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UNITED STATES DEPARTMENT OF AGRICULTURE

BULLETIN No. 988

Contribution from the States Relations Service
A. C. TRUE, Director.

Washington, D. C. PROFESSIONAL PAPER December 5, 1921

HEAT PRODUCTION OF HONEYBEES IN WINTER.

By R. D. Mixner, formerly Assistant Chief of the Office of Home Econonucs,
States Relations Service, and Gro. S. DemMutH, formerly Apicultwral Assist-
ant, Bureau of Entomology.

CONTENTS.
Page. Page.
Source of heat in winter cluster____ 3 | Method of measuring the work done
Outline of the experiment_______~ 4 bia ther Chis terse es Dae eeee 6
Discussion of the temperature re- Results obtained in the experiment_ 8
sponses in this experiment______ Eas aa ASS 1) 009 0 2 Yr 7 a ale Sa 14

Studies of the behavior of honeybees in winter’ show that these
insects do not hibernate, but throughout the entire winter they con-
sume their stores of honey and generate heat. The results of these
studies further show that after the winter cluster is formed, at 14° C.,
there is an inverse relationship between the temperature inside and
outside the cluster, and that the generation of heat to warm the
winter cluster is solely by muscular activity, such as fanning of the
wings and other movements. These results do not agree with the
conclusions of Parhon? that the honeybee is in part heterothermic.
The work on behavior of the bees during winter, from which the
practical conclusions as to the needs of bees in winter were drawn,
was chiefly on temperature responses, and no data were available
as to the actual heat production of the bees during this season. The
work herein recorded was begun in order that the missing data might
be in part obtained.

From many observations it has long been iow that the duration
of life of the individual worker bees is determined by the work which

1U. 8S. Dept. Agr. Bul. 93 (1914), The Temperature of the Honeybee Cluster in Winter.
By Phillips and Demuth. See also Farmers’ Buls. 695, 1012, and 1014.

?Parhon, Marie, 1909. Les échanges nutritifs chez les abeilles pendant les quatre
Saisons. Paris: Masson et Cie. 57 pp.

55663°—21 1

2 BULLETIN 988, U. S. DEPT. OF AGRICULTURE.

_ they are called upon to do. When there is a heavy honey flow and the
bees are at their greatest activity their lives are limited to about 6
weeks, while during the winter season, if every condition is favor-
able, they may live 6 months. On the other hand, it is clear from
the experience of beekeepers and from the investigations previously
mentioned that if the conditions in wintering are unfavorable the
bees are aroused to great activity. Under these conditions they are
greatly reduced in strength, and even though they may live through
the actual period of winter, they are so depleted in vitality that they
are unable to do the heavy work incident to building up the colony
to full summer strength, and they die off faster than their places
are taken by the emerging bees of the brood reared in the spring.
In the honeybee organism either the power of constructive metab-
olism is entirely lacking or it is far less effective than that of de-
structive metabolism, and the rate of the latter is apparently accel-
erated by the activity of the bees, thus bringing on more rapidly the
impairment of functional capacity which ends in death. The physio-
logical changes which occur in worker bees during this process of
aging are not well understood, but certain facts have been observed
which are significant. Mr. Goodrich-Pixell* has found that the
nerve cells in bees dying of exhaustion are highly vacuolated and the
cytoplasm greatly depleted, thus substantiating the work of Hodge #
and of Smallwood and Phillips.®

Chief among the factors that influence the activity and consequent
welfare of a colony of bees in winter are the condition of the colony
at the beginning of winter (physiological age of the individuals), ex-
ternal temperature, quality of the food used during confinement,
ventilation, humidity, and various causes of irritation. The experi-
ment here recorded was undertaken to study the responses of bees to
some of these stimuli, as measured by heat production, being a con-
tinuation of the work of Phillips and Demuth (loc. cit.) on the be-
havior of bees in winter, in which work the temperature responses
were of greater significance. It was carried out in December, 1915,
and the intention was to continue with similar experiments in other
seasons under a wider variety of conditions than was maintained in
this instance. Such investigations can be conducted only after brood
rearing has normally stopped, and they must be concluded before the
bees are filled with feces, in order that the data may not be com-
plicated by activity due to this disturbing factor. It is therefore

8 Quart. Jour. Micros. Sci. [London], n. ser., 64 (1920), No. 254, Pt. 2, pp. 191-206.
ill. Determination of age in honeybees.

