EFFECT OF HIGH AMBIENT TEMPERATURE IN EARLY
AND LATE LACTATION ON LITTER GROWTH
AND SURVIVAL IN RATS
By HELEN HEARNSHAW*t and MANIKA WODZICKA-ToMASZEWSKA*
[Manuscript received 13 March 1973]
Abstract
The effect of heat stress, applied either during the first half or during the
second half of lactation, on total litter production per rat was studied. The survival
of the pups, the growth rate of the surviving pups, the liveweight changes, and feed
intakes of the rats were also measured. Some observations were also made on maternal
and pup behaviour.
Heat stress, whether applied in the first or the second half of lactation, significantly depressed total litter weight at weaning per rat. At weaning, the total weights
of pups produced by the groups which had been subjected to heat stress during either
the first half or the second half of lactation were not significantly different. Heat in the
first half of lactation resulted in a high rate of pup mortality (62% as compared with
2% in the control group). However, those pups which survived in the early heat group
grew as well as the pups in the control group. Heat stress in the second half of lactation
didnot cause a significant increase in pup mortality, but the growth rates of the surviving
pups were slower than those of the controls. Heat in early lactation had adverse
effects on the maternal behaviour of rats. Large numbers of pups were rejected by their
mothers and this probably contributed to the high death rate in this group. Liveweights and feed intakes of mother rats were depressed by heat. Physiological and
behavioural mechanisms underlying these results are discussed.
I. INTRODUCTION
There are many reports on the adverse effects of heat stress during gestation
and lactation in the rat and mouse (e.g. Hsu 1948; Macfarlane et al. 1957, 1959;
Biggers et al. 1958; Fernandez-Cano 1959; Howard et al. 1959; Aldred et al.
1961; Skreb and Frank 1963; Pennycuik 1964a, 1964b, 1964c, 1965, 1966a, 1966b).
However, no information is available on the effects of heat stress applied at different
times during lactation. Also, where growth rates of pups kept in hot environments
during lactation have been examined (e.g. Biggers et al. 1958; Pennycuik I 964a,
1964b, 1966a), pups have not been identified individually, whole litters only being
measured.
This paper describes a study of the effects of heat stress applied during the first
or the second half of lactation on the growth rates and survival of individual pups,
* Department of Physiology, University of New England, Armidale, N.S.W. 2351. (Address
for reprints.)
t Present address: Department of Agriculture, c/o Division of Animal Physiology, CSIRO,
P.O. Box 239, Blacktown, N.S.W. 2148.
Aust. J. bioi. Sci., 1973,26,1171-8
1172
H. HEARNSHAW AND M. WODZICKA-TOMASZEWSKA
the total litter weight per rat till weaning, and on the liveweights and feed intakes
of the mother rats.
II.
MATERIALS AND METHODS
(a) Outline of Experiment
A total of 24 litters of female albino rats were allocated to three equal groups using a table of
random numbers (Fisher and Yates 1953). The randomization was stratified within days oflittering
and according to litter size. The three groups were as follows:
(1) The control group kept during the entire experiment under the environmental conditions
of the rat colony at the University of New England at an ambient temperature of approximately 23°C.
(2) The early heat group (EH) kept under control conditions for the first 48-60 hr after
parturition and then put into an environment of approximately 33°C and kept there
until the pups were 12 days old. From the twelfth day till weaning (at 22 days) they were
kept in the control environment. These rats were not put into the hot environment at
birth because Pennycuik (1966a) found that mice exposed to heat at parturition lost
90% of their pups.
(3) The late heat group (LH) kept in the control conditions for the first 12 days after parturition and then transferred to the hot environment until weaning.
All three groups were kept at a constant photoperiod of 14 hr light, 10 hr darkness.
(b) Animals and Management
All rats and pups were weighed 48-60 hr after parturition and the pups were individually
marked for future identification. All litters were inspected each morning. Pups which had fallen
out or been pushed out of the cage were replaced and food and water replenished if necessary.
