Thermal control of mother-young contact in rats

Physiology & Behavior, Vol. 21, pp. 793-811. Pergamon Press and Brain Research Publ., 1978. Printed in the U.S.A.

MONOGRAPH

Thermal Control of Mother-Young Contact in Rats t

M I C H A E L L E O N , P A T R I C K G. C R O S K E R R Y A N D G R A N T K. S M I T H

Department of Psychology, McMaster University, Hamilton, Ontario, Canada

(Rece ived 13 July 1977)

L E O N , M., P. G. C R O S K E R R Y A N D G. K. SMITH. Thermal control o f mother-young contact in rats. PHYSIOL. BEHAV. 21(5) 793-811, 1978.---The factors involved in the control of nest bout duration by mother rats were analyzed. Disruption of prolactin or adrenocortical secretions halted the normal progressive decline in daily nesting time, but the behavioral effects of hormone disruption were not mediated by a suppression of milk delivery. Rather, these hormones stimulate a chronic elevation in the body temperature of mother rats which makes mothers vulnerable to the acute thermal consequences of huddling with their litter. The temperature of a mother rat rises further after her ability to dissipate heat decreases when she comes in contact with her young, a rise that eventually limits the duration of nest bouts. Decreasing the efficiency of heat dissipation during nest bouts by elevating room temperature, pup temperature, or by removing the tail (a major pathway for heat dissipation) decreased nest bout duration. Increasing the rate of maternal heat loss by decreasing room temperature, pup temperature, or by partially removing the insulating fur of dams increased nest bout duration. We conclude that nest bouts are normally limited by a rise in maternal temperature, and that the rate of temperature rise determines the duration of each bout.

Mother young interactions Hormonal control of behavior Thermal development Adrenal steroids

Prolactin Thermoregulation

ALTRICIAL mammalian young are dependent on their mothers both for the provision of milk and for the maintenance of thermal homeostasis. Nurturance can occur only when mother and young unite, and two major strategies have evolved to ensure contact between mother and young. Young either remain in continuous contact with their mother, or are nurtured intermittently. The maintenance of continuous contact between mother and young re- quires specialized adaptations that enable the mother to forage for sufficient food to support the increased energy demands of lactation while caring for her young. Marsupial infants, for example, are kept in continuous contact with their mother by remaining in her environmentally stable pouch [27,43]. Placental mammals, on the other hand, typically do not have adaptations permitting the maintenance of continuous contact with their offspring, and mothers must periodically leave their young to engage in life supporting activities away from the nest. In the mother's absence, the young of placental mammals are buffered from extreme temperature fluctuations and hidden from predators in a pro- tected, insulated nest site.

Placental dams of different species have a wide range of nesting patterns, returning for a different number and duration of nurtural unions with their young. At one extreme of the range of nesting patterns is the tree shrew which visits its young briefly every other day [33], and at the other extreme is the Norway rat which initially spends about 75% of the day

in contact with its litter [24]. Little is known about the proximate causation of the nesting behavior of mother rats, and in the present monograph, we investigate the bases for the control of this nurtural interaction.

STIMULUS CONTROL OF NESTING BEHAVIOR

EXPERIMENT 1

The most striking aspect of the maternal nesting pattern in rats is a progressive decline in daily nest time during the first 2 weeks postpartum. This decline results not from a steady decrease in the number of bouts that a mother initiates but from a systematic decrease in the length of each bout [14,24]. One possible explanation for the observed decline in nesting time is that, following parturition, rat mothers spend progressively less time in their nests, independent of the stimuli from growing young. If this were the case, then dams whose pups were removed immediately following parturition would initiate the same number of bouts as dams kept with a normal litter. If, to the contrary, the nesting pattern of mothers is influenced by cues emitted by pups, then the behavior of mothers deprived of pups should differ from those of mothers with pups. In the present experiment we compared both the number of bouts and the cumulative nesting time of dams with and without pups during the two weeks following parturition.

~This research was supported by National Research Council o f Canada Grants A8578 to M.L. and A7465 to G . K . S . P . G . C . was supported by an N.R.C. Postdoctoral fellowship. We thank C. R. Brown, A. Lindsey, M. L. Sherriffand E. Wright for their technical assistance, and B. G. Galef and B. Woodside for their excellent comments. Prolactin was kindly provided by NIAMD.

Copyr igh t © 1978 Brain Resea rch Pub l i ca t ions Inc.--0031-9384/78/110793-19502.00/1

794 LEON, CROSKERRY AND SMITH

M e t h o d laO

All mothers in this and in subsequent experiments were l eo

primiparous Wistar rats, obtained as virgins from Canadian Breeding Farms, St. Constant, Quebec, bred in our labora- >- tory, and weighing 275-325 g at parturition. On the day of z o 1~ parturition, 8 mothers had their litters reduced to 8 pups, and

m 120 8 dams had all of their pups removed. [

Each of the 16 pregnant females were placed in a nesting ~- behavior recording cage prior to parturition. These recording ~ 100 cages, described in detail elsewhere [15], were used to con- ~ ao tinuously monitor maternal nesting behavior. Each consisted z of a polycarbonate laboratory cage, fitted with a nest tray that was invariably chosen by the mother for nesting. Each eo tray was balanced on a fulcrum, and when the female entered the nest tray, a microswitch closed and activated equipment 4o

which recorded both the number of nest entrances and the accumulated time spent with the litter. The equipment was 1100 programmed so that nesting bouts less than 5 see in duration were not recorded. The small nest tray size (88 sq cm) ensured that mother and litter were in contact during rec- 10oo

orded bouts; nipple attachment and suckling almost invariably occurred shortly after nest bout initiation [15]. _ aoo

