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Cony. Biochem. Physiol. Vol. 7iA. No. 4, pp. 679 to 689. 1982 0300-9629’S?/ I ?0679- I 1$03.00/O
Printed in Great Britain. 0 1982 Prrgamon Press Ltd
TORPOR AND METABOLISM IN HUMMINGBIRDS
KKISTINE KRUGER. R. PRINZINGER* and K.-L. SCHUCHMANN
Institut fiir Biologie III, Lehrstuhl Zoophysiologie. Auf der Morgenstelle 28.
7400 Tiibingen 1, W. Germany
(Rrcrirrd 8 Marc,h 1982)
Abstract- -1. Body temperature and diurnal cycle of energy metabolism at different ambient tempera-
tures (+2 to +25 C) were tested for 18 different hummingbird species from different biotopes and of
different body-masses (2.7-17.5 g).
2. The level of resting metabolism (night time) reaches 3(&70”,, of activity metabolism values (day
time). The mean is almost exactly 50”,.
3. The mean metabolism-weight regression line of the night time values follows the equation
M = 0.67 x l+‘“.73 (bf = energy metabolism in kJ/hr, W = body weight in g). That of the day time
values is hf = 0.83 x W”.53.
4. Resting metabolic rate of the hummingbirds is considerably higher than the theoretically expected
value for nonpasserine birds, even when torpor values are taken into account.
5. All species tested show torpor during night time, independent of ambient temperature. feeding
situation or the environment (biotope) of the species.
6. In comparison to the resting metabolic rate. metabolism during torpor decreases by 6&90”,, to a
relatively constant level. This level is not (linearly) temperature-dependent (~1 = 0.008. r = 0.22). never-
theless many species show a minimum at cu 15-20~~‘.
7. During torpor. body temperature normally reaches the level of the ambient temperature but never
falls below 18 -20 C. The normal values during the daytime range from 38 to 40 C and during night time
from 35 to 37 C.
INTRODUCTION
Torpor is a numbness-like state with a large reduction
of body temperature and metabolism. Entrance into,
maintenance of and arousal from torpor are active
and regulated processes. Torpor has no ill effect on
the animal (Swan, 1974). Torpor has been described in
many animal groups. Raths & Kulzer (1976) list
among the Mammalia bats, tenrecs, prosimians.
various rodents and shrews. In birds torpor is known
from species of the orders Caprimulgiformes, Apodi-
formes. Trochiliformes and Coliiformes. among
others.
Hummingbirds, which include the smallest bird
species, inhabit nearly all climatic and altitude zones
of America and are one of the most species-numerous
groups among the nonpasserines. Corresponding to
their low body size and their mode of life-they feed
mainly on nectar during flight, hovering from blos-
som to blossom-they show a high energy demand
but only minimal storage capacity. The ability to
reduce energy consumption drastically during the
night may be the only possibility for these birds to
survive in, for example, the Andes at a height of
4000 m and a mean ambient temperature of +_5 C.
For these reasons, investigations of torpor are of
special interest in humming-birds.
From many previous works on hummingbirds
came the assumption that these birds only undergo
torpor in the course of acute energy deficiency and
low ambient temperatures. From the considerations
stated above, we held another question to be more
reasonable: Do hummingbirds practice this ability
* To whom requests for offprints should be sent
only in a state of emergency or is torpor used
1. for
conserving energy as often as possible, i.e., regularly
each night. not controlled by a physiological de-
ficiency but (perhaps) by a endogenous rhythm’! To
clarify this question we investigated the energy metab-
olism of hummingbirds of different body sizes from
different climatic zones (tropics. desert, temperate
zone).
hlATERlALS AND ,METHODS
Seventeen different hummingbird species and one hybrid
were used in this study. All birds were wild-caught with the
exception of the hybrid. and underwent a complete moult
in captivity. Their exact age is unknown but is estimated to
be between 3 and 5 years. The hummingbirds were housed
at a constant temperature of 20’ C with a 12: 12 hr photo-
period (lights on 07:3@19.30 hours). Some birds were kept
individually in small, cloth cages (0.5 x 0.5 ml and others
were housed. several birds together. in an aviaiy (3 x 5 m).