*Jour. Physiol, 17 (1894) Changes in ganglion cells from birth to senile death
observations on man and honeybees.

5 Jour. Comp. Neur., 27 (1916). Nuclear size in the nerve-cells of the bee during the
life-cycle.

.
2

HEAT PRODUCTION OF HONEYBEES IN WINTER. 3)

possible to carry out but one experiment a year with a given colony.
Circumstances incident to the war prevented continuation of this
work, but the results obtained in this experiment are of such economic
importance, as well as scientific interest, that it seems desirable to
publish them without further delay.

SOURCE OF HEAT IN THE WINTER CLUSTER.

The effect of external temperature on the activity of a colony of
bees is conspicuous. The bee is similar to other cold-blooded animals
in that it lacks the means for internal regulation of body temperature
that are found in birds and mammals, and hence the temperature of
its body is affected by that of the surrounding air. As the tempera-
ture of the air in the hive falls in winter the bees become less active
until a certain critical temperature (14° C.) is reached, at which
they undertake by muscular activity, not unhke that of shivering, to
produce heat in order to keep warm. Between the combs and some-
times extending above or below them they form an approximately
spherical and fairly compact cluster, with the bees on the outside
comprising a sort of shell with their heads turned toward the center.
This shell may be several layers thick, the number of layers and the
compactness of the cluster depending upon the size and condition of
the colony and the temperature of the air in the hive. The bees in
this shell remain quiet, except for an occasional shifting of position,
but those in the space inside the shell become very active, moving
about, shaking their bodies, and fanning vigorously with their wings,
thus producing heat to warm the cluster.

By means of many thermocouples fastened in different parts of
the hive Phillips and Demuth (loc. cit.) were able to measure the
temperatures at various points within and around the winter cluster.
They found that when the temperature of the air within the hive
and surrounding the bees was between 14° and 20° C. the bees remain
quietly on the combs but not clustered, their. body temperatures
being, of course, approximately that of the surrounding air. While
the upper temperature limit of this quiescent condition is not defi-
nitely fixed, varying with the condition of the bees and the weather
outside the hive, the lower limit is quite accurately determined by
the needs of the bees. When the air temperature falls to 14° C. the
bees come together to form the winter cluster. If the temperature
falls still lower, they begin to generate heat within the cluster, and
frequently the inner temperature rises considerably above those tem-
peratures at which the bees were able to exist without activity.
Temperatures as high as 30° to 35° C. are not uncommon, and, indeed,
were observed even when the air outside the cluster was as low as

4 BULLETIN 988, U. S. DEPT. OF AGRICULTURE.

0° C. In locations where the outer temperatures fall much below this
the bees are still able to maintain high temperatures, more bees taking
part in heat production. That such high temperatures can be main-
tained in these circumstances indicates that the shell of bees is effec-
tive as a heat insulator, but there is obviously a serious drain on the
vital capacity of the bees employed in producing heat. This is shown
by the rapid slowing down of the fanning of the wings as it con-
tinues.
OUTLINE OF THE EXPERIMENT.

To obtain information regarding the actual amount of work done
by a colony of bees while in the winter cluster, a small colony on
four combs having natural honey stores was placed in the chamber of
a small respiration calorimeter and their carbon-dioxid production
and oxygen consumption were measured for 10 days, while the tem-
perature of the air surrounding the bees was kept just low enough
so that the bees at all times would remain clustered. Throughout
the experiment the temperature of the air surrounding the bees and
at several points within the cluster was taken in order that this
work might be made comparable with the work on the behavior of
bees in winter as indicated by temperature responses. The bees were
located in a box within the calorimeter so constructed that while they
could not escape from it there was opportunity for abundant ventila-
tion. There were 14 thermocouples distributed in the hive in the
calorimeter in such manner that the temperatures in different places
inside and outside the cluster could be ascertained, the leads from
the thermocouples being extended through the outlet in the wall of
the chamber to a potentiometer on the outside. The temperatures
were read every half hour, day and night, for nearly 12 days.

