
in pollen and nectar foragers. We can directly compare our nectar-loading data
to prior studies. In addition, because pollen foragers do not consume their
load, there should be no confounding effects of variation in hemolymph sugar
levels when comparing loaded and unloaded pollen foragers. Second, we examine
the effect of load carriage on wing kinematics and calculated mechanical power
output in hovering pollen foragers. In bumblebees, loading causes similar
increments in metabolic rate and mechanical power output
(Cooper, 1993
); it seems
reasonable that honey bees should show a similar pattern.
The type of load carried (pollen or nectar) may also affect the metabolic
cost of load carriage during flight. First, pollen is carried externally on
the pollen baskets of the legs, whereas nectar is carried internally in the
abdominal honey stomach, potentially causing the center of mass of the animal
to differ with the two load types. Changes in the center of mass may alter
body angles, wing positions and angles of attack, or may require correctional
changes in the flight movements to prevent such changes in body angle. We
measured wing kinematic patterns in pollen foragers, and compare these to
previous data (Ellington,
1984a,b
,c
).
Secondly, honey bees tend to specialize on either pollen or nectar and such
specialization is partially genetically based (Calderone and Page, 1989;
Robinson and Page, 1989;
Fewell and Page, 1993
). Flight
metabolic rates differ among genotypic lineages within a colony
(Harrison et al., 1996a
),
leading to the question of whether genetic differences between pollen and
nectar foragers are also associated with metabolic variation. To partially
address this possibility, we tested for genetic differences between our pollen
and nectar foragers using variation in allozymes of malate dehydrogenase-1
(MDH-1), which has been linked to varation in flight metabolism in honey bees
(Coehlo and Mitton, 1988; Harrison et al.,
1996a
).
Thirdly, variation in metabolic rates between pollen and nectar foragers
could be generated by variation in cooling capacities during flight. Nectar
foragers can extrude their crop contents for evaporative cooling of the body,
while pollen foragers lack this option
(Heinrich, 1979;
Cooper et al., 1985
). Thoracic
temperatures affect flight force output and metabolic rate
(Feller and Nachtigall, 1989
;
Nachtigall et al., 1989
;
Coelho, 1991
;
Hrassnigg and Crailsheim,
1999
; Harrison and Fewell,
2002
). With our respirometry chambers, we could not obtain
thoracic temperatures quickly enough to ensure that the measured temperature
would be similar to that during flight. Therefore, to test for an effect of
load type on thermal balance in flying honey bees, we compare the thorax
temperatures of pollen, nectar and water foragers while returning to the hive
across a range of ambient temperatures.
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Materials and methods |
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Thoracic temperature of pollen, nectar and water foragers in the
field
We measured thorax temperatures of 512 pollen, nectar and water foragers
returning to a single hive in the field. After measurement of thorax
temperature, all the bees were frozen for later measurement of malate
dehydrogenase (MDH-1) phenotype. Testing for these effects within a single
colony was critical as this allowed us to control for variation among
colonies.
Returning foragers were collected from 07.00 to 09.00 h, 12.00 to 14.00 h and 17.00 to 19.00 h over 4 days in mid-June. Shaded air temperatures near the colony entrance were 2438°C; all temperatures were measured in shade, though the bees had generally just flown in the sun. 24 pollen and non-pollen foragers were collected during each period using forceps insulated with styrofoam. We measured their thoracic temperature within 5 s of capture using a Physitemp MT-29/T hypodermic needle microprobe (29 gauge, time constant=0.025 s) and a Physitemp BAT-10 thermocouple thermometer. Shaded ambient temperature was recorded at 30 min periods throughout the sampling period using another thermocouple. Bees with visible pollen loads on their legs were classified as pollen foragers. To determine whether non-pollen foragers were nectar or water foragers, we separated the thorax and abdomen with a razor blade, and removed the crop contents by gently squeezing the abdomen. We determined the solute concentration of the crop contents with a Fisher hand refractometer (Santa Clara, CA, USA). We classified bees with crop solute content concentrations of less than 5% as water foragers. Bees with less than 5 µl of liquid in their crops were discarded. For this and all subsequent experiments, statistical analysis was performed using SYSTAT version 9, with P<0.05 considered significant.