They were fed with Drosophi/a and a lo”,, water-solution of
saccharose. fructose and glucose as well as amino acids and
aromatic substances. Water for drinking and bathing was
available ud lihitum.
Body temperature was measured (under the plumage) in
the axilla of the birds with a digital thermometer (Testo-
therm KG) (accuracy about +O.Z’C). To prevent a warm-
ing of the birds, and by this a falsification of the results, the
completely motionless hummingbirds (during torpor) were
layed in a cloth while measurements were taken and not
touched directly by hand.
During the measurements of metabolism, the birds were
isolated in cylindrical Plexiglas respirometer chambers
(20 x 40cm) containing a perch and a feed tank (Fig. 1).
The respirometer chambers were placed in a tempersture
controlled cabinet, regulated with an accuracy of + 1 ‘C.
The air flow through the chambers was adjusted to the
679
680 KR~ST~NE KRUGER et LI/.
Fig. I. Schematic vie\\ of the plexiglas rcspirometer
chamber used in this study. Shown is a vrr> small hum-
constant value of 20 l:hr. Oxygen consumption and carbon
dioxide production were measured in an &en flow system
with a Hartmann & Braun MAGNOS 2 T and URAS 2 T
paramagnetic 0,.analyzer. and an infrared COz-analyzer.
All gas volumes were reduced to standard pressure and
temperature conditions (accuracy of all data: _t I”,, of the
measuring range).
During the measurements of metabolism the photo cycle
was adjusted to provide the birds with the normal diurnal
cycle. For this reason they were also fed at the same times
as under normal housing conditions (9.00 hr and 16.00 hr):
food was given trd lib. The hummingbirds normally became
quickly familiar with the testing conditions, Usually within
one day they sat calmly on the perch and showed a normal
food intake. As they could fly in the respirometer chambers
(hovering-flight). measurements of awake ( =active) birds
do not represent basic energy metabolism.
To test for a possible effect of a circadian rhythm of
metabolism on the results, each ambient temperature was
monitored continuously for at least 24 hr. Each experiment
lasted 46 days without interruption. During the duration
of the experiment the birds remained in the respiratory
chambers. and were only removed for taking measure-
ments of body temperature and body mass. The results of
the first day measurements were not evaluated, as it was
observed that the birds were very nervous at the beginning
of the testing procedure and that they needed some time to
habituate to the changed housing conditions.
The measuring apparatus, with six changing channels.
allowed the tebting of 5 individuala at the same time. The
last channel served as a zero-point control.
RESL’LTS
As expected, there were greater fluctuations in
energy metabolism during the day time. caused by the
fact that the hummingbirds could fly in the respirat-
ory chamber. Nevertheless the values show a temper-
aLure-dependent characteristic mean (Table 1). With
the beginning of darkness, energy consumption of all
hummingbirds decreased very quickly to an essen-
tially lower level, remaining relatively constant during
the entire night provided that the bird did not
undergo torpor. The values of night time metabolism
range between 30-70”” of the mean day time values
with an average of almost exactly 50”,,.
When entering torpor, energy consumption de-
creased once more by 6&90”,, to a very steady level.
Duration and level of the torpor phase are different in
the individual species.
At the beginning of the light phase energy con-
sumption increased very quickly. After a short time
(5-20min.) the hummingbirds had reached their nor-
mal day time level. The rate of metabolic increase
during this awakening phase may amount to over
loo”,, per 5 min.
The respiratory quotient (RQ) during day time had
a constant value of 1 (which means that mainly sugar
was metabolized). After the beginning of the dark
phase (end of food intake) RQ remained at this value
for about 10min and then decreased within about
40 min to a lower mean value of 0.8 (fat metabolism).
which then remained steady during the entire night.