The thermocouples were so placed in the hive as to make it im-
possible for the clustered bees ever to occupy space in which some
of the thermocouples were not located, thus insuring that the
temperatures of the cluster might be obtained wherever the cluster
might move in the hive. The temperatures of all parts of the
hive outside the cluster could also be obtained by the arrangement
of these thermocouples. One of the thermocouples (No. 15) was
located outside the hive and 2 inches from it, thus giving the
temperature of the air of the chamber at this point. The readings
obtained with this thermometer are plotted in the charts on pages 15
to 18. A resistance thermometer was also placed in the chamber, but
at some distance from the thermocouple. Measurements made with
this thermometer are shown in the table on page 8. The two
records did not always exactly agree because the thermometers
were not together.

HEAT PRODUCTION OF HONEYBEES IN WINTER. 5

DISCUSSION OF THE TEMPERATURE RESPONSES IN THIS
EXPERIMENT.

The colony used in the experiment here reported was taken to
Washington from the suburbs some time prior to the beginning of
the experiment. The bees were placed in the calorimeter and then
it was found that the apparatus was defective and it was necessary
to remove them. During the interval before the experiment here
recorded was begun, they were placed outside where they were
free to fly when the weather permitted, and they had several flights
and carried out the dead bees. They were therefore in good condi-
tion at the beginning of the experiment.

For several hours after the hive was again placed in the respira-
tion chamber, the temperatures of the hive and bees were high,
chiefly as a result of the disturbance arising from the handling
necessary at this time. They were put in place at 3 p. m. on Decem-
ber 11, and during the night the temperature of the bees on one
occasion, and in one point only, rose to 35° C. During the night
the temperature of both the chamber and the bees drifted down,
until shortly after noon on the 12th, when they may be considered
as having reached normal quiescence. Just when the bees definitely
formed a winter cluster is not clear from the data, but certainly
when they had reached quiescence they were clustered.

In the graphic charts of temperatures of this colony, records are
included for thermocouples 6, 7, and 12, these being the ones which
were in the center of the cluster, which was located near the top
and slightly to one side of the hive. For comparison with these
the record for thermocouple 15 giving the temperature of the air
of the chamber at one point outside the hive is also included.

It will be observed that on several occasions the temperature of the
center of the cluster (which shifted between thermocouples 12 and 7,
according to the movement of the cluster during the experiment)
rose somewhat abruptly but temporarily, not, however, reaching the
temperatures observed at the time that the bees were placed in the
chamber. While some of the rises may be attributed to mechanical
disturbances, it was not always possible to determine the exciting
cause. This is in accordance with numerous observations made in the
work on the behavior of bees in confinement to which reference has
already been made. Throughout the experiment, of course, heat
production never ceased, and with the bees in this condition of
activity it took but a small disturbance to induce them to generate
shghtly more heat. This is comparable with the periods of activity
that have long been observed in bees wintered in cellars.

It is more important to note that during the 12 days that the
bees were in the respiration chamber the temperature of the cen-

6 BULLETIN 988, U. S. DEPT. OF AGRICULTURE.

ter of the cluster gradually rose from an average of 16° C. on
December 13 to an average of 30° C. on the 22d, though the air outside
the hive kept in the range of temperature from 6° C. to 9° C. This
is in agreement with results obtained by Phillips and Demuth (loc.
cit.) with bees wintered in a cellar which were interpreted as indi-
cating that such an upward drift of temperature of the colony dur-
ing confinement is the result of irritation because of an accumulation
of feces. In the case of the colonies recorded in an earlier publica-
tion,’ one colony showed a slower rise than was found in this colony,
while another, wintered on honeydew stores, showed a more rapid
rise. Since it has been shown that disturbance of any sort causes a
rise in cluster temperatures, it is not entirely clear to which disturb-
ance the rise of this colony should be attributed. Of course, as this
colony was located in a respiration chamber in a busy laboratory,
it was exposed to greater disturbance than would have been the case
in some other experiments or in the average bee cellar, although all
practicable precautions were taken to avoid jar and the apparatus
was cushioned. It is not improbable that the sudden and temporary
increases in temperature may have been due to physical disturbance
and that the cause of the continued rise was physiological dis-
turbance. :