Effect of load type on malate dehydrogenase-1 variation
Each bee used in the thorax temperature experiment was tested for malate
dehydrogenase-1 (MDH-1) phenotype using cellulose acetate electrophoresis
(Helena Laboratories system). We crushed the head in 100 µl of extraction
buffer (B-nicotinamide adenine dinucleotide phosphate and
DL-dithiothreitol: 0.13 and 6.5 mmol l1 respectively,
0.00001x Triton X-100). We used a discontinuous buffer system; cellulose
plates were soaked with tris-glycine soaking buffer, and gels were run on a
tris-malate running buffer (Hebert and
Beaton, 1989). Homogenates were run for 22 min at 250 V, stained
for MDH-1 (Hebert and Beaton,
1989
), and scored for the presence of three possible alleles,
which we recorded as `slow', `medium' or `fast'.
Effect of loading on hovering metabolic rate and wingbeat
frequency
Experimental design
We collected workers foraging from artificial resource stations set up
several meters away from the colonies. The resource stations consisted of
dishes with sponges soaked in 40% sugar solution or with freshly ground
pollen. Pollen and nectar were replenished throughout the day. Bees that
arrived at the pollen dish and began to collect pollen were considered pollen
foragers; those bees landing on the nectar dish and extending their proboscis
for nectar uptake were considered nectar foragers.
In one experiment (designated the Regression experiment), load mass varied continuously. We collected 86 bees that had spent varying amounts of time on the resource dishes. In this case, data were analyzed with an analysis of variance (ANOVA) design in which behavior (pollen versus nectar load type) was considered an independent categorical factor and total mass (bee + load) was a covariate. In a second experiment (termed the ANOVA experiment), load mass varied discontinuously. 20 unloaded foragers were collected as they arrived at the resource dishes, and 20 foragers were also collected when they showed signs of departing from the feeding station (preening, retraction of proboscis in nectar foragers, or fast walking). For this second experiment, data were analyzed by a two-way ANOVA, with behavior (pollen versus nectar) and loading status (unloaded versus loaded) as categorical factors. Data for these two experiments were analyzed separately for statistical purposes, but all other techniques were identical.
Carbon dioxide emission during hovering
Bees were placed in the respirometry chamber within 2 min of capture from
the foraging stations. We measured metabolic rate indirectly using
flow-through carbon dioxide respirometry at 24±1°C in a clear
lucite cylinder, 50 mm in diameter and 14 cm long. Air free of CO2
and H2O flowed through the chamber at 3.70 l
min1, monitored by an Omega FMA-5613 mass flow meter
(Stamford, CT, USA). Calculated flow velocity in the chamber was 3.6 m
s1. To obtain CO2- and H2O-free air,
room air flowed through a Balston 75-52 FT-IR purge gas generator (Haverhill,
MA, USA) and was further scrubbed by a column of Drierite and ascarite just
prior to entering the flight chamber. The excurrent air passed through a
column of magnesium perchlorate to remove the water vapor before the
CO2 content was measured with a Licor LI-6252 CO2
analyzer (Lincoln, NE, USA), interfaced with Sable Systems hardware and
software (Las Vegas, NV, USA) and a computer.
To encourage bees to hover in the center of the tube, flight trials were conducted in a dark room, and a light (battery-powered steady white light) was used to illuminate the chamber from above and induce bees to hover in one area. Metabolic rates were only calculated during periods in which bees hovered spontaneously with little vertical or horizontal movement. Because the effort of flight decreases near the chamber's edge (Raynor and Thomas, 1991), we also discarded data from bees that flew within a 1 cm of the chamber wall. Hovering periods typically lasted 34 min, and all but five of the hovering periods were at least 40 s in length, with the majority representing at least 1.5 min of continuous hovering. Hovering metabolic rates were repeatable within individuals (data not shown).
Wingbeat frequency
We recorded wingbeat frequencies onto audio tape during hovering flight in
the respirometry chamber using an optical tachometer (constructed at Arizona
State University) (Unwin and Ellington,
1979). The tape was later analyzed using Sound EditTM on a
Macintosh computer by measuring the duration of 1015 wingbeats at five
intervals throughout the trial and averaging these intervals.