In the morning after food intake, RQ again increased
to 1 within half an hour after the beginning of the
light phase.
Body mass of the hummingbirds also underwent a
distinct diurnal rhythm. On average the birds weighed
(dependent on the weight class) about O.lLO.3 g in the
evening more than in the morning. This amount was
stored during the day time as an energy reserve for
the night.
Duriny cccti&y. The mean values of activity metab-
olism increased with falling ambient temperature. The
changes of energy metabolism were different from
species to species (see Table 1). The mean of the corre-
sponding regression line during the day time is
M = -6.67 T, + 515 ( + 151). The metabolic adap-
tation to changing ambient temperature was ac-
complished very quickly. often within a few minutes.
The increasing rate was in this case largely propor-
tional to the temperature fluctuations.
Dwingg rhr night. The dependence of energy con-
sumption on the ambient temperature was distinctly
less during the night than during the day. If the
regression line is calculated without taking into
consideration the torpor values. it lies at bl =
- 3.85 T, + 252 () 180). The average temperature
dependence of night time metabolism including tor-
nor values is remarkablv small (m = -0.84). In many
hummingbird species the slope of the regression line
(ru) even inverted (see Table 1) during the night. Al-
together there existed greater interspecific differences
in night time metabolism than in day time metab-
olism.
Durirly! torpor. A temperature dependence of energy
metabolism during torpor could not be confidently
confirmed statistically. This means that the level of
torpor metabolism is not linearly temperature-depen-
dent. The mean regression line follows the equation
M = 0.008 T, + 48 (i 36). Nevertheless. for many
species there seemed to be a minimum of energy con-
sumption at about 15-20 C (the range of deepest ob-
served body temperature). There was also no statisti-
cally significant correlation to be found between
ambient temperature and the occurrence of torpor
IOOO- O.UNOERUOOOIf
coo-
;
: 600.
:
2
~ 400.
Torpor and metabolism in hummingbirds
1000 1
R.MULSRNT
800
-
1’5 21 3 9 15
1000 1 U.BENJRMINI
800~ ,“,
oJ
15 21 3 9 I5
10007 TSCIIULbS
I5 2’1 i i 1’5
1000, CH.OENONE
""li Ui:iEIPNDRI '2'
3 iI 1'5 I5 21 3 9 15
1000 1 HYBRID
800
z i
200 1
15 y-1
21 3
Fig. 2.
682
1000
Fig. 2-cont.
F.NELLiVORR ;
1; ---
21 3 9 ‘5.
O.ESTELLR I
1s 2, ;
R.CUPPIPENNIS 1000
800
;
: 600
:
1
400
z
200
0
CT
: 600
_
7
z 400
“00
0
'oool O.ESTELLR 2
1 I
800
800
-
;
: 600. I 0
: 600
;
7 i
IS 21 3 9 is
1000
I
E.JUGULRRIS
800
800
1 I
0
: 600
400
r
200
1000
tloo-
;
: 630.
;
7
400.
x
200
Fig. 2. Diurnal cycles of energy metabolism at dmerent amblent temperatures. measured contmuously
on consecutive days. The black and white bars represent the dark light cycle. 25 (-. ‘0 c.
15 c. ----- 10 c. 5 c.
Torpor and metabolism in hun~nlln~birds 683
fr = 0.22) or the length of the torpor phase (I’ = 0.1) The mean metabolism-weight regression line of
(see Table 1). In the same way as during the resting the day time values follows the equation M =
time (night), metabolism adjusted during torpor to a 0.83 x W”.5h (!\I = metabolism in kJ)hr and L+’ =
very steady level. This adjustment did not usually body weight in g). That of the night time values is
begin until about 13.OCOI.00 hr. Before the beginning M = 0.67 x I+” ‘3. During the day time the heavier
of the light phase. torpor normally first passed over to hllmmingbirds were calmer than the smaller species.
normal sleep, but several birds also awoke directly This results in a lower regression exponent for the day
from torpor shortly before day-break. time values.