Tt will be noted that beginning at 6.30 p. m. on December 22 the
temperatures of the cluster began to drop. At this time the carbon-
dioxid content of the air in the chamber was high and the oxygen
deficient, as will be explained later. Under these conditions the bees
were more quiet (generated less heat) than when under conditions
which would usually be considered more favorable. The temperature
of the center of the cluster dropped until it reached 23° C. The
reason for the decrease in activity at this time has not been dis-
covered. It was thought that the bees were dying because of un-
favorable atmospheric conditions, but at 5 a. m. on the 23d the tem-
perature again began to rise and continued until it again reached
34° C. Whether this increase in activity was a reaction in response
to physical disturbance or to change in atmospheric conditions made
at this time (see p. 13) is not clear.

METHOD OF MEASURING THE WORK DONE BY THE CLUSTER.

At noon, December 12, measurement of the metabolic activity of the
bees was begun. The respiration calorimeter used for this experi-

ment has been described in a publication of the department,’ but to ~

aid in explaining the conditions of the experiment the principles of

6U. S. Dept. Agr. Bul. 98. The Temperature of the Honeybee Cluster in Winter.
7 Jour. Agr. Research [U. 8.], 6 (1916), No. 18, pp. 7038-720.

HEAT PRODUCTION OF HONEYBEES IN WINTER. 7

the apparatus may be briefly summarized. The respiration chamber
in which the hive was inclosed was ventilated by withdrawing air
from the lower portion, passing it through sulphuric acid to remove
water vapor and through soda lime to remove carbon dioxid, and
returning it to the upper part of the chamber. The increase in the
weights of the sulphuric acid and the soda lime during a given
period indicates respectively the quantities of water vapor and car-

- bon dioxid removed from the chamber. These represent the quan-

tities produced during the period when due allowance is made for
change in the water vapor and carbon-dioxid content of the air as
ascertained from analyses made at the beginning and end of the
period. Oxygen to replace that removed by the bees was supplied
to the chamber from a cylinder, the gas being introduced at a rate
sufficient to maintain a certain volume in the system, as indicated by
a tension equalizing device which served to keep the air in the cham-
ber at the same barometric pressure as that of the laboratory. The
quantity of gas admitted was ascertained from the loss in weight of
the cylinder or by reading a meter through which the gas was passed.
This showed the quantity of oxygen consumed by the bees when cor-
rection was made for change in the residual oxygen content of the air
of the chamber. In making these corrections for variations in re-
sidual gases, changes in temperature and barometric pressure of the
air of the system were also taken into account. By proper attention
to these means of ventilation, any desired conditions with respect to
water vapor, carbon dioxid, or oxygen content of the air could be
maintained.

The temperature of the air surrounding the hive could also be
controlled to a certain extent. In a space adjacent to the metal
walls of the respiration chamber, and protected by a thick heat-
insulating cover, were means for heating and cooling the walls; also
within the chamber was a coil of copper tubing through which cold
water could be circulated to take heat from the air about the hive.
By weighing the water flowing through this coil and measuring its
increase in temperature, the quantity of heat carried out could be
ascertained, which, with necessary corrections for heat from other
sources, would be that imparted to the air by the bees.

8 BULLETIN 988, U. S. DEPT. OF AGRICULTURE.
RESULTS OBTAINED IN THE EXPERIMENT.

Data indicating the physiological activity of the bees are sum-
marized in the following table with others showing the experimental

conditions.
Summary of experimental data.