Optical tachometers are potentially subject to errors associated with
counting the fore and hind wings separately, and with counting pulses
associated with wing tip fluctuations. We were particularly concerned with
these possibilities as our measurements of wingbeat frequency substantially
exceeded those reported by Ellington
(1984a,b
,c
)
for honey bees. Therefore, we induced several honey bees to fly in a glass
cylinder and measured their wingbeat frequency using the optical tachometer
while simultaneously recording their hovering with hi-speed video photography
at 3000 frames s1. Using NIH Image, we digitized the video
and measured the wingbeat frequency by counting the frames required per
wingbeat. The two methods gave identical measurements of wingbeat
frequency.
Mass measurements
After metabolic measurements, we froze bees in liquid nitrogen and
transferred them to a 70°C freezer. We later thawed and weighed
them to obtain total mass (bee + load) using a Mettler AE 240 analytical
balance (sensitivity ±0.01 mg). Pollen loads were removed from the legs
and weighed. We estimated load mass of nectar foragers by subtracting the
average unloaded bee mass from the total mass of the loaded bee. For the one
bee in which load calculated in this manner was (slightly) negative, we used a
load mass of zero. We also measured thorax mass (including wings and legs
without pollen loads).
Respiratory exchange rates and calculation of metabolic rates
Calculation of metabolic rates from CO2 emission requires
knowledge of the ratio of CO2 produced to O2 consumed
(respiratory exchange rate, RER), which depends on the substrate catabolized.
Although reported RERs are generally 1.0 for honey bees
(Rothe and Nachtigall, 1989),
prior studies have not reported whether the bees measured were pollen or
nectar foragers. Potentially, substrate utilization might differ among pollen
and nectar foragers if they consume and metabolize part of the loads they
carry. Therefore, we measured the RERs of ten flying pollen and nectar
foragers using closed system respirometry.
We collected loaded pollen and nectar foragers from resource dishes and placed them in a 500 ml Erlenmeyer flask. After adding the bee, the flask was flushed for 1 min with air lacking CO2 and water. The flask was then sealed, and the bee was mildly agitated to fly for 10 min at 24.7±0.1°C. We only analyzed data from bees that flew for at least 9.5 min during the 10 min trial. We extracted a 60 ml gas sample from the flask via a septum. The gas sample was then injected into a stream of dry, CO2-free air and drawn sequentially through a magnesium perchlorate column (to remove water), the CO2 analyzer (Anarad AR-411, Santa Barbara, CA, USA), an ascarite column (to remove CO2), the oxygen analyzer (Ametek S-3A, Pittsburg, PA, USA) and an Ametek R-1 flow controller. Output of the CO2 and O2 analyzers was digitized and analyzed with Sable Systems Datacan V hardware and software. The RER was calculated as the ratio of the volume of CO2 emitted relative to the volume of O2 consumed.
Effect of pollen loading on wing kinematics and mechanical power
output
Honey bees have a pattern of periodic stationary hovering while collecting
pollen. We induced pollen foragers to exhibit this stationary hovering by
luring them to collect pollen off a spoon. During the foraging interval,
pollen foragers exhibited virtually perfect hovering, only briefly darting to
the spoon to collect 12 mg of pollen before hovering again while they
packed the pollen onto their pollen baskets. Bees repeated the
foragingpacking cycle 1015 times until fully loaded, at which
point we captured them with forceps. The spoon was positioned within a filming
stage, which consisted of a Panasonic VHS video recorder (Los Angeles, CA,
USA) simultaneously filming the top and side views of the bee using a mirror
positioned at a 45° angle to horizontal. After capture, we froze the bees
in liquid nitrogen and later weighed their pollen loads and bodies. In this
case, thorax mass was measured after removing both pairs of wings and all of
the legs.
The wingbeat frequency of individual foragers were measured from the moment of arrival until the bee showed signs of departure, using an optical tachometer recording onto the videotape. Then wingbeat frequency was analyzed at 5 s intervals throughout the loading trial using Sound EditTM.