Behavioural observations were carried out at the time of the daily checks. All rats were kept in
individual standard wire cages with wood shavings for nest making. Wood shavings were replenished
when necessary. All rats were fed a standard pelleted ration ad libitum and had unlimited access
to water.
The dry-bulb temperature of the control environment was 22· 5-24,5 DC. Humidity was
uncontrolled and the wet-bulb temperature ranged from approximately 16 to 19°C. When heatstressed, the rats were kept in a thermostatically controlled chamber with continuous air circulation,
a dry-bulb temperature of 33 ± O' 5°C, and a wet-bulb temperature of 19± 1°C.
Every Wednesday and Saturday each rat and each pup was weighed and food intakes were
measured. Rats were weighed to the nearest gram, pups to 0'05 g, and food to the nearest gram.
All pups were weaned at 22 days.
"Rat" refers throughout this paper to mother rat; "pup" refers to the offspring of "rats";
a "rejected pup" was one which was found out of the cage, having either crawled out or been pushed
out of the cage.
III.
RESULTS
(a) Survival and Growth Rate of Pups
The total productivity of the mother rat may be measured by the total weight
of pups produced at weaning. The two components of this are pup survival and the
growth rate of the surviving pups.
(i) Pup Survival
The average number of pups per litter (± S.D.) at the beginning, the end of the
first half, and the end of the second half of lactation was as follows:
HEAT STRESS AND LITTER SURVNAL IN RATS
1173
Beginning
End of first
half
End of second
half .
Control
lO·6±2·7
1O·4±2·6
9·5±2·4
EH
LH
1O·6±2·0
4·0±3·0
3·9±2·9
lO·1±2·7
1O·1±2·7
8·8±3·0
Group
It is apparent from these values that heat stress in the first part of lactation caused a
considerable increase in pup mortality (62 % versus 2 % in controls). Heat stress
applied in late lactation did not significantly decrease survival. A x2 test indicated
that the ratios of living pups for the three groups at the beginning of the experiment,
at the end of the first half, and at the end of the second half of lactation were significantly different. This was mainly due to the X2 of the EH group. All rats in the EH
group lost some pups; one lost the entire litter within 3 days of the beginning of the
heat treatment.
TABLE
1
COMPARISON OF THE TOTAL NUMBER OF PUPS PER GROUP AT BIRTIl WITIl THE
TOTAL NUMBER OF DEATHS AND THE TOTAL NUMBER OF REJECTIONS IN THE FIRST
AND IN TIlE SECOND HALF OF LACTATION
First half
No. of
pups
at birth
Group
Control
EH
LH
No.
dead
Second half
No.
rejected
No.
dead
No.
rejected
85
2
7
41
85
53
148
1
81
0
0
17
2
32
Table 1 shows that many pups in the EH group were rejected while in the hot
environment. Rejections also occurred in the second half of lactation in the control
and LH groups. The majority of those in the control group were from one litter
towards the end of lactation and may have been "escapes" of pups which were
becoming very active rather than rejections.
TABLE
2
PARTITION OF DEATIlS IN THE TIIREE GROUPS DURING THE FIRST AND SECOND HALVES OF LACTATION
First half
Group
Dead
Dead and
partly eaten
,,-,
Missing
Dead
Dead and
partly eaten
Missing
5
0
1
1
1
36
4
0
0
0
0
13
0
9
7
Control
EH
LH
Second half
----"
It can be seen in Table 2 that 13 pups in the EH group were missing, presumed
dead and eaten. All the pups "dead and eaten" or "missing, presumed dead" had
previously been rejected.
1174
H. HEARNSHAW AND M. WODZICKA-TOMASZEWSKA
Dead pups had lighter body weights on the day of death than their live litter
mates (P<O·OOI). However, pups that died had not on average been smaller at
birth than pups that survived. There was no relationship between pup mortality
and feed intake or weight of the rat.
(ii) Mean Weight of Surviving Pups
The mean weights ± S.D. (g) of the surviving pups at the beginning, end of the
first half, and end of the second half of lactation were as follows:
End of second
half
3S·10±6·45
Group
Beginning
Control
6·75±1·20
End of first
half
lS·40±2·75
EH
S·20±2·00
16··00±3·10
3S·50±6·15
LH
S·00±1·45
lS·90± 3· 50
29·00±5·25
The weights of the surviving pups in the EH group were not significantly different
from those of pups kept under control conditions. At weaning, however, the surviving pups of the LH group were significantly lighter than those in the control and
in the EH group.