.c Total nesting time and number of bouts were transcribed E daily at 0900 hr when the lights came on, and at 2100 hr when the lights went off. The test room was kept at 22 ° + I°C in _~ t m - - i--

t h i s and subsequent studies, unless otherwise specified. No nest material was provided in order to maintain constant the z 7o0

physical properties of the nest tray. Powdered Purina Lab- oratory Chow and water were continuously available, but could not be reached by the dam unless she left the nest. We ~o calculated two-week mean values of dally nest bout number and weekly mean nest time for each dam. These means were so0

used to determine statistical differences between the two groups using the Mann Whitney U test.

Resu l t s and Discuss ion

The main results of Experiment 1 are to be found in Fig. 1, which shows both the mean total dally nest time and mean number of nest bouts for females with or without young. Mothers with litters initially spent 79% of the day with their pups, a proportion that declined steadily to 37% at the end of the second postpartum week. Mothers deprived of their litters initially spent only about 55--65% of the day in the nest tray, but continued to spend about the same amount of time each day in the nest throughout the 2 week test period. Mothers with pups spent significantly more time in the nest during the first week postpartum (p<0.05) but significantly less time during the second week (p<0.01) than dams without pups. The number of nest entries remained relatively stable for both groups, but dams without pups consistently initiated 2-3 times as many nest entries as those caring for litters (p<0.01; Fig. 1 and Table la). The data indicate that pup stimulation is important for the determination of maternal nesting patterns. In fact, the pups eventually suppress nesting time of their dams below that of dams nesting without pups.

ENDOCRINE CONTROL OF NESTING BEHAVIOR

E X P E R I M E N T 2

In the previous experiment, we found stimuli from the pups to be necessary for the normal expression of maternal

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FIG. 1. Mean daily nest bout initiation frequencies and nest times of mother rats kept with or without their 8-pup litters. SEMs are

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nesting behavior. Pups could induce the normal decline in maternal nesting time in either of two ways. Either mothers respond to short term changes in the condition of the pups, or pups induce chronic changes in the internal state of the mother that in turn result in the normal decline in nesting time. In Experiments 2-6, we investigated the continuing effects of pup stimulation on maternal nesting and we subsequently investigated the role of immediate influences of the young in Experiments 7-11.

Continuing changes in behavioral responsiveness can be produced by hormonal action, and young rats stimulate an increase in the secretion of several hormones in their mothers [4, 25, 35]. If the elevated circulating levels of the hormones stimulate the normal nesting behavior of mother rats, then selective suppression of maternal endocrine activity should continue to alter maternal nesting behavior, perhaps over a period of several days.

In response to pup stimuli, prolactin levels are elevated in mothers during the first two weeks postpartum [4, 22, 44], and we therefore considered this hormone as a possible mediator of pup stimuli in determining maternal nesting behaviors. If prolactin plays a continuing role in the control of nesting then its suppression should produce chronic changes

MOTHER-YOUNG CONTACT 795

TABLE 1 MEAN DAILY BOUT FREQUENCY DURING THE FIRST TWO WEEKS

POSTPARTUM

a. Without pups 133.88" With pups 50.70*

b. Ergonovine 44.56 Control 43.51

c. Ergonovine + prolactin 41.60 Control 55.46

d. Ergocornine 45.38 Control 55.46

e. CB 154 43.15 Control 44.77

f. Adex-Ovex 31.74 Control 36.48

g. Ovariectomized 45.71 Control 44.36

h. Adrenalectomized 36.77 Control 37.96

i. Adrenal-demedullated 46.77 Control 40.00

j. Teat-sealed 54.51 Control 50.22

k. Cool Room 46.11 Control 51.69

1. Warm Room 44.70 Control 45.65

m. No tail 57.88 Half tail 49.43 Tail Intact 55.18

n. Shaven 44.69 Furred 50.73

*indicates p <0.01

in the behavior of mothers. To suppress circulating prolactin levels, we administered either ergocornine, ergonovine or CB 154 to mother rats because all three drugs suppress serum prolactin levels in lactating females while differing in their side effects [8, 21, 36, 42]. If similar effects on nesting behavior are produced by the 3 drugs, the drug-induced changes in nesting could be attributed to a common action on prolactin rather than to secondary actions of any one pharmacological agent. Furthermore, if it is the prolactin sup- pressive action of ergot drugs which is responsible for any observed changes in maternal nesting behvior, then exogen- ous prolactin should restore normal nesting behavior to drug-treated dams.