We used these data to calculate the mechanical power output for 17 pollen
foragers hovering in the unloaded and maximally loaded state. Wingbeat
frequencies were averaged for three different sequences at the beginning and
at the end of the trial. Videotape images were analyzed by capturing an image
in Apple Video recorder and importing into NIH Image where stroke amplitude,
inclination of stroke plane, and body angle were measured (defined as in
Dudley, 1995). The horizontal
movement of the wings can be observed as a blur when viewing the hovering bee
from the top. Stroke amplitude was measured as the angle between two lines
drawn parallel to the top of outer edges of this top-view wing blur. The wing
blur can also be seen in the side view and inclination of stroke plane was
measured as the angle between this side-view wing blur and true horizontal
(90° to a suspended, weighted string in the viewing frame). By convention
(Dudley, 1995
), positive
inclinations of stroke planes correspond to angles above horizontal. A line
parallel to the body axis was drawn and the angle between this line and true
horizontal was used as the body angle; by convention, positive body angles
correspond to angles below horizontal.
Wings were removed from filmed bees and weighed to within 0.1 µg using a Cahn C-31 microbalance (Cerritos, CA, USA) after rehydration for 24 h by placing wings in a chamber with water-saturated air. Wing area and length was determined by filming one pair of wings for each bee and analyzing images using NIH Image.
We calculated mean lift coefficients averaged over the stroke and power
output for the filmed bees at the beginning using Ellington's model
(1984a,b
,c
)
by substituting our morphological and kinematic parameters into a Matlab
program, written by Lance Tammero and Erica Feuerbacher (University of
California, Berkeley, USA). The wingbeat frequency, stroke amplitude, body
angle and body mass were all substituted directly into the model. Power
outputs g1 flight muscle were calculated, assuming that
muscle mass constitutes 75% of the thorax mass
(Heinrich, 1980
). We
calculated aspect ratio AR using the formula
4R2S1 (Ellington,
1984a
), where R is
wing length and S is the surface area of one set of wings (i.e. right
wings or left wings). Wing moments were obtained from averages for honey bees
reported by Ellington (1984a
).
Traditionally, profile power outputs during insect flight have been calculated
assuming that drag=7/Re1/2, where Re is Reynold's
number (Ellington,
1984a
,b
,c
);
we term this the E Drag Estimate. Recently, much higher estimates of drag and
profile power have been made for insect flight. In Drosophila
melanogaster the lift to drag ratio=0.55
(Sane and Dickinson, 2001
), so
we also calculated profile power assuming this ratio applies to honey bees,
which we term the SD Drag Estimate.
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Results |
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Malate dehydrogenase phenotype
Bees carrying different types of load had different MDH-1 phenotypes
(Fig. 1). MDH-1 phenotype
differed significantly between nectar and pollen foragers (G test:
G=57.2, P<0.001), between nectar and water foragers
(G=6.4, P<0.05), and between pollen and water foragers
(G=27.4, P<0.001).
|
Body and load sizes
Mean body mass of unloaded bees was approximately 73 mg
(Fig. 2). Mean pollen loads
were 24% and 27% of unloaded body mass in the regression and ANOVA
experiments, respectively. Mean nectar loads were 32% and 40% of unloaded body
mass in the two experiments (Fig.
2). Some nectar loads nearly equalled mean bee mass (the largest
load was 70 mg).
|
There was some inconsistent evidence that pollen foragers are smaller than nectar foragers. Pollen foragers had significantly smaller thoracic mass than nectar foragers in the ANOVA but not the Regression experiment (Fig. 2, Tables 2, 3). Pollen and nectar foragers did not differ significantly in head width (pollen foragers: 3.76±0.015 mm; nectar foragers 3.77±0.013, t-test, Table 2). There was also a tendency for nectar foragers to carry heavier loads (Fig. 2, Table 3, significant BehaviorxLoad status interaction effect). There was evidence that larger bees carry heavier loads, as total mass significantly (positively) affected thorax mass and head width (Table 3).