375
Control
Fig. I.-Total weight of
pups per litter during the
experiment; pups were first
weighed at 2-3 days after
birth then twice weekly
till weaning.
250
~
~
f
§
125
01
1
,
3
!
5
Weighing No.
(iii) Total Weight of Pups Produced per Litter
Figure I shows the total weight of pups produced per litter (or per rat) in the
three experimental groups during the experiment. Student's t-test was carried out
on the total weight of pups per rat both at the end of the first half and the end of
the second half of lactation. At the end of the first half of lactation the total pup
weight per rat of the EH group was significantly less (P<O'OOI) than those of the
other two groups (which were under control conditions). At weaning, both the EH
and the LH groups had significantly smaller total pup weights per rat than the
control group (P < 0·01). The total pup weight per rat of the LH group was greater
than that of the EH group, but the difference was not statistically significant.
HEAT STRESS AND. LITTER SURVIVAL IN RATS
1175
(b) Liveweight and Feed Intakes of Rats
The initialliveweights and liveweight changes of rats (g), ± S.D., at the end of the
first and second half of lactation are given in the following tabulation:
Change in liveweight at end of
Group
Initial
liveweight
Control
247±25
EH
LH
256±39
First half
+ 15± 11
-24± 15
271±38
+2±18
Second half
-10±13
+14±22
-51 ±21
These values show that heat stress, whether applied during the first or the second
half of lactation, caused a significant decrease in the liveweight of the rats. The loss
of liveweight was significantly greater in the EH group than in other groups in the
first half of lactation (P<O'OOI), while the decrease in liveweight was significantly
greater in the LH group than in the other two groups during the second half of
lactation (P<O·OOI).
The mean feed intakes (g) per rat per day during the first and second halves
of lactation are tabulated below (the number of pups sucking per litter at the beginning and end of each period is shown in parentheses):
Group
Control
EH
LH
During first half
42·3±9·6 (10,6,10,4)
20'6±5'5 (10'6, 4'0)
40'4±9'9 (10'1,10'1)
During second half
67·9± 13·6 (10'4, 9'5)
40'9±21'4 (4'0, 3'9)
52'1±1l'3 (10,1, 8'8)
As lactation advanced the control group ate more (P<O·OOI). Heat stress in the
first half of lactation reduced intakes by half when compared with the control group
(P<O'OOI) but when the stress was applied in late lactation its depressing effects
were much smaller (P < 0·05). The intakes of the EH group during the second half
of lactation were lower than those of the other two groups. However, it should be
remembered that, at this time, the EH group had only a few surviving pups per litter
and hence probably were producing less milk.
Figure 2 shows that, irrespective of treatment, there was a relationship between
feed intake of the rat and weight gain of her litter over that period. As the litter
increased in weight so did the feed intake of the rat.
(c) Behaviour
As much observation as possible was carried out on the behaviour of the rats
and their pups. Whilst these observations were qualitative, it was hoped that they
would be of some help in explaining such results as pup survival. It was noticed:
(1) During the first half of lactation pups tended to crawl out of the nest when
heat-stressed, whereas the pups kept under control conditions stayed in the nest.
Thus, many pups in the EH group were observed falling or crawling through the
wires of the cage. In the second half of lactation the pups were too big to fall through
the wire of the cage. In general, the pups were more restless under heat-stressed than
in control conditions during the first half of lactation. All pups were active during
the second part of lactation.
1176
H. HEARNSHAW AND M. WODZICKA-TOMASZEWSKA
(2) The mother rats were much more restless in the heat, especially during early
lactation. They would not remain still, or let the pups suck, and were often seen
carrying the pups in their mouths and pushing them out of the cage. Maternal
behaviour was much better in the rats heat-stressed in the second half of lactation
than in those heat-stressed in the first half.