Method

On the day of parturition, 7 primiparous Wistar dams were assigned to each of 3 groups that received daily injections of 1 of 3 ergot derivitives at doses previously shown to suppress circulating prolactin levels [42]. Dams were injected daily with either 8 mg ergonovine maleate (Lilly), 0.5 mg ergocornine maleate (Sandoz), or 0.5 mg CB 154 (2-

bromo-a-ergokryptine; Sandoz) each suspended in 0.15 cc corn oil. Each drug-treated dam was paired with a control dam that received a daily injection of 0.15 cc corn oil.

Another group of 7 mothers was subjected to daily admin- istration of 8 mg ergonovine, but received 25 I.U. prolactin (NIH-P-S l l , ovine, supplied by NIAMD) 2 X/day in 0.2 cc PVP and saline (50% WN) . Their controls each received daily 0.15 cc injections of the corn oil vehicle and 0.2 cc of the PVP-saline vehicle. All drug and corn oil injections were administered subcutaneously for 14 days at 0900 hr, while prolactin and PVP-saline injections were given subcutaneously both at 0900 and at 2100 hr each day.

Prolactin suppression interferes with lactation, and to ensure that experimental and control mothers experienced equivalent litter stimulation, 1 drug-treated, 1 vehicle- treated, and 1 colony mother had their 8-pup litters rotated among themselves each day. Each mother therefore cared for pups taken from the same pool of healthy, growing young.

Mothers were kept in nesting behavior recording cages under the same conditions as described in Experiment 1. Continuous records of nesting time and bout frequency were again transcribed every 12 hr at 0900 and 2100 hr. The mean daily nest times for each mother were computed for the first and the second weeks pospartum, and compared with the Mann Whitney U test. Daily nest bout numbers were compared in the same way, but over the entire two week period.

Results and Discussion

The main results of Experiment 2 are presented in Fig. 2 which shows the mean dally nest times of prolactin- suppressed and control mothers. In the first week postpartum, prolactin-suppression induced no significant change in the nesting behavior of dams relative to their con- trois (p's>0.05). However , in the second week, prolactin- suppressed dams did not exhibit the normal decline in nesting time. Rather, throughout the second week they continued to spend about as much time with the pups as they did at the end of the first week. Controls continued to spend less time with the pups each day and were in contact with the pups for significantly less time than prolactin-suppressed dams during the second week (p's<0.01). Nest bout number did not differ between prolactin-suppressed dams and their controls (p 's>0.05, Table lb, ld, le).

Prolactin replacement in ergonovine treated dams suc- cessfully reinstated the progressive decline in nest time shown by controls, and there was no statistical difference between these groups either in the amount of time spent with the young or in the number of bouts initiate/] in either week (p 's>0.05, Table lc). Both these data, and the data demon- strating a common effect of the three ergot derivitives on nesting behavior, indicate that drug effects on nesting behavior were prolactin mediated.

The above data indicate that the elevated circulating prolactin levels that pups stimulate their mothers to release is necessary for the normal expression of maternal nesting behavior, but only in the second week postpartum. Other factors therefore must be involved in the expression of normal maternal nesting behavior during the first postpartum week.

EXPERIMENT 3

In the previous experiment, we found that prolactin played a role in producing the normal decline in maternal


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FIG. 2. Mean daily nest times of ergonovine-, ergonovine + prolactin-, ergocornine-, CB 154-treated mothers and their vehicle-injected controls. SEMs are shown.

nesting time during the second week postpartum. The delay in the occurrence of a behavioral change following ergot injections might have been due to a delay in the effective suppression of prolactin until the second week. However, circulating levels of prolactin fall shortly after ergot adminis- tration is begun and are not further depressed after repeated daily injections [32].

An alternative explanation for the delay in response to prolactin suppression involves the action of another endocrine system. It is possible that circulating prolactin normally stimulates the release of another hormone which directly suppresses nesting time. Withdrawal of prolactin may suppress circulating levels of this second hormone only by the end of the first week. If gradual removal of the second hormone by prolactin suppression halts the decline in nesting time after a week, perhaps direct removal of the second hormone would immediately halt the decline in nesting time. Ovarian and adrenal steroid secretion are both elevated during lactation [25, 44, 50], are both stimulated by prolactin [5, 11, 38], and can both be depressed by prolactin suppression [50,51]. It seemed possible that secretions from these glands mediated the effects of prolactin on nesting behavior. We therefore excised both ovaries and adrenals of puerperal females and determined whether the loss of the hormones from these glands would prevent the normal decline in maternal nesting time.

Method

On the day of parturition, 8 primiparous Wistar mothers were subjected to surgical removal of both ovaries and adrenals, and 8 other dams were given sham operations.

Surgery was performed under light ether anesthesia. The glands were removed from two incisions through the un- shaven dorsomedial flanks. Experimental and control dams received 0.9% saline to drink. Litters were exchanged daily between the 8 adrenalectomized-ovariectomized mothers, their 8 sham-operated counterparts, and 8 colony mothers. Daily nesting time and number of bouts were continuously recorded for 14 days as previously described. Mothers were subsequently subjected to autopsy to confirm the loss of adrenal glands. The Mann Whitney U test on individual 14-day mean data was used in statistical comparisons.