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Flight metabolism
Pollen and nectar foragers did not significantly differ in respiratory
quotient (RQ) values (pollen foragers, RQ=1.00±0.014, N=10;
nectar foragers, RQ=1.01±0.025, N=10; t-test,
t=0.781, P=0.40), suggesting that both pollen and nectar
foragers utilize only carbohydrate for catabolism during flight. Therefore, we
converted CO2 emission rates to metabolic rates (in mW) assuming
21.4 J ml CO21
(Gordon, 1982).
Pollen foragers had higher metabolic rates than nectar foragers. In the Regression experiment, pollen foragers had a significantly higher metabolic rate per bee than nectar foragers (averaging across all loads; pollen foragers: 60.5±0.79 mW; nectar foragers: 56.5±0.86 mW, ANOVA, Fig. 3, Table 2). Similarly, in the ANOVA experiment, unloaded pollen foragers had a metabolic rate approximately 10% higher than unloaded nectar foragers (Fig. 4, Table 3). Load status (loaded versus unloaded) did not affect the difference in hovering metabolic rates between pollen and nectar foragers (Fig. 4, Table 3; no significant behaviorxload status interaction).
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|
Wingbeat frequencies
Average wingbeat frequencies did not differ significantly between pollen
and nectar foragers (Regression experiment, averaging across all loads;
234±1.5 Hz and 233±1.9 Hz, respectively;
Fig. 5A, Table 2).
|
Effects of load mass
Flight metabolism
There was no significant effect of load mass on metabolic rate in the
Regression experiment for either pollen or nectar foragers
(Fig. 3, Table 2). However, metabolic
rates significantly increased in loaded foragers by 6% in the ANOVA experiment
for both pollen and nectar foragers (Fig.
4, Table 3).
Wingbeat frequencies
In the Regression experiment, wingbeat frequencies did not significantly
vary with total mass in either pollen or nectar foragers
(Fig. 5A,
Table 2). The wingbeat
frequency decreased significantly with increasing thorax mass in both pollen
and nectar foragers (Fig.
5B).
During the mechanical power output measurements, bees varied in the amount of time they spent loading (average 146 s, range 83272 s) and so we examined how wingbeat frequencies changed as a function of the percentage of total flight time at the resource station. Mean final pollen load size in this study was 13.2±1.39 mg (18% of mean body mass, N=19), range 5.226.4 mg (635% of mean unloaded body mass). The mean wingbeat frequency did not vary significantly with time spent loading (Fig. 6, regression, P>0.05), nor did it vary significantly during pollen loading for 19 of 24 pollen foragers tested (linear regressions on the data for individuals). Of the five that did show significant changes in wingbeat frequency, four had significantly lower wingbeat frequencies with increasing load size. Mean wingbeat frequencies of pollen foragers were higher in the field (during the mechanical power output measurements) than in the respirometry chamber (268.0±2.13 Hz and 233.3±1.24 Hz, respectively; ANOVA, F=134.2, P<0.0001).
|
Stroke amplitude and angles of the wing and body
Stroke amplitude, inclination of stroke plane and body angle did not differ
significantly between bees with zero load and those carrying their maximal
load before capture (Fig. 7,
paired t-tests; stroke amplitude: T=0.29,
P>0.75; inclination of stroke plane: T=1.48,
P>0.15; body angle: T=0.59, P>0.50). Mean
stroke amplitude was 118±2.4°, mean inclination of stroke plane was
31±1.3°, and mean body angle was 14±0.8°. The mean final
pollen load for the 17 bees successfully filmed for these parameters was
14.1±0.88 mg, 19% of the mean unloaded body mass of foragers.
|
Mechanical power output
Induced power (power used to generate lift) significantly increased from
the unloaded to the loaded state, regardless of which estimate of drag we used
(Table 4). The Reynold's number
Re for the wings was calculated as 1669±38.9 for unloaded bees
and 1664±37.3 for loaded bees. The coefficient of drag and the profile
power (power used to overcome drag on the wing) did not significantly differ
between loading states using the E drag estimate
(Table 4). Using the SD drag
estimate, the coefficient of drag is proportional to the coefficient of lift,
so both the coefficient of drag and profile power significantly increased when
calculated using the SD drag estimate
(Table 4). In neither case did
inertial power (power used to accelerate and decelerate the wings)
significantly increase with loading state. Estimates of profile power and
total power were approximately threefold greater when we used the SD drag
estimate compared to when the E estimate of drag was used
(Table 4). The substantial
disparity between these two methods for calculating profile and total power
illustrates the need for further research in this area.
|
Total power was calculated assuming perfect elastic storage of inertial
power, an assumption generally considered to be nearest to reality
(Dickinson and Lighton, 1995).