. Water | |
torent er CO2in | Oxygen| vapor | Carbon | Oxygen| Heat
Date. Stespsael| See ae airin | inairin| taken dioxid con- | gener-
Ghee ichnien chamber.) chamber.) from the |produced.| sumed. a ated.
= air.
| °C. | Percent.| Per cent.| Per cent.| Grams. | Liters. Liters. | | Calories:
Dec atsiseran sas secce Wad LOIS: Sileceenee see 0. 53 15. 2 L7G Ol 642ee Ss aosea eee
ID Yerchal t: BAG taro ernie 6.4to0 8.0| 75to 90 1. 42 16.8 3.4 103.45 Pesedh ess. ee eee
Deesla le. ra = ee 6.1 to 8.2| 77to0 90 . 87 1 eat 5.0 1H By a) OB eee ees oe
CGN Ga = Seca ees |6.3t07.0| 77 to 95 . 81 21.1 8.1 13 s84|522 2 eee
DICCP Las eae ee |6.3t07.6| 72 to 93 1.08 22.6 8.3 L228 pst Sce eae eee
ID CCHIS 235s Set pees | 7.8t09.2| 76 to 95 . 52 24.5 6.9 12.8 |) 52753. Se eae
DCC slO eee ee |7.1t07.8| 50 to 86 . 63 26. 4 26.5 12-9) ees ee
Wee 20s 255 et oe re |6.9to7.9| 49 to 66 . 23 28.9 25. 9 TAS 5a) 22S ee eee
Dee 2S eee |6.8t0 8.3} 47 to 66 1. 40 24.5 22.2 Wd Qs eee Se eee
ID ECR IIA sana ace |7.4t07.7| 45065 <k| 18.2 25a, LGSSeh oe ee | dye eee
Becket eee ee 17.6 to 8.8| 50t055 "29 | 7.3 15.9 14,05. oh eee
| | 3 L
-Potalsiomithingeefirstday: 2= 552 ee eee eee eee eee ee 129.9 138. 4 | 683

With the warm conditions prevailing in the laboratory, the cool-
ing capacity of the apparatus, which had been constructed for work
at higher temperatures, was not sufficient to chill the hive as much

as had been desired when this experiment was planned, consequently-

the bees were not subjected to very low temperatures. ‘Those shown
in the table were measured with an electrical resistance thermometer
suspended in the air above the hive, which was as warm as that in
any part of the apparatus, but the readings on two thermometers
in other parts of the chamber did not differ materially from these.
The figures shown are the lowest and highest temperatures observed
each day, but there was no uniformity in the time at which these
occurred. The fiuctuations in temperature are shown in the curve
for thermocouple No. 15 on pages 15 to 18. The maximum range,
from 6.1° to 9.2° C., was in the vicinity of the temperature which
beekeepers usually consider favorable for bees wintering in cellars.
The daily production of carbon dioxid shown in the table is an
index of the amount of work performed by the bees. This quantity
was derived, in the manner previously explained, from the weight
of the carbon-dioxid absorber, which was taken every 24 hours. Any
error in these figures, with the possible exception of those for Decem-
ber 21 and 22, which are explained later, is believed to be of small
magnitude. The most significant error that could occur would be
due to the fact that the circulation of air was not directly through
the hive, but through the chamber in which the hive was inclosed.
In some cases there might be an accumulation of carbon dioxid in
the hive in one period which would escape in a later period, with a

:
:

HEAT PRODUCTION OF HONEYBEES IN WINTER. 9

corresponding error in the measurements of the quantities for the
two periods; but as there was free space in the small experimental
hive for only a few liters of air, a relatively large change in the
carbon-dioxid content of the air in the hive would introduce only
a very small error in the quantity measured in any period.

The determination of the carbon-dioxid production for the experi-
ment asa whole is accurate. In footing the total the quantity for the
first day is omitted, because the oxygen consumed was not measured
that day. In the 10 days the bees produced 130 liters of carbon dioxid
and consumed 138 liters of oxygen. The corresponding respiratory
quotient is 0.94, which indicates that their metabolism was almost
entirely that of carbohydrate. Their heat production, calculated
from these data, was 688 calories. The quantity of heat measured by
the calorimeter was larger than this, but it involved an error due to
leakage of heat through the walls, owing to the wide difference be-
tween the temperature of the air in the chamber and that in the lab-
oratory, which the apparatus as used could not overcome. Making
such allowance for this error as was indicated by subsequent test of
the apparatus under somewhat similar conditions, the corrected
amount of heat measured was but slightly different from this com-
puted value.

