The Lives of Bees
Page 29
wing movements—adult bees can boost greatly their metabolic heat pro-
duction, up to about 500 watts per kilogram (230 watts per pound)! It is,
therefore, the adult worker bees’ metabolic rate that is varied to adjust a
colony’s heat production. Workers engaged in heat production for the
colony are almost indistinguishable from resting bees; both stand motion-
less on the combs. However, one can sometimes observe a worker press
her thorax firmly onto the cap of a cell containing a pupa and then stand
motionless in this posture for several minutes. She is generating heat (with
her flight muscles) to raise her thorax temperature to approximately 40°C
(104°F). This will boost the temperature of the brood cell by a few de-
grees. Also, measurements of single bees or small groups of bees confined
in a controlled- temperature respirometer show clearly that worker bees
will resist chilling by dramatically raising their metabolic rate. For exam-
ple, whereas a group of 10 workers at 35°C (95°F) showed little elevation
of thorax temperature (36°C/97°F) and had a low metabolic rate (29
watts per kilogram/13 watts per pound), bees in a group held at 5°C
(41°F) boosted their metabolic rate to 300 watts per kilogram (136 watts
per pound) and so maintained a thorax temperature of 29°C (84°F), far
above the ambient temperature.
In nature, where each bee works together with thousands of fellow
colony members in resisting cold, the process of the workers increasing
their heat production operates together with the process of them reducing
their heat loss by clustering. Figure 9.7 shows how the bees coordinate
these two thermoregulatory mechanisms. Heat production increases as the
ambient temperature drops from 30°C (86°F) to about 15°C (59°F) and
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20
stable cluster
clustering
no cluster
15
10
5
Metabolic rate (watts per kilogram of bees)
0–20
–10
0
10
20
30
Temperature (˚C)
Fig. 9.7. Metabolic rate of an overwintering colony in relation to ambient tem-
perature. The colony was inside a controlled- temperature cabinet that served as
a metabolic chamber. Each data point shows the minimum metabolic rate of the
colony during a 24- hour run at a fixed temperature.
does so again when it falls below about 10°C (50°F), but it decreases
sharply when the ambient temperature declines from about 15°C to 10°C.
As discussed above, this is the temperature range over which a colony
coalesces into a well- insulated cluster and can resist deepening cold en-
tirely by reducing its heat loss. Because clusters continue to shrink down
until temperatures reach about −10°C (14°F), reducing heat loss continues
to play a role in colony thermoregulation down to this temperature,
though as we can see in Figure 9.7, it is accompanied by rising heat produc-
tion. Presumably the reason colonies do not initiate their cluster formation
at higher temperatures, and thereby reduce their nest- heating costs, is that
coalescing into a tight cluster disrupts other colony operations, such as
foraging and food storage.
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234 Chapter 9
COOLING THE NEST
A honey bee colony sometimes needs to cool its home to keep the tem-
perature in the brood- nest region below about 36°C (97°F). Prolonged
temperatures just 2°–3°C (about 3°–5°F) higher than this will disrupt the
development of immature honey bees undergoing metamorphosis—that
is, the transformation of a larva to an adult. Some of the danger of nest
overheating arises from the colony’s inevitable production of metabolic
heat. The resting- level heat production by brood and non- incubating adults
is much lower than that of actively incubating bees; nevertheless, even
non- incubating bees produce enough heat that, when the air temperature
outside a colony’s nest rises above about 27°C (80°F), the colony can face
a threat of its brood nest overheating. This danger is more common for
managed colonies residing in thinly insulated hives fully exposed to direct
sunlight than for wild colonies inhabiting well- insulated tree cavities
shaded by neighboring trees. But whenever overheating of the brood nest
arises, the bees deploy mechanisms for keeping their nests cool that are as
effective as the ones they use to keep them warm. For example, when
Martin Lindauer placed a colony of bees living in a thin- walled, wooden
hive in full sunlight on a lava field near Salerno, in southern Italy, the
colony’s maximum temperature inside its hive never exceeded 36°C
(97°F), even though the temperature of the air at hive level outside rose
to 60°C (140°F) and the temperature inside an unoccupied hive nearby
rose to 41°C (106°F). To prevent nest overheating, colonies deploy a bat-
tery of cooling mechanisms in a graded response. They start with the
adults spreading out inside the nest and partially evacuating it (to reduce
internal heat production and foster heat loss by convection), followed by
fanning (to create forced convection), and finally spreading water on the
combs (to turn on evaporative cooling).