During torpor the minima1 body temperature of all
tested hummingbirds lay between I8-70°C and during
night time (normal sleep) between 3S-37°C (see
Table 3). Values beyond 18’C did not occur. Table 3
shows only those values where comparable day and
night time data were available.
DISCUSSION
When disturbed during torpor. the hummingbirds
showed a number of typical displays occurring at cer-
tain phases of torpor. In the deepest phase they
showed no visible reaction at all. However. a short
time after a waking-stimulation (for example a touch)
they strcched their wings abruptly and gave shrill
cries (like the syueak of a mouse) that are never heard
from waking birds. At the same time we observed
clamp-reflexes of the birds’ feet. When placed on :I
perch they grabbed it and remained sitting in an
upright position. During the deep phase of torpor and
during the awakening period the claws were closed
and no clamp-refIex occurred. During undisturbed
awakening from torpor there also occurred several
distinctively different phases of display in which
breathing frequency, body stance and movement of
the hummingbirds changed characteristically from
minute to minute until the bird was awake.
Both during day and night. and at all temperatures
tested. the metabolism of all the hummingbirds was
considerably higher than the theoretically expected
value for nonpasserine birds. This fact could also be
~~n~rrned for periods during which torpor occurred.
From the in~~estigations of other authors, the fol-
lowing equations were obtained (defined as above):
M = 0.1 SS x I+“.” for passerines (Dawson & Hud-
son. 1970); M = 0.081 x Cyo.” for nonpasserines
(Aschoff & Pohl. 1970). It is commonly assumed that
the basal metabolic rate of passerine birds is higher
than that of nonpasserines. Prinzinger & HCnssler
(1980) investigated small representatives of nonpasser-
ine birds and found no difference in the metabolism-
weight relationship in comparison to passerine birds
of the same weight. The calculated equation (M =
0.138 x 14’O-lh) lies between those given above. but
closer to that of the passerines. The slope of all three
lines is about the same. The values for our much
smaller hummingbirds are significantly higher although
the slope is again the same. The correlation between
basal metabolic rate and body weight al night time
with an ambient temperature of 2S’C (calculated in-
cluding torpor values) follows the equation hl =
0.2 i 2 x f4’“-T’. Hummingbirds therefore. though
belonging to the nonpasserine birds. have ;I higher
basal metabolic rate than all other birds species inves-
tigated thus far. The values are equal to those of
mammals of the same weight. for example shrews
(Fans & Sicart. 19X0) (Table 5). This means that the
level of n?et~lbolism does not depend on the system-
atic order ;I bird belongs to inonpasserincs~~ passer-
ines) but rather on its body mass. The corresponding
level of metabolic rate of hummingbirds and Croci-
durinae indicates that this rule is valid over ;I very
Nide range; while in the same lveight class. humming-
birds and ~hitet[~~~thed shrews belong to {cry difkr-
Table 1. Mean energ! tnctabolism (J g,hr) at direrent ambient temperatures during &I! (Iqt number).
during night (2nd number1 and during torpor (3rd numberl
715
37x
60
6X9 636
261 311
101 76
475 450
217 242
85 75
- 456
- 308
9x
532 598
249 390
* 61
- 615
- 349
52
604 507 J9.1 \ = -15.3 r: -i-S47
791 234 203 ; = -I 1.6 X +3x0
36 44 * ?’ = - 1.6 x +71
- 419 479 42x y= -13.6 x +734
778 203 180 y = -5.4 x +318
15 26 26 y = -4.0 x fill
- 416 477 375 y = -3.5 x f491
- 205 231 I44 y = -3.2 x +X5
51 26 63 y = -1.9 x +8x
- 5’0 313 300 y = -13.5 x +624
- 228 165 140 y = -11.3 x +408
*
.- 25 68 4 = -2.1 x +99
- 6’6 - 471 Y = -3.3 x -+605
.- ‘97 - 235 ; = -2.9 x +33-!