The number of bees in this colony, by actual count, was 9,635. The
average weight of empty worker bees is about 0.075 gram; their total
weight, in round numbers, would be 720 grams. The heat output of~
this colony, 688 calories, was therefore equivalent to 0.97 calorie
per gram for the 10 days, or virtually 0.1 calorie per gram per day.
This is equivalent to a heat output of 7,000 calories per day by a man
weighing 70 kilograms (154 pounds), which is found only in unusual
circumstances. The average individual of this size actively engaged
in hard work at least 8 hours a day would give off about 4,000 cal-
ories in 24 hours. The heat output of lumbermen working hard in ©
the northern woods in a cold winter was found to be about 7,000
calories per man per day, as indicated by their food consumption.
During the period that they were working hardest their hourly ex-
penditure of energy may have been double the average for the rest
of the day, possibly as high as 600 calories per hour, although this
seems doubtful. In certain experimental conditions a well-trained
man engaged in muscular activity sufficient to cause a heat output of
650 calories per hour, which was measured in the same manner as the
heat output of the bees was measured in this experiment, but this
was considered to be severe, exhausting work, almost at the limit of
human endurance, and was continued only for short periods. This
output, per unit of weight, would be larger than that of the colony
of bees taken as a whole, but it will be recalled that the bees actually

55663 °—21——2

10 BULLETIN 988, U. S. DEPT. OF AGRICULTURE.

engaged in the excessive activity of heat production at any one time
are only a small part of the total colony, the rest of them being
crowded together in the shell of the cluster or in empty cells of the
honey comb or standing quietly. The amount of work done by the
bees that are really active is comparable with that done by the man
in unusual conditions, and is therefore relatively enormous; and this
is maintained not only for short periods but through the whole day
and the whole winter.

Moreover, it will also be observed that the temperature conditions
during this experiment were those in which bees are the least active.
In fact, as mentioned previously, the temperature in the respiration
chamber during the experiment was about the same as that which
beekeepers usually maintain in cellars for wintering bees. Colonies
wintered outdoors, especially if unprotected, must endure in many
cases much more severe temperature conditions. Furthermore, this
experiment was conducted at a time of the year when bees are
naturally more nearly quiescent. Bees are usually more active dur-
ing the latter part of winter than during late fall and early winter.
The figures obtained in this experiment, therefore, represent about as
low an expenditure of energy as is ever found in a colony of bees,
except for short intervals. In a preliminary test with this colony
the quantities of carbon dioxid measured were decidedly larger than
these, owing to less favorable conditions.

A hygrometer suspended in the chamber was read at frequent.

intervals. The maximum and minimum readings for each day
are shown in the table. During the first five days the humidity was
allowed to remain at a high level. This was accomplished by keep-
ing the air of the system in circulation only part of the time,
virtually every other hour. During the other five days the humidity
was kept much lower by maintaining a constant circulation of air
threugh the sulphuric acid. There was a very noticeable difference
in the quantities of water vapor removed from the chamber in the
several days of the two periods, owing to the fact that the relative
dryness of the air in the later period was causing a loss of water
from the wood of the hive. No difference in the activity of the bees
that could be ascribed to the difference in water-vapor content of the
air was noticeable in the temperature curves or in the carbon-dioxid
output of the various days. |

The barometer was read at noon each day. There were no sig-
nificant changes in barometric pressure during the course of the ex-
periment. The reading on the 13th was 755 millimeters, which rose
each succeeding day to 769 on the 16th, then fell to 750 on the 18th.
It was 767 on the 19th and for the rest of the experiment remained
within 4 millimeters of this pressure.