(3) In the heat, both the rats and their pups had scruffy coats whereas those
of the control animals were shiny and sleek.
a
'00 [
80
D
Fig. 2.-Relationship
between feed intake per
rat during a period and
litter weight at the end
of that period.
Control.
D
o LH group .
• EH group.
,~60 f
0
0
0" goo
0
1
-c 40
"
~
20L
o
IV.
""
100
200
300
400
Litter weight (g)
DISCUSSION
Heat stress, whether applied during the first or the second half of lactation,
considerably depressed total productivity per rat. The extent of this reduction in
productivity was not significantly different between the EH and LH treatment
groups, but the factors contributing to this were different in each group. Heat stress
in the early part of lactation resulted in greatly increased mortality rates, but the
surviving pups grew as well as those kept under control conditions. Mortality in
the LH group was not significantly different from that of the controls, but the growth
rates of the surviving pups were significantly lower than those of the controls.
Many factors could have lowered pup survival of the EH group. Firstly there
were many rejections in that group and rejection would reduce a pup's chance of
survival. Pups were replaced in the cages only once a day and would be without food
whilst out of the cage. Many of the rejected pups may have died of starvation whilst
out of the cage or because they were too weak to suck when returned to the nest.
The mothers contributed to the numbers of rejections by not allowing the pups to
suck, by not retrieving pups which were out of the nest, and in many cases by actively
pushing the young out of the cage. In fact, as in the experiments of Macfarlane et al.
(1959) and Pennycuik (1964c), cannibalism occurred in the rats heat-stressed in early
lactation. It is interesting to note that, in this experiment, all pups which were found
partly eaten, or were missing presumed dead, had been rejected previously. Most
HEAT STRESS AND LITTER SURVIVAL IN RATS
1177
had low body weights at the time of death. Pennycuik (l964b) found that heatstressed pups had lowered sucking reflexes, which also would tend to lower survival.
Heat stress in late lactation did not significantly increase pup mortality, probably partly because the pups were more robust and could withstand periods of
starvation, partly because they were too big to fall out of the cage, and partly because
maternal behaviour of the rats was better than when heat stress was applied early
in lactation. Maternal behaviour was probably well established at this late stage of
lactation and was thus not unduly upset by the heat stress.
The lowered growth rate of pups in the LH group was also probably the result
of numerous factors. Pennycuik (1964a, 1964b) found that heat stress resulted in
lowered feed intakes and liveweights of rats, less secretory tissue in the mammary
glands, and hence lowered milk production. Since lowered intakes and liveweights
followed heat stress in this experiment, it is likely that the heat-stressed animals had
lowered milk production. The depressing effect of heat stress on milk production was
probably greater earlier in lactation (Pennycuik 1966b), but the number of pups
competing for this milk in the EH group was so small (38 % only survived) that they
obtained enough milk to grow well. However, in the LH group mortality was low
and the number of pups competing for the limited milk supply was large. This would
probably cause the low growth rates of pups in this group.
It would be interesting to find out how the survival and growth rates of pups
heat-stressed in early lactation would be affected if the cages were so constructed
that it was impossible for the pups to either crawl or be pushed out of the cage. If
pup mortality were reduced, would the growth rates of the larger number ofsurviving
pups be also depressed because of the reduced milk supply?
Finally, detailed behavioural studies, such as frequency of suckling, the amount
of motor activity of the mother and of the pups, etc., might help to explain the
contributory causes to pup mortality and growth.
V.
ACKNOWLEDGMENTS
Grateful acknowledgments are made to Professor J. V. Evans for making the
facilities of the Department of Physiology available, to Professor N. T. M. Yeates
of the Department of Livestock Husbandry for the use of the thermally controlled
chamber, and to Mr. M. E. D. Webster for the loan of equipment used for some
of the measurements. We wish to thank Drs. V. Bofinger and N. Dawson for advice
on statistical analysis, Mr. A. Jones for the help with the experimental animals, and
Miss Helen Sewell, Mrs. Gabrielle Wallis, and Mr. P. Mortell for technical assistance.
VI.
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