- * * *
._ 533 535 478 y = -8.2 x -+638
228 ‘28 170 y = -10.7 x l-431
* * * *
I -0.96)
I - 0.90)
t-0.65)
(-0.86)
(-0.84)
( - 0.90)
( - 0.64)
i-0.65)
t -0.64)
I-0.81)
t -0.98)
I - 0.50)
I - 0.44)
i-0.35)
( - 0.94)
( - 0.92)
684 KRETINE KRUGER et al.
?‘;thic I -- cont.
Species, sex. mesn body mass tg)
_
_
_
_
_
_
_
45x
196
*
_
_
_
_,2h
*
37
396
2%
35
Mean day time regression
Mean night time regression
(without torpor valuesf
Mean torpor regression
_
370
17S
I‘
367
726
*
3X6
344
*
3%
228
52
35x 48? 446 y = -C.l x +547
194 244 751 y = -1.3 x +XS
35 30 36 y=O.? x +28
596 358 ‘14 y = -20.9 x +814
260 1.51 120 y = -9.8 X +377
* I3 * *
SlZ 5’79 513 y = 4.8 x +415
215 2% 254 y = 3.5 X i 171
29 3s 102 ]i=CW x 1-62
421 4’10 431 y = -2.8 x +a4
2417 2% 1’1 )’ = -2.0 X +1x0
47 -il 97 y = 1.2 x 1-311
All)
185
29
398
225
*
306
141
10
310
188
*
314
‘03
*
411
224
35
439
166
9
298
173
*
7132
If>,?
I6
767
157
15
231
I SO
X
240
13%
*
?;” -3.x5x +‘Sl(i_lRO)
y = 0.008 x +3Y (t?h)
ent taxa. These results atso show fh;tt values for the
relation h = LI x M/h amnor be caiculated exactly
over a wide range of body weights. and values cannot
be extrapolated correctly beyond the actual measur-
ing range as the position of the line is shifted in the
direction of zero by great changes in body weight and
the factor LI increases when bodk mass decreases, It is
of interest that even Hhen torpor metaboiic values are
included in the calcukttion. the metabolic rate of
htzmmirtgbirds is much higher than that of heavier
birds.
These tiny birds cannot compensate for their higher
metabolic demands by using their normal ability to
save energy. This means that they have to use their
ability to fall ir~io torpor as often as possible. even at
it high ambient tompertlture and with sufEcient food
Torpor and metabolism in hummingbirds 685
IO00 I 0.UNOERH0001, 1000 R.RLEYRNOR, 2
01 .
0 IO 20 30
1
1000
800
;
; 600
;
7
z 4oc
200
c
1000
800
u
; 600
;
2
z 400
200
0
1000
BOO
z
0
: 600
;
_’
z 400
200
0
1000
801
CT
: 6OC
;
7
r 400
200
0
0 I'0 i0 40
0.CRISTRTUS 2
b I’0 20 li0
R.RLEXRNOR, I
CH.MELLISUGUS
-- --ii
-----.
--N
800
;
; 600
;
7
_
r 400
200
0
1000
800
0
: 600
;
3 400
r
IO00
800
;
: 600
;
1
_
r 400
200
0
,000
800
;
: 600
;
7
400
z
200
0
.
0 IO 20 30
".BENJRMlNI
0 IO 20 70
T.SCITULUS
-- I
0 IO 20 30
CH.OENONE
Fig. 3.
Fig. 3-~011~.
I DO0
- i
800
0 IO 20
lOOal R.NIGRICOLLIS
30
““1 E.JUGULRRIS
7----- --- r----- ,
0 :3 20 30 0 10 20 30
T LOCI T DC
Fig. 3. Energy metabolism (M) as a function of ambient temperature (T,) of different hummingblrd
species during day (-). during night (N) and during torpor (0) (see also Table 1).