The main results of Experiment 3 can be seen in Fig. 3, which shows the mean daily nest time of adrenalectomized- ovariectomized dams and their sham-operated controls. As can be seen in the figure, adrenalectomized-ovariectomized dams did not exhibit the decline in nesting time shown by sham-operates and spent significantly more time with the young than their controls (p<0.01). However, the mean number of bouts initiated each day did not differ between groups (p>0.05; Table If), indicating that average bout length was greater for adrenalectomized dams than for controls.

Adrenal and/or ovarian hormones are therefore necessary for the normal expression of maternal nesting behavior. Moreover, since the normal decline in nesting time was prevented immediately, rather than after a week delay, these data lend some support to the suggestion that prolactin exerts its action on nesting through its trophic effects on hormones from these glands.


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FIG. 3. Mean dally nest times of adrenalectomized-ovariectomized and sham-operated control mothers. SEMs are shown.


Ovariectomy combined with adrenalectomy altered the course of maternal nesting, but we did not know whether both glands are necessary for normal maternal contact patterns. If ovarian secretions play no role in mediating maternal nesting responses, then ovariectomy should not interfere with the normal decline in daily nesting time. Dams were therefore subjected to ovariectomy and their nesting behavior compared to sham-operated controls (Study I).

If ovariectomy alone does not alter maternal nesting behavior, it is likely that the adrenals are necessary for the normal expression of maternal nesting. Adrenalectomy would then have the same consequences for nesting as adrenalectomy-ovariectomy. We therefore compared the maternal nesting behavior of adrenalectomized and sham- operated dams (Study 2).

The adrenal cortex produces steroid hormones, and the effects of adrenalectomy might be attributable to these secretions. However , the adrenal medulla is also excised by total adrenalectomy, and it is possible that adrenal medullary, rather than adrenocortical secretions are necessary for normal maternal behavior. If medullary secretions are the effective agents of the adrenal, then excision of the medullae alone should have the same consequences for maternal nesting as total adrenalectomy. Conversely, a lack of effec[ of adrenal demedullation would indicate that the adrenal secretions involved in the control of nesting come from the cortex. We therefore excised only the adrenal medullae of dams and compared their nesting behavior to that of sham- operated controls (Study 3).

Method

Study 1. Sixteen late pregnant dams were placed in continuous nesting behavior recording cages. On Day 1 postpartum, 8 dams were ovariectomized and 8 other dams were subjected to a sham operation under light ether anesthesia. The mothers were returned to their cages and their cumulative daily nesting time and number of bout entries were recorded continuously for 14 days.

Study 2. Eight other dams were adrenalectomized on Day 1, and 8 mothers were subjected to a sham operation. Indi-

viduals in both groups were returned to their cages and given 0.9% saline to drink. Their nesting behavior was then recorded for a two week period. Adrenalectomy was subsequently verified by autopsy.

Study 3. Seven nulliparae were subjected to adrenal demedullation, using a modification of the method of Farris and Griffin [18] and 7 other females were subjected to a sham operation. After 2 weeks, these females were mated and after they had given birth, their nesting behavior was recorded continuously for 14 days.

All purturient females in this experiment had their litters reduced to 8 pups, and the litters were rotated daily among 1 experimental, 1 control and 1 colony mother to equalize the quality of pup stimulation. The Mann Whitney U test on individual 14-day mean data was used to assess statistical differences between groups.

Results

The main results of the Study 1 are presented in Fig. 4, which shows the mean daily nest time ovariectomized dams relative to their sham-operated counterparts. Ovariec- tomized dams exhibited the normal decline in nest time and spent as much time as controls in contact with the young during the two weeks postpartum (p>0.05). The mean number of bouts initiated each day also did not differ between groups (p>0.05; Table lg). Ovarian secretions are therefore not critically involved in the control of maternal nesting behavior.

The main results of Study 2 are presented in Fig. 5, which shows the mean daily nest time of adrenalectomized dams and their sham-operated controls. Adrenalectomized dams did not have the normal decline in nest time shown by controls, and spent significantly more time with the young during the two weeks postpartum (p<0.01). Mean daily number of bouts initiated did not differ between groups during the test period (p>0.05; Table lh), indicating that average bout length was longer for adrenalectomized dams than for con- trois. Adrenal secretions therefore are necessary for the normal expression of maternal nesting.

The main results of Study 3 are presented in Fig. 6, which shows the mean daily nest time of adrenal demedullated dams relative to their controls. Adrenal demedullated dams exhibited the normal decline in nest time and their mean daily nest time was no different from controls during the two week period (o>0.05). The two groups also did not differ in the number of nest bouts they initiated (p 's>0.05; Table li).

Discussion

The data indicate that ovarian and adrenal medullary secretions are not critically involved in the control of maternal nesting. Rather, the secretions of the adrenal cortex are im- plicated as a factor involved in inducing the decline in nesting time. Indeed, the tendency of demedullated dams to be with pups for somewhat longer than controls may reflect a depression in adrenocortical response induced by removal of the medullae [6].

Thoman and Levine [48] observed that adrenalectomized mothers nursed more often than controls, and the present experiment confirms their report. Their findings also are extended, since both adrenalectomized and control mothers in the present study cared for the same pool of growing pups. The effects of adrenalectomy therefore cannot be attributed to a maternal response to stunted pups, despite the suppression of lactation by adrenal removal.


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FIG. 4. Mean daily nest times of ovariectomized and sham-operated control mothers. SEMs are shown.