The dispersal of adult worker bees inside their nest cavity as its tem-
perature rises is an extension of the cluster expansion that starts when
the temperature inside the nest cavity rises above about −10°C (14°F).
The air temperature outside the nest at which fanning behavior begins to
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Temperature Control 235
supplement this dispersal by the bees is variable and depends on such
factors as the sun exposure of the nest structure (tree or hive), the insula-
tion of the nest cavity’s walls, and the strength of the colony (which influ-
ences its overall rate of heat production). What matters to the colony,
ultimately, is keeping the internal nest temperature below 36°C (97°F),
and many observers have reported bees initiating strong ventilation of
their nest when its internal temperature approaches this critical limit for
the brood- nest temperature. The fanning bees deploy themselves through-
out the nest, aligning themselves in chains to drive the air along existing
(unidirectional) currents. Additional fanning bees stand outside the en-
trance opening with their abdomens pointing away from it, pulling air out
of the nest.
Recently, Jacob Peters and colleagues at Harvard University have mea-
sured the velocity of airflow at the entrance and have reported that it can
be as high as 3 meters per second (ca. 10 feet per second), hence 10.8
kilometers (6.7 miles) per hour. This occurs when the temperature of the
air that is being expelled is over 36°C (97°F), indicating that the colony’s
brood- nest temperature is dangerously high. An earlier investigator, Engel
H. Hazelhoff, working in the Netherlands,
measured the volume of airflow
through a nest created by the fanning bees. He constructed a hive with two
openings, one at the top, connected to an airspeed indicator (anemome-
ter), and one at the bottom, serving as the hive’s entrance. With this hive,
he could accurately measure the airflow through the hive produced by the
fanning bees. Once, when there were 12 bees fanning steadily, all spaced
evenly across the 25- centimeter- (10- inch- ) wide entrance of his hive,
Hazelhoff measured the rate at which cool, fresh air flowed in through the
top opening: up to 1.0–1.4 liters (0.26–0.37 gallons) per second!
Hazelhoff also discovered that a high level of carbon dioxide in the air
inside a nest, not just a high temperature, will stimulate strong fanning by
bees for ventilation. This means that nest ventilation by honey bees func-
tions not only in social thermoregulation but also in social respiration.
Remarkably, the carbon dioxide content of the air inside a colony’s nest
cavity when it is not being actively ventilated by the bees is 0.7%–1.0%,
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236 Chapter 9
which is 20–30 times above the normal percentage of carbon dioxide of
atmospheric air (0.03%–0.04%). The ability of honey bee colonies to
thrive under such “stuffy” conditions shows us how remarkably these social
bees are adapted to living in large numbers inside tree cavities with small
openings.
In chapter 5, we saw how the tree- cavity nests of wild colonies often
have just one entrance opening and that often its size is quite small, only
10–20 square centimeters (1.5–3.0 square inches). This raises the ques-
tion of how the bees achieve sufficient airflow through just one small en-
trance opening to prevent buildup of excess heat (and carbon dioxide) in
the nest. The work of Jacob Peters and his colleagues shows us how the
bees solve this problem. The fanning bees distribute themselves asym-
metrically around the entrance opening so that air continuously enters
and exits at different locations around its perimeter (Fig. 9.8). The clus-
tering of the fanning bees also has the benefit of reducing the fluid friction
of the air, which increases the ventilation efficiency. Evidently, the fanning
bees achieve this efficient partitioning of the inflow and outflow air-
streams by sensing the air temperature—which is highest where the air
is shooting out—and aligning themselves with the direction of the airflow
wherever the air is hottest. In other words, the fanning bees use the air-
flow itself, not direct interactions with the other fanners, to efficiently
partition the outflow of hot air and the inflow of cooler air through their
nest’s entrance.
When the bees cannot cool their nest adequately by means of worker
dispersal and nest ventilation, they bring the power of evaporative cooling
to the problem. Water absorbs a great deal of heat (energy) when passing
from a liquid into a gas, so water evaporation is a wonderfully powerful
means of cooling objects. We all know this from our experience in sweat-
ing when our bodies are overheating. A vivid demonstration of the power
of evaporative cooling in the context of beekeeping comes from a report
of a catastrophe by P. C. Chadwick, a beekeeper in southern California.