-

HEAT PRODUCTION OF HONEYBEES IN WINTER. i

There was no apparent effect on the activity of the bees from
variations in the carbon-dioxid content of the air in the hive, at
least within very wide limits. One column in the table shows the
percentage of carbon dioxid in the air at the time the residual analy-
sis was made each day. These figures tell little of the condition of
the air at any other period during the day; they merely show what
it was after the air of the chamber had been passing through the
soda lime for at least an hour; but unless the bees had been actually
more active at the time the residual analysis was made (which,
according to the thermocouples, did not occur in any instance) there
must have been at least as much and probably more carbon dioxid in
the air previous to the time of the analysis than is indicated by these
figures. It would appear, then, that throughout the whole of the
experiment the carbon-dioxid content of the air in the hive was
appreciably greater than that of normal air, which is probably the
usual condition in a hive; also there were outside variations in the
proportion of this gas in the air, as shown by the data in the table.
On December 21 and 22 arrangements were made to insure a consider-
able excess of carbon dioxid in the air. During most of the time on
these days the soda lime was removed from the train for purifying
the circulating air and the carbon dioxid was allowed to accumulate
within the respiration chamber while the water vapor was removed.
Starting with the content of nearly one-quarter of 1 per cent on the
20th, or almost eight times that in normal air, the increase continued
until in the whole air system of the apparatus, which was about 170
liters, there was included over 10 liters of carbon dioxid before the
period ended on December 21, a proportion more than 200 times that
in normal air. There is no significant change in the curves on page 18
showing the behavior of the bees, to indicate that they were materially
affected by these abnormal conditions. The curve for thermocouple -
No. 7 continued at the same level for nearly 12 hours, then began to
rise slowly ; those for Nos. 12 and 6 fell somewhat for about 12 hours
and then maintained a level for the remainder of the period. There
would appear to be on the whole a quieting of the bees for this day,
but this could be hardly attributed to the quantity of CO, present,
for on the following day, when there was a still greater concentration,
the activity of the bees increased.

From the character of the curves in these two days it would be
expected that the carbon-dioxid production on the 22d would exceed
that of the 21st, but not necessarily by nearly 50 per cent as shown
in the values in the table. It is not unlikely that some of the carbon
dioxid measured on the 22d was produced on the 21st. Uninten-
tionally, replacement of the soda lime in the air circulating system
was delayed until one hour before the close of the first period, and

12 BULLETIN 988, U. S. DEPT. OF AGRICULTURE.

this was not sufficient time to remove all the carbon dioxid from the
system, as was shown by the high percentage of the gas found in the
residual air. It is possible that in this circumstance the air in the
hive had a larger percentage of carbon dioxid than that of the
sample analyzed. On the 22d the air was passed through the soda
lime for nearly three hours prior to the end of the period, in which
case the air in the hive had greater opportunity to become like that
of the system. Even with a carbon-dioxid content of at least 6 per
cent, which was the case on the 21st, the quantity of the gas carried
over in the hive to the next period would be much less than 1 liter,
which would still leave a wide difference between the figures for
carbon-dioxid production in these two days. There is nothing in the
data at hand to suggest a reason for this difference. It is interesting
to observe that the total of carbon dioxid produced for these two
days was almost identical with that of the two days preceding them,
when the carbon-dioxid concentration of the air was low.

The proportion of oxygen in the air at the end of each period is
also shown in the table. These figures simply show the condition at
a given time each day, but they give no definite idea of the propor-
tion of oxygen in the air during the whole day. This would vary
hour by hour with the admission of oxygen, the absorption of water
vapor and carbon dioxid, and with changes in the temperature of
the air, but on the whole would be somewhere in the range between

the proportion at the end of one period and that at the corresponding |

time in the period preceding or following. The figures therefore
show that there was a continual increase in the proportion of oxygen
from the 18th to the 20th, then a decrease to the 23d.

The low proportion of oxygen in the air at the beginning of the
experiment was due to the fact that air rather than oxygen was
supplied to the system to replace the carbon dioxid and water vapor
removed during the preliminary period and to maintain a sufficient
quantity of air in the system while the apparatus was being chilled
before the experimental conditions were established. After the ex-
periment began, replacement was made by oxygen until the 20th,
when the requisite volume was again maintained by admitting air,
in order to reduce the proportion of oxygen in the air of the system.
No effect that could be ascribed to changes in the oxygen content
of the air was observed until the last day of the experiment. On
that day not only water vapor and carbon dioxid, but oxygen also
was removed from the system by passing the circulating air through
a solution of potassium pyrogallate before returning it to the
chamber. This was continued until the proportion of oxygen in
the air, which was only 18 per cent at the beginning of the period,
was very greatly reduced. After a few hours the circulation of air
was stopped and the water vapor and carbon dioxid allowed to ac-