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Adrenalectomy depresses locomotor activity [31,41] and it is possible that a reduction in general motor activity produced was responsible for the disrupted pattern of maternal nesting. However, the maintenance of the posture involved in maternal nesting should probably not be considered inactive in the same sense as a decline in locomotion. Indeed mothers were often observed to lie prone or to sleep on the wire mesh away from their nest tray, and one would expect an increase in these inactive behaviors at the expense of nesting time if a reduction in general motor activity were involved in mediating the effects of adrenalectomy in dams. We will discuss this possibility further in Experiment 10.

There is evidence to support the suggestion that the elevation of adrenal steroid levels in the second week postpartum depends, in part, on the action of prolactin on the adrenal. Chatterton, Chien and Ward [11] found that elevation of endogenous prolactin levels with the drug per- phenazine tripled the circulating levels of corticosterone in virgin female rats, indicating the sensitivity of the adrenals to prolactin stimulation. Witorsch and Kitay [52] found that a week of prolactin, ACTH, or growth hormone injection increased the circulating corticosterone levels of hypophysec- tomized virgin rats. These hormones elevate corticosterone by depressing adrenal 5c~-reductase, which is a microsomal enzyme that controls the balance between corticosterone and its A-ring reduced metabolites. Perhaps during normal lactation, high prolactin levels suppress adrenal 5c~- reductase and thereby stimulate levels of corticosterone that are effective in depressing maternal nest time through the second week postpartum. The data presented in the above experiments indicate that hormone secretion induces a chronic change in the internal state of the mother rat that influences the duration of contact with the litter.

T H E RM A L CONTROL OF NESTING BEHAVIOR


In the previous experiments, we have established the necessity of adrenal corticosteroid secretions for the normal depression of maternal nesting time during the two weeks postpartum. After this level of understanding of the hormonal control of a behavior is attained, researchers tradi- tionally have shifted to an analysis of the neural mechanisms that mediate the endocrine effects on that behavior. This research perspective has led to the conception of hormones as acting directly on CNS neurons to facilitate the responsiveness of an organism to particular external stimuli. Such a research orientation has been richly rewarded with a detailed knowledge of the mechanisms involved in many endocrine- dependent behaviors. However, hormones enter various non-neuronal tissues and might affect behavior indirectly by altering peripheral physiological processes, which in turn affect the CNS neurons that control ~oehavioral responses to external stimuli. We therefore examined the possibility that adrenal hormones produce chronic changes in other physiological systems which might provoke the maternal brain to respond to the developing young with the normal pattern of nesting.

Prolactin and adrenal steroids are involved in the production of milk [13] and our hormone manipulations interfered with milk delivery. If mothers utilize the cues associated with milk delivery to regulate the amount of time that they spend with their pups, then the hormone effects might have been secondary to the effects on milk flow. For example,


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FIG. 7. Mean daily nest times of teat-sealed and sham-operated control mothers. SEMs are shown.

mothers may terminate bouts when they have delivered a particular amount of milk to the young. As dams proceed through lactation, they become more efficient in delivering milk [17]. Perhaps dams deliver this critical amount of milk in progressively less time and therefore terminate bouts in progressively less time. If a mechanism of this sort determines nesting time, then disruption of milk delivery by hormone manipulations might have affected maternal nesting indirectly. To assess this possibility, we sealed the nipples of mother rats to prevent the delivery of milk while maintaining the maternal hormonal state by allowing pups to continue to suckle these dams [22]. Cessation of milk delivery cues should disrupt the normal expression of maternal nesting if such cues are critical for the control of mother-young contact.

Method

On the day of birth, 8 dams had their teats sealed by cautery of the nipple tips, and 8 control dams had 12 points on the ventrum cauterized, approximately 1 cm medial to each nipple. Mothers were allowed to recover from the light ether anesthetic and returned to their 8 pups. In continuous nesting behavior recording cages, the number of nest bouts and the cumulative dally time spent on the pups were recorded for 14 days and analyzed as previously described. Since the nipple-sealed mothers could not supply milk, pups were exchanged between these females and their lactating controls each morning. The lack of weight gain when the pups were with teat-sealed dams verified the complete block of milk delivery by teat cautery.


The main results of Experiment 5 are presented in Fig. 7, which shows the mean daily nest time of test-sealed and control dams during the 14 days after parturition. Despite their inability to deliver milk, nipple-sealed females exhibited the same progressive decline in dally nesting time as controls. They did not differ statistically from controls in either mean nest time or number of nest bout initia-

tions (p's>0.05; Table lj), although the teat-sealed dams consistently spent more time over pups. These data indicate both that stimuli associated with milk delivery do not play a critical role in regulation of nesting activity, and that changes produced by hormone disruption are unlikely to have been mediated chiefly by alterations in milk delivery.