Total power increased significantly by 1617% regardless of the method
of estimated drag and profile power (Table
4). This increase is quite similar to the 18% increase in body
mass due to loading that these bees experienced. The lift coefficient (lift
per wing stroke) also increased significantly from unloaded to loaded status
(18% increase; t-test, T=4.1, P=0.002), regardless
of the manner in which drag was estimated.
We calculated efficiency (power output/power input) for unloaded and loaded pollen foragers since the body masses and maximal loads were similar during the ANOVA experiment and the power output measurements (Table 4). Efficiencies were threefold higher when calculated using the SD drag estimates, and tended to be higher for loaded bees. However, we could not test these trends statistically as the power outputs and metabolic rates were measured on different bees.
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Discussion |
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Effects of load type (pollen versus nectar foragers)
Pollen foragers had metabolic rates during hovering that averaged 10%
greater than nectar foragers (Figs
3,
4; Tables
2,
3). Differences in flight
metabolic rates between pollen and nectar foragers were similar whether loaded
or unloaded bees were compared (Fig.
4, Table 3). Thus,
the higher hovering metabolic rates of pollen foragers cannot be explained by
biomechanical effects of the load type such as differences in drag or center
of mass.
These metabolic differences may reflect physiological or behavioral
plasticity. For example, pollen foragers might be more behaviorally activated
(excited or motivated) by their resource than nectar foragers. In support of
this hypothesis, hovering metabolic rate is correlated with sugar reward rate
in honey bees (Moffat, 2000).
Pollen may be considered a more rewarding resource by these bees.
The metabolic differences between pollen and nectar foragers could also be
due to genetically based variation in muscle, wing morphology or
neuroendocrine properties which influence hovering metabolism and/or wing
kinematics. This possibility is supported by the differences we found in
malate dehydrogenase phenotype, and the many papers which have documented
genetic effects on foraging behavior in honey bees
(Calderone and Page, 1988;
Robinson and Page, 1989
;
Fewell and Page, 1993
).
Bees specializing on pollen versus nectar collection might use
different wing kinematics; such differences could be genetically based or due
to phenotypic plasticity. The only kinematic parameter that we measured for
both pollen and nectar foragers was wingbeat frequency, and the mean wingbeat
frequency did not differ between pollen or nectar foragers. However,
comparison of our data for pollen foragers with the data reported for honey
bees by Ellington (1984b)
suggests that pollen foragers may fly with their abdomens much nearer to the
horizontal plane, and with a more inclined stroke plane than nectar foragers.
The bees studied by Ellington were probably nectar foragers, as these are the
most common type of forager. In our study, pollen foragers had a mean
inclination of stroke plane of 31°, compared to a mean inclination of
stroke plane of 11° in the Ellington study, while our pollen foragers had
body angles of 14° compared to 56° in the Ellington study. Note that
the absolute angles between the wing and body were identical (45°) in both
studies, indicating that the pollen foragers in this study are simply rotated
forward along the pitch axis relative to the bees studied by Ellington.
Perhaps a more horizontal body angle facilitates the packing of pollen into
the corbicula. A more inclined stroke plane directs less of the lift
generating air vortices vertically downward, such that less lift is generated
for the same power output. Thus, the higher metabolic rates during hovering of
pollen foragers could be due to pollen foragers requiring more power output to
generate the same vertical lift as nectar foragers.
There was also some evidence that pollen foragers had smaller thoraxes than
nectar foragers; since mass-specific metabolic rates tend to decrease with
size, these mass differences could contribute to the differences in metabolic
rates between pollen and nectar foragers. However, unloaded body mass and head
width did not differ between pollen and nectar foragers (Tables
2,
3), and thorax mass was not
correlated with flight metabolic rate (Pearson test, P>0.05), all
of which suggest that body size differences are unlikely to explain the
differences in flight metabolism between pollen and nectar foragers. Wingbeat
frequency of both pollen and nectar foragers were negatively correlated with
thorax mass (Fig. 5B), but
wingbeat frequency was not correlated with metabolic rate (Pearson test,
P>0.05). Wingbeat frequency and body mass are also negatively
correlated in euglossine bees (Casey et
al., 1985).