Torpor and metabolism in hummingbirds 6X7
Table 2. Occurrence of torpor at different ambient temperatures
Ambient temperature (‘C) 5 IO I5 20 ‘5
Number of birds in torpor II 15 13 14 14
No observed
torpor 6 4 7 5 6
Percentage of birds in torpor 65 79 65 74 70
Table 3. Mean body temperatures at different physiological stages
Species Mean body temperature ( ‘C)
day time night time torpor Ambient temperature
L. clemenciae
0. esfella
U. benjamini
0. c,risratus
Ch. ocnonc
39.6 35.7 19.6 15
40.8 35.7 20.3 15
39.0 28.0* IO
40.0 20.8 15
39.0 35.0 18.0 I5
* Bird not in a deep torpor.
supply. Torpor occurred in all hummingbird species
investigated. No correlation between occurrence of
torpor and body mass was found. The lowering of
metabolic rate was the same in all weight classes
(about 80-909k of the active metabolic rate). Inhabi-
tants of deserts fell into torpor as often as those of
tropical, temperate or cold zones. Length of torpor. as
well as occurrence, is not correlated with the biotope
of the different species. Torpor seems to be a common
characteristic of the entire family of Trochilidae and is
not limited only to those living in extreme biotopes.
Probably the evolution of this ability was mainly
dependent upon the low body mass of these smallest
representatives of the birds whose body temperature.
of 4&42’C, lies above even that of the mammals.
Even in tropical regions with favourable living con-
ditions the small hummingbirds can only achieve a
positive energy balance sufficient for survival with the
use of torpor. In contrast to mammals (e.g. shrews)
hummingbirds can consume food only during the day
time, thus they have greater difficulty maintaining
their energy balance during the night.
According to calculations by Berger & Johanson
(1980) a hummingbird can save up to 85”,, of energy
expenditure in torpor. compared to the homoiother-
mic stage, a fact which is confirmed by our own
results. The maximum amount of this energy saving is
at relatively high ambient temperatures. of 15--20 C.
At these temperatures torpor is. therefore, very effec-
tive for the birds. The minimum energy consumption
lies in this range, because the birds can decrease their
body temperature passively to correspond to this level
of ambient temperature; below this level they again
need more energy to regulate their body temperature
to the minimum value of about 18-20 C. which then
are above ambient temperature.
Table 4. Metabolism-weight relationship at different ambient temperatures during da\
and night (mean values of I7 different species) (,$I in kJ hr. I$’ (body mass) in gl
Ambient
temperature Weight-metabolism regression (coefficient r)
day time night time
5 ‘C
IO c
I5 c
20 c
25 c
Mean
n-r = 0.926 x W0 ‘” (0.88) iLf = 0.273 x U.” ‘- (0.69)
M = 0.975 x u’“.SD (0.81 I hf = 0.23 I x 12’” T6 (0.67)
M = 0.866 x W” 54 (0.84) I!{ = 0.2X6 x 12’” ” (0.64)
M = 0.726 x W” ‘.’ (0.87) nr = 0.221 x U’” I(1 (0.X5)
R,f = 0.708 x w” SJ (0.84) R1 = 0.21’ x LZ’[’ -6 (0.79)
M = 0.803 x W”.sb .If = 0.670 x 14’” -’
Table 5. Energy consumption of shrews (S) and hummingbirds (H) which have
similar size and which were measured under nearly identical conditions (during
resting period at an ambient temperature of 25°C) [shrew data from Fons 81
Sicart (1980)]
Species Mean body mass
(g)
Mean resting metabolism
(J/g x hr)
Suncus etruscus (S) 2.5 320
Ocreatus underwoodii (H) 2.7 300
Crocidura russula (S) 8-10 150
Lampornis clemenciae (H) 8.3 120
688 KRISTINE KRUGER et al.
During torpor the hummingbirds do not abandon
temperature regulation but the whole metabolic rate
is regulated to another (lower) level. It is of interest in
this regulation that there is no adjustment of new.
periodically changing nominal levels, but rather an
adjustment of a single nominal value of 18-2YC. Up
to this point no regulation takes place. When this
point is reached regulation begins automatically and
so body temperature does nat fall below this level.