In the previous experiment, we found that the effects of hormone disruption on nesting behavior were not mediated by a cessation of milk delivery cues, but it is possible that changes in other physiological processes precipitated the changes in maternal nesting. The loss of adrenal hormones in nonlactating rats results in a depression of body temperature [10, 40, 53], a hormonal action that might also occur in lactating females. If maintenance of normal body temperature by mothers is necessary for the normal expression of nesting behavior, then a depression in maternal temperature might mediate the changes in nest time produced by adrenalectomy. For example, mothers may remain on their young long enough to warm them. Mothers with depressed body temperatures, caused by adrenalectomy, would be less efficient in warming their offspring, and might therefore remain on the pups for prolonged periods. To determine the validity of this hypothesis, it is necessary to determine whether maternal body temperature is depressed by the loss of adrenal steroids. We therefore recorded the core and ventral temperatures of dams subjected to loss of adrenal hormones. In addition, if prolactin suppression alters nesting behavior by depressing adrenal steroids, then prolactin-suppressed dams should have depressed body temperatures in the second week when they have elevated nest times. Moreover, since teat sealing did not affect nesting behavior, one would not expect these dams to have altered body temperatures.

Method

Mothers and litters previously used to assess the effects on nesting behavior of prolactin suppression and replacement (Experiment 2), ovariectomy-adrenalectomy (Exper- ment 3), adrenalectomy (Experiment 4), and teat sealing (Experiment 5) had their core and ventral temperatures recorded daily for 14 days between 0900 and 1100 hr. Each female stood in a cloth sack while her temperature was recorded on a YSI Model 43 telethermometer, by placing a YSI 401 probe 3 cm into the rectum. Ventral skin temperature was then recorded by placing a YSI 408 surface probe against the sternum of the mother. Individual mean data were computed weekly and groups were compared with the Mann Whitney U test.

Results

The mean dally core and ventral temperatures of each group are presented in Figs. 8 and 9, respectively. Core temperatures of both adrenalectomized-ovariectomized and adrenalectomized females were suppressed throughout the two week recording period (p's<0.001). Prolactin-suppressed dams and their controls had no significant core temperature differences during the first week postpartum. On the other hand, the prolactin-suppressed females had depressed core temperatures and elevated nest times in the second week (p's<0.01), while prolactin replacement therapy reversed these disparities (p's>0.05). Teat-sealed dams had core temperatures no different from controls (p>0.05). Ventral tem-


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FIG. 8. Mean daily core temperatures of ergonovine, ergonovine + prolactin adrenalectomized-ovariectomized, adrenalectomized,

teat-sealed mothers and their respective controls.

3, I 36

37 .0

E r g ~ v i ~ - - c ~ w o l . . . . / ~ ~ , , . / ' ) '

I I I I I I I I I I I I I

Ergonovine and I ~ l ~ - - Ctm*~¢ - - - -

LIJ 11 " '~

i/ \ 3

~., I I I I I I I I I I I I I I

I I I I I I I I I I I I I I

Z

W / ~ x d / \ / /

I I I I I I I I I I I i r i

DAYS POSTPARTUM

FIG. 9. Mean dally ventral skin temperatures of ergonovine, ergonovine + prolactin adrenalectomized-ovariectomized, adrenalec-

tomized, teat-sealed mothers and their respective controls.

peratures correlated with shifts in core temperature, with the exception that adrenalectomized-ovariectomized and adrenalectomized dams did not differ statistically from controls in the first postpartum week (p's>0.05).

Discussion

The data from Experiment 6 indicate that adrenal steroids, perhaps stimulated by prolactin, chronically elevate the temperatures of mother rats. Adrenal steroids probably raise maternal temperature by increasing the metabolic rate of tissue of lactating females [39,54], which results in a two-fold increase in maternal heat production [7]. The additional heat that is produced by dams is apparently not com- pletely dissipated, resulting in a chronic elevation of maternal temperature [49].

The functional significance of maternal temperature changes for the control of maternal nesting time is suggested by the relationship between these parameters, which is summarized in Fig. 10. A striking correlation emerges; when core temperatures are depressed, nest times are increased. It

is therefore possible that maternal nesting time depends on the temperature of mother rats.

EXPERIMENT 7

The data presented in the Experiment 6 suggest a clear dependence on adrenal secretions for the elevation of maternal temperature during lactation, but the question of how a change in maternal temperature could affect nesting behavior remains. Since one of the functions of nesting is to main- taln an elevated pup temperature, mothers might monitor pup temperature and remain on the pups for as long as it takes to warm a litter. Hormone-disrupted dams have low body temperatures and would warm pups with decreased efficiency. These dams may remain on the pups for extended periods in order to warm them to a particular temperature. If mothers are monitoring pup temperature, then the increased nesting time would reflect a compensatory response to the needs of the pups, given the altered internal state of the mothers.


Direction of Significant Change Relative to Controls

Rrst Week / Second Week

EXPERIMENTS 2-5 EXPERIMENT 6

GROUP NEST T I M E

a. E rgonov ine = /

b. C S 1 5 4 = / ~

c. E rgocomine = /

d. E rgonov ine + Pro lact in = / =

e. A d e x - O v e x ~ /

f. A d e x t /

g. Teat -Sealed = / =

M A T E R N A L CORE T E M P E R A T U R E

=/~

=/~ =/(.