Differences in the thoracic temperatures of free-flying pollen and nectar
foragers appear to be at least partially mediated by differences in flight
metabolic rates. As in the study of Cooper et al.
(1985), pollen foragers had
significantly higher thoracic temperatures than nectar foragers. We also found
that water foragers had the coolest thoracic temperatures. Nectar foragers
extrude a droplet of nectar from their crops into their mouthparts during
flight at high temperatures to evaporatively cool
(Heinrich, 1979
;
Cooper et al., 1985
). Thus, the
observed differences in thoracic temperature among pollen, nectar and water
foragers could be due variation in the ability of foragers to evaporatively
cool themselves, as nectar and water foragers may have greater liquid crop
content. Another possible explanation for higher body temperatures in pollen
foragers is that pollen may add drag during forward flight, slowing forward
speed and convective cooling. However, if heat loss rates are similar for
nectar and pollen foragers, the 5% difference in thorax and air temperatures
(at 24°C) between pollen and nectar foragers
(Table 1) can be completely
accounted for by the greater metabolic rates of pollen foragers. This suggests
another possible evolutionary hypothesis for the higher flight metabolic rates
in pollen foragers: since pollen foraging is more likely to occur at cooler
morning air temperatures (J. Fewell, personal observations), pollen foragers
may possess higher flight metabolic rates to aid thermoregulation during
flight.
Effects of load mass
Load mass appears to increase flight metabolic rate, but only slightly.
Loaded pollen and nectar foragers had significantly higher metabolic rates
than unloaded pollen and nectar foragers in the ANOVA experiment
(Fig. 4,
Table 3), but the increase in
metabolism (6%) was small compared to the 27% (pollen) and 40% (nectar)
increases in body mass. We found no effect of load on metabolic rates in the
Regression experiment (Fig. 3,
Table 2). It is likely that
this second result simply stems from the fact that the effect of load on
metabolic rate was so small that it was obscured by individual variation; in
other words, the regression experiment lacked sufficient statistical power
despite reasonably large sample sizes (N=86). The coefficients of
variation for hovering metabolic rate were 7.9% and 10.6% for pollen and
nectar foragers, respectively, in the Regression experiment, both of which
were greater than the change in metabolic rate with load of 6% observed in the
ANOVA experiment.
Thus our results are intermediate between those of Wolf et al.
(1989) who reported a stronger
effect of loading on metabolic rate and those of Balderrama et al.
(1992
) and Moffat
(2000
), who reported no effect
of load on flight metabolism. The regression equations provided by Wolf et al.
(1989
) predict that the mean
loads observed in our ANOVA experiment would lead to an increase in mean
metabolic rates of 25% for nectar foragers and 14% for pollen foragers. The
comparison of the results of our Regression experiment (no effect of load)
with the results of Wolf et al.
(1989
) is quite striking,
since some of the bees in the Regression experiment carried loads as great as
the maximum carried in the study by Wolf et al.
(1989
), while having metabolic
rates similar to unloaded bees (Fig.
3). One possible explanation for the differences between our
results and those of Wolf et al. is that their data were based primarily on
repeated measures of individuals after consuming different amounts of nectar,
while our data were based on measures of different individuals who carried
different masses of nectar or pollen. In other words, inter-individual
variation may have obscured the loading effect in our study. Arguing against
this possibility is the observation that wing kinematic variables did not
differ with loading, even within individuals (Figs
5,
6,
7).
An alternative possibility is that the difference between these studies
reflects the different air temperatures used during the metabolic measurements
(Wolf et al., 1989, 32°C;
this study, 24°C). The major difference between our results and those of
Wolf et al. (1989
) are the
metabolic rates measured for unloaded bees. Our unloaded nectar foragers had
metabolic rates 36% higher (57 versus 42 mW bee1)
than hovering bees in Wolf et al.