During torpor, metabolic rate is also reduced to a
very low level which is obviously independent of
ambient parameters (e.g. ambient temperature). It
seems to represent a minimum levef for maintaining
the vital functions of the hummingbirds. Prinzinger et
~rl. ( 1981) investigated the red-backed mousebird
(Co[ius L.U.~~LIIZOIK~) and found that during times of
food scarcity these birds decrease their body tempera-
ture and metabolic rate continuously. When falling
short of a critical body mass of about 5Og they
undergo a nightly torpor by reducing their metabolic
rate abruptly to &out 5”. of normal values. They also
maintain a minimal critical body temperature of
about 18~‘C. Unlike hummingbirds, mousebirds do
not faI1 into torpor when food supply is sufficient.
They are essentially heavier (55-70 g) than humming-
birds and so have a more favorable relation of body
mass to energy expenditure.
In small mammals torpor occurs in nearly the same
way as in birds. It seems to be a common pattern that
is only slightly changed to fit the special demands of
different groups of animals. In bats (e.g. Il$j*oris
m_~oris. Heldmaier. 1970) a periodic occurrence of tor-
por could aiso be found even when they were main-
tained under constant ambient conditions (DD). He
shows that this is not only a simple reduction of
metabolism but a number of complicated regulatary
processes in which body temperature and metabolism
can be regulated to different levels. In bats, as in
1~L~mmingbirds. the same parallel changes of O2 con-
sumption, decrease of RQ to 0.75 and decrease of
body temperature to wa’tues of 2@ 22 C can be ob-
served. The same minimum of body temperature dur-
ing torpor can also be observed in white-toothed
shrews whose body rnzs (3-2Og) corresponds to that
of hummingbirds. Nngel (1977) induced torpor in
Sunc~~,s c’trusr~rs by reducing food supply. and also
found that body temperature is not reduced below a
lrvcl of’ 18 30 C. Frey & Vogel (1979) found that S.
r~uzc~~rs falls into torpor when food supply is reduced.
as well as spontaneously in a diurnal rhythm occur-
ring mostlv at night between 1.00 hr and 6.00 hr. The
small Crodidurinae. S. c*t~trscus and C~*r)cidl~r~r ~trssultr,
have a metabolic ratt‘ that lies high ahox the theor-
etically expected values. The values for energy expen-
diture (Fans & Sicart, 1980) correspond remarkably
well to those of hummingbirds with the same body
size.
Torpor as a means of energy saving is used by
many diRerent groups of animals in times of food
shortage. as well as periodically in the resting phase of
the diurnal cycle. Body temperature, metabolism and
behaviour are changed in a very similar way accord-
ing to a common pattern with only slight variations.
Therefore. we can agree with Raths & Kulzcr (1976)
that torpor seems to he a polyphyleticnlly evolved
ability. It allo~vs a lowering of metabolism and body
temperature in primarily homoiothermic animals that
occasionally ~mo~sebirds~ or continuously (humming-
birds, shrews) live near the subsistence leve1. These
animals can lower their body temperature secondar-
ily, thereby saving a great amount of energy without
having to give up their ability to regulate tempera-
ture. On the contrary. their ability to regulate tem-
perature. expanded as metabolism, can be regulated
on several different levels, depending on the actual
demands. The theory of torpor being a secondary
character is confirmed by the fact that newborn
shrews do not develop the ability to fall into torpor
until they have developed hom~iother~~i~ reactions at
the age of about one week (Nagel, 1977).
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