=/m

~/~

=/=

EXPERIMENT 7

PUP SKIN TEMPERATURE

(./=

(,/(,

(,i(,

=i=

=i= ~/i

FIG. 10. Comparisons of the changes in mean weekly nest times, maternal core temperatures, and pup skin temperatures in ergonovine, CB 154, ergocornine, ergonovine + prolactin, adrenalectomized-ovariectomized, adrenalectomized and teat- sealed dams relative to their respective controls. The Mann-Whitney

U test was used to assess statistical differences.

If mothers limit fluctuations in litter temperature by regulating the amount of time they spend with the young, one would expect pups to be maintained within a normal temperature range irrespective of the experimental treatment to which their mothers are subjected. To determine whether pup thermal state was regulated by dams having different nesting behavior, we recorded the temperature of pups of hormone-disrupted and teat-sealed dams.

Method

Pups of litters whose mothers were previously used to assess maternal nest time after prolactin suppression and replacement, adrenalectomy-ovariectomy, adrenalectomy and teat sealing had their temperatures recorded each day. At 0900-1100 hr, each mother was removed from the nest and one pup from each litter was chosen at random. The skin temperature of the pup was recorded with a YSI 408 surface probe placed on its sternum. Mean weekly pup temperatures for experimental and control pups were compared with the Mann-Whitney U test.


The main results of Experiment 7 are presented in Fig. 11, which shows the mean pup temperature of young from litters of hormone-altered and teat-sealed mothers. If pup temperature plays a primary role in controlling maternal nesting behavior, one would expect a consistent relationship to emerge between maternal nesting or maternal temperature and pup thermal state, but neither relationship was found. CB 154 and ergocornine injections to mothers resulted in significantly depressed pup temperatures during both weeks (p's<0.01), while the temperatures of pups in the ergonovine group were depressed in the first week (p<0.01), rising to control levels by the second week (/9>0.05). Prolactin replacement in ergonovine-treated dams reestablished normal temperature levels to their pups in both weeks (p's>0.05). Adrenalectomy-ovariectomy of dams depressed the mean temperature of their pups during both weeks (p's>0.05), but adrenalectomy of mothers had no significant effect on pup temperature in either week (p's>0.05). Teat-sealed females

0

u l

G .

0 , .

i i J r r i i i i i i ~ i i

A

i i t t i t t i i i L i i J

t/ ~x.O~,ex - -

i t r t I i t i t t i h i i

3 2 t i ~ i ~ i i t t I i i I i

FIG. 11. Mean daily pup skin temperatures from offspring reared by ergonovine, ergonovine + prolactin, adrenalectomized- ovariectomized, adrenalectomized, teat-sealed dams and their

controls.

kept pups at depressed temperatures in both weeks (p's<0.01).

Rat pups were clearly not maintained within the same temperature range by dams in different conditions. Moreover, pup temperatures were not correlated with maternal nesting time or with maternal temperature s. Fig. 10 contains a summary of pup temperature changes relative to controls for the 2 weeks postpartum. Pup temperatures were depressed in the teat-sealed group when maternal temperature and nest time were no different from controls. When maternal prolactin was suppressed, pup temperatures were depressed not only when mothers had depressed temperatures with elevated nesting times, but also when maternal temperatures and nesting times were no different from controls. Both adrenalectomized and adrenalectomized- ovariectomized mothers had depressed core temperatures and elevated nest times. However, adrenalectomized dams had pups at normal temperatures, while adrenalectomized- ovariectomized dams had pups with depressed temperatures.

The common factor that accompanied pup temperature


depression was a failure of dams to deliver milk to the young, as revealed by their weight loss. The pups of teat-sealed, prolactin-suppressed and adrenaleetomized-ovariectomized dams could not periodically ingest warm milk into their core, and these young had depressed temperatures. On the other hand, adrenalectomized dams continued to deliver milk, and their young maintained normal temperatures. Despite the great difference in the temperature of their young, adrenalectomized and adrenalectomized-ovariectomized dams had similar nesting times and depressed body temperatures. The data indicate that pup temperature is probably not a critical determinant of maternal nesting.

EXPERIMENT 8

In Experiment 7 we found no support for the proposal that dams remain in contact with the young to regulate pup temperature. However , it is possible that hormone disruption produced changes in maternal responsiveness to pups that might have overshadowed the normal mechanisms for regulating nesting time. If under normal conditions, pup thermal stimulation contributes to the determination of maternal nesting time, then direct experimental manipulation of litter temperature should provoke changes in nesting behavior. Pups are normally kept at about 33--35°C during the first two weeks postpartum, and if mothers regulate their nest time to maintain pups within this thermal range, then cooler pups should induce them to have increased nest time and warmer pups should elicit very little nest time. We therefore monitored nesting time for mothers kept with pups that were either cooled, or with pups warmed to a temperature considerably higher than would ever normally be reached in the nest.

Method

Fifteen late pregnant Wistar female rats were placed in continuous nesting behavior recording cages. After parturition, each dam had its litter reduced to 8 pups, and the dams were then place.d in one of 3 equal-sized groups. On Day 2 females were subjected to a minor surgical procedure (see Experiment 9) and then returned to their cages. On Day 7, we began to record nesting behavior of dams with litters either cooled to 22.5°C or warmed to 39°C, which is warmer than the nest ever reaches. Control dams cared for pups whose temperature was not altered from the normal 33--35°C. Litter temperature was controlled by passing warmed or cooled water through a 9.5 cm dia. Tygon coil on the floor of the nest tray.