(1989
), while our maximally
loaded nectar foragers had metabolic rates virtually identical to those of
Wolf et al. Unloaded hovering honey bees have lower metabolic rates and
wingbeat frequencies at higher air temperatures, which aids in stabilization
of thoracic temperature (Harrison et al.,
1996a
,b
;
Roberts and Harrison, 1999
).
Perhaps unloaded bees hovering in cooler air (as in this study) are generating
excess power and heat (relative to that required to hover) for
thermoregulatory purposes, but are using steering mechanisms to maintain
position. Bees flying in cool air could then use this excess power to carry
loads without increasing metabolic rate, thereby apparently increasing
efficiency. In contrast, bees flying at warmer air temperatures (as in the
Wolf et al., 1989
study) may
reduce metabolic heat production and power output toward the minimum required
for bees to hover (approximately 0.3 W g1, Nachtigall, 1989;
Harrison and Fewell, 2002
),
and thus must increase their power output and metabolism in order to carry
loads. This hypothesis could be tested by examining the interactive effects of
load mass and air temperature on hovering metabolic rate and wing
kinematics.
In the filmed pollen foragers, total power output significantly increased
by 1618% (depending on the drag estimate used), correlating well with
the 18% increase in body mass of loaded foragers
(Table 4). This increase in
total power output for loaded foragers is greater than the 6% increase in
metabolic rate, suggesting an increase in muscle efficiency (power
output/power input) during loading. Increases in efficiency with loading have
also been reported for bumblebees (Cooper,
1993).
Honey bees were able to generate this increased mechanical power output
without any detectable changes in wing kinematics (Figs
5,
6,
7). Long-eared bats increased
their wingbeat frequency and stroke amplitude with increasing load
(Rayner and Thomas, 1991).
However, in honey bees, wingbeat frequency of loaded foragers flying in the
metabolic chamber remained constant with increasing load size. These
laboratory data were corroborated by the data from the power output
measurements in which pollen foragers in the field did not increase their
wingbeat frequency while loading. The consistency in the laboratory and field
studies indicates that loaded foragers in the respirometry chamber did not
preferentially utilize air currents near the chamber wall
(Rayner and Thomas, 1991
) in
order to carry loads without added cost, since there were no surfaces within
several cm of the bees being filmed. Similarly, stroke amplitude, inclination
of stroke plane and body angle all remained constant for hovering pollen
foragers, as load increased from 0 to 18%. In contrast, bumblebees with a load
equal to 18% of body mass (this was the mean load of the filmed pollen
foragers) increased their metabolic rate by 15%, their wingbeat frequency by
5% and their stroke amplitude by 4% (estimated from equations provided by
Cooper, 1993
). Changes in
wingbeat frequency and stroke amplitude have been shown to be tightly coupled
to changes in metabolic rate during tethered flight in flies
(Dickinson and Lighton, 1995
).
The observation that hovering honey bees can carry substantial loads without
increasing wingbeat frequency or stroke amplitude may explain the relatively
small effects of loading on metabolic rate.
How are honey bee foragers able to carry significant loads without changing
wingbeat frequency, stroke amplitude or inclination of stroke plane? In loaded
individuals, the mean lift coefficient averaged over the stroke significantly
increased by an amount similar to the increase in induced power within
individuals (Table 4). The rise
in mean lift coefficient averaged over the stroke is likely to have occurred
due to increased use of unsteady power generation mechanisms such as have been
demonstrated for Manduca sexta and Drosophila melanogaster
(Ellington et al., 1996;
Dickinson et al., 1999
). The
hypothesis that the hovering pollen foragers utilize such aerodynamic
mechanisms is also supported by the extraordinary body and inclination of
stroke planes of hovering pollen foragers. Only the hoverfly has been reported
to fly with such an inclined stroke plane
(Ellington, 1984c
). Hoverflies
utilize variable and complex wing movements whose aerodynamic and energetic
consequences remains unclear (Ellington,
1984c
). Pollen foragers of Apis mellifera should provide
an easily available and tractable model for future studies of the aerodynamics
of hovering flight at high stroke planes.
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References |
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