HEAT PRODUCTION OF HONEYBEES IN WINTER. 13

cumulate in the air of the system in which there was a deficiency
of oxygen. The effect on the activity of the bees was soon apparent ;
the temperature curves, which for some reason had begun to rise,
very shortly turned in the opposite direction and continued to fall
for about 12 hours. The proportion of oxygen was then 12 per cent
and it was thought that the bees had probably been suffocated. Eight
hours before the time at which the period would regularly end the
air of the system was again put in circulation and water vapor and
carbon dioxid removed, oxygen being also removed at the same time.
This was continued until the close of the period (which was also
the end of the experiment) in order that the air of the system might
be quite thoroughly freed of carbon dioxid. After the circulation
of air was resumed the bees again indicated that they were living,
and during the time that the air-purifying system was operating
their activity increased until by the end of the experiment the tem-
perature curve had reached as high a point as at any time during
the course of the experiment, even though the proportion of oxygen
in the air was low. Analysis of the sample taken at the end of the
period showed only 7.3 per cent of oxygen. |

If the decrease in the activity of the bees in this instance was due
to atmospheric conditions in the hive, the cause was probably excess
of carbon dioxid and water vapor rather than deficiency of oxygen.
Though the proportion of oxygen in the air was decreased from 18
to 12 per cent in 16 hours, it is doubtful if this alone would have
an appreciable effect upon the physiological activity of the bees. In
experiments with men in atmospheres about as deficient in oxygen as
this, there was no noticeable effect upon their metabolism. In these
experiments, however, there was no such excess of carbon dioxid and
water vapor as in the experiment with the bees.

It is possible, as intimated on page 6, that the reason for the
increase in activity of the bees after the circulation of air was re-
sumed may have been physical disturbance. Since it was thought
that the bees were dying, movement about the laboratory was some-
what less restricted when the air-circulating device was started, al-
though care was still taken to avoid jarring the calorimeter. ‘The
circulation of air through the calorimeter could hardly have caused
any disturbance of the bees, because the low rate, while sufficient to
keep the air in motion, could not produce any current that would stir
the hive. It is also possible that, since the removal of oxygen from
the air was continued during this period, the proportion of oxygen
in the air eventually became so low that the bees had to respire more
rapidly to obtain a sufficient quantity of this gas. It would be ex-
pected, however, that this effect would be manifested somewhat later
in the period than the time at which activity was renewed.

14 BULLETIN 988, U. S. DEPT. OF AGRICULTURE.

In considering the circumstances on this last day of the experi-
ment with bees it is interesting to recall observations made in the
study of the effect of ventilation on men, that the sensations produced
by “bad” air are not experienced when the air is stirred. If this
indicates an actual difference in physiological conditions in the differ-
ent circumstances, then it is not inconceivable that something analo-
gous to this was true of the bees on this day. The stirring of the
air when the circulation was resumed may have served to remove
some cause of depression that was effective when the circulation was

stopped.
SUMMARY.

In the colony of bees under observation in the respiration chamber
the expenditure of energy was reduced to the lowest limit by the
maintenance of favorable temperature and by the avoidance of all
disturbing factors, so far as possible. Under these circumstances,
rarely found in the apiary, the energy produced by the bees, as
measured by the carbon dioxid and-water produced and the oxygen
consumed, was greater, according to body weight, than that pro-
duced by a man when working at hard manual labor, when we take
into consideration the fact that the work was done by only a rela-
tively few of the bees in the cluster. Even assuming that the work of
the period were equally divided among the bees, their energy output
per unit of body weight is higher than that of the average laborer.
When we take into consideration the fact that usually the bees do
not have such favorable conditions in winter as these bees had, it is
clear that the energy output 1s enormous in the average apiary.

HEAT PRODUCTION OF HONEYBEES IN WINTER.

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BULLETIN 988, U. S. DEPT. OF AGRICULTURE.

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CHaArT III.—Temperatures shown by thermocouples 6, 7, 12, and 15, from 10.30 p. m., December 17, to 2.30 a. m., December 21.

17

18

BULLETIN 988, U. S. DEPT. OF AGRICULTURE. |

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