One day in June 1916, when the midday air temperature rose to 48°C
(118°F), his bees brought large amounts of water into their nests and pre-
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x
1.2
3
Te
36 mperature (˚C)
0.8
2
bee density
air velocity
0.4
1
34
air temperature
0
locity (m/s) 0
32
Ve
–0.4
Density (fanners/cm)
–1
30
0
5
10
15
20
25
30
35
x (cm)
Fig. 9.8. Top: Worker bees ventilating at the entrance of a hive, in a cluster on
the left side of the entrance opening. Bottom: Air velocity (green), bee density
(black), and air temperature (red) across the hive entrance. The outflow and
inflow velocities are indicated by positive and negative values, respectively.
vented the melting of their combs. By 9:00 p.m., the temperature had
dropped to 29.5°C (85°F), but at midnight a hot breeze blowing in off the
desert raised the air temperature back up to 38°C (100°F). The supplies
of water in the colonies were soon exhausted, no more water could be
collected until daylight, and the combs of many of Chadwick’s colonies
softened and collapsed during the night.
The details of the acquisition, handling, and storage of water in nests,
and the regulation of its collection, have received close attention in recent
years. For example, it is now clear that some of a colony’s workers that are
old enough to work outside the hive will specialize in water collection,
and that they can travel to water sources up to 2 kilometers (1.2 miles)
away, even though they must make their return flights with their crops
(fuel tanks) filled with water. That some of the workers in a honey bee
colony are water- collector specialists makes sense, because colonies need
water daily, sometimes for evaporative cooling, other times for diluting
stored honey and producing glandular secretions to feed brood, and still
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238 Chapter 9
other times to humidify their nests to prevent desiccation of the brood.
There are times, too, when the adult bees need water simply to relieve
personal thirst—that is, to maintain osmotic homeostasis in their bodies.
I witnessed bees in a broodless colony responding to powerful sensa-
tions of personal thirst one January day in Ithaca, when deep snow covered
the land but a strong sun had warmed the air enough that some of the bees
living in the observation hive in my office were making flights outside. At
first, I figured that these bees were simply conducting cleansing flights, but
then I noticed several bees inside the hive performing extremely vigorous
waggle dances for a site just outside the building. These were water collec-
tors! They had found puddles of snowmelt in the parking lot, and upon
entering the observation hive they were being mobbed by thirsty bees.
One of these water collectors produced the most exuberant and persistent
waggle dance that ever I have seen; it lasted for 339- plus dance circuits
without a pause. I must write “339- plus,” because I did not see the start of
her eye- catching performance.
At the time, I thought that the extraordinary wintertime thirst of these
bees was not natural. I figured it was an artifact of their living in an obser-
vation hive located in a heated room, an arrangement that prevented their
metabolic water from condensing on the walls of their
nest cavity (the
observation hive). Since then, I have learned that even bees living outdoors
in cold wooden hives can get extremely thirsty in winter. Ann Chilcott, a
beekeeper in northern Scotland, has recorded bees collecting water in
January and February, even under cloudy skies, as long as the air tempera-
ture is above 4°C (39°F) (Fig. 9.9). In a related study, Helmut Kovac and
colleagues at the University of Graz, in Austria, used an infrared camera
to measure the thorax temperatures of wintertime water collectors while
they were drinking from a water source near their hives. They discovered
that when a wintertime water collector is at a water source, busily loading
up on water, she activates her flight muscles (i.e., she shivers) to keep her
thorax temperature always above 35°C (95°F), even when the air tempera-
ture is as low as 3°C (37°F)! Evidently, this ensures that the water collector
can complete a short flight home before her thorax temperature falls—
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Fig. 9.9. Worker bee loading up at a moss- filled water source on a chilly January
morning, near Inverness, in northern Scotland.
from the wind chill during flight—below the critical 25°C (77°F) needed
to produce a wingbeat frequency high enough for flight.
These reports of bees desperately collecting water at low temperatures
in winter make me wonder whether some condensation on the walls inside
a hive or tree cavity might be beneficial to a colony during winter. Perhaps
this helps explain why colonies in the wild avoid nest cavities with top
entrances, which allow warm, moist air to escape. Derek Mitchell has
explained that if a nesting cavity is well insulated and lacks a top vent hole,
then overwintering bees will not have cold condensation dripping down
on them, because the temperatures of the ceiling and walls above the bees
will be above the dew point. There will be condensation on the cooler
walls below the bees, but this may be beneficial, because it provides them
with a ready source of fresh water. This may help explain, too, why bees
living in natural nest cavities coat their walls with propolis: the condensed
water is not lost to the bees by soaking into the wooden walls of their
home.
During the warm months, most honey bee colonies have ready access