Cumulative nesting time and number of nest bout initiations were recorded for 24 hr starting at midday on Day 7. Ambient temperature was 22°C - I°C and lights came on for 12 hr at 0900 hr.


The main results of this experiment are to be found in Fig. 12, which shows the mean nest time of dams maintaining cool, control, or warm pups during the 24 hr period. Mothers on warmed pups spent as much time as controls on their pups (p>0.05), while mothers with cooled pups exhibited significantly greater nest times than control dams (p<0.01). Mothers with cooled pups initiated as many nest bouts as controls (Y(=26.0; p>0.05), but dams wit_h warm pups initiated many more bouts than controls (X=62.2; p<0.01). Mean nest bout lengths for dams with cool, control and

1300 -

1100

c . m

E 90O v

ug

m

k- V- 700 g] ug z P~

~. 500

¢3

3OO

100

I

II II

I ±

I I

COOL CONTROL W A R M PUPS PUPS

FIG. 12. Mean nest time in a 24 hr period spent with Cool (22.5°C) Pups, Control (unmanipulated temperature of 33-35°C) Pups or

Warm (39"C) Pups.

warm pups were 65.68 min, 37.9 min, and 14.4 min, respectively, and both cool and warm groups differed significantly from controls (p's<0.01). Dams with cool pups therefore had prolonged nest bouts and dams with warm pups had many short encounters with their young.

If mothers remain on their pups to warm them, one would expect the observed increase in nesting time by dams on cooled pups. However , one would not expect dams with warm pups to spend as much time nesting as controls since the warmed litters were kept at a temperature considerably higher than controls, Chronically warmed litters did not re- quire any maternal contact to maintain pup temperature, but dams spent as much nest time with these pups as with control pups. It is improbable that pup temperature would be the critical determinant of maternal nesting only when pups are cool but not when warm. We therefore interpret these data to indicate that regulation of pup temperature by mothers is not the mechanism by which nesting time is determined.

EXPERIMENT 9

The data from Experiments 7 and 8 suggest that mothers do not monitor pup temperature and nest for a bout duration sufficient to maintain the litter within a particular tempera-


ture range. On the other hand, it also seems clear that the temperature of the pups can influence the duration of bouts and thereby affect dally nesting time.

Croskerry, Smith and Leon [16] have proposed a model for the control of nesting bout duration which may explain the observed effects of pup temperature on maternal nesting behavior. Specifically, we suggested that when the mother initiates a nest bout, she becomes part of a huddle and thus occludes a portion of her ventrum. Her surface area available for passive heat loss would decrease at bout onset, and she would dissipate heat with decreased efficiency. Since dams have a chronic elevation of body temperature [49], the additional increase in heat retention should increase maternal temperature even further. Bouts might then be limited in order to maintain maternal thermal homeostasis.

Cooled pups constitute a heat sink and contact with these pups should avert a rise in maternal temperature that would necessitate the termination of nesting encounters. Con- versely, mothers with warmed pups terminate bouts rapidly, and these dams should experience a rapid temperature rise after bout initiation because the litter would serve as a heat source. Control dams have bouts of intermediate length, and should experience a temperature rise, but one not as steep as mothers with warmed pups.

To determine whether maternal temperature actually changes during nest bouts, and whether those changes would be consistent with the suggestion that increasing maternal temperature limits nesting time, we continuously recorded ventral and core temperatures of dams maintaining cool, control and warm pups during nesting behavior. Litter temperature should also rise as a consequence of maternal nesting, because the pups are members of the nurtural huddle. We therefore also monitored pup temperature continuously under control, cool and warm pup conditions.

Method

On Day 2 postpartum, each of the 15 mothers used in Experiment 8 had a YSI 44003 thermister implanted subcutaneously over the sternum to measure maternal ventral temperature, and another termister sutured in place above the left kidney to measure maternal core temperature. The leads emerged at the head, and were placed in a head-cap, affixed to the skull with acrylic dental cement. Surgery was performed under light ether anesthesia.

Mothers with litters of 8 were then returned to nesting behavior recording cages and left undisturbed in a room kept at 22°C. At midday on Day 7 nest temperature was shifted to 22.5°C, 39°C, or left undisturbed (33-35°C). To monitor nest temperature, three thermisters, 7.0 cm apart were connected in series and embedded in a thin layer of epoxy in the nest tray. Maternal ventral, maternal core, nest temperature, and nest bout duration were recorded for 24 hr on a Beckman Model 700 polygraph. Maternal core and ventral temperatures were subsequently analyzed at bout initiation and ter- rnination for bouts greater than 10 min. Changes in nest temperature during bouts greater than 10 min were analyzed to determine the extent to which pup temperature was altered by maternal contact.

Results

Sample recordings of maternal core, maternal ventral and pup temperature during nesting with cool, control and warm pups are shown in Fig. 13. The main results of Experiment 9 are presented in Figs. 14 and 15 which show, respectively,

ON OFF ON

I I i i i i t , i r t r i i

CONTROL

385 '

38."]

ON OFF ON OFF

, , , I , f I r , , , , , f i

WARM P U P S ~: 38.1° 1 ~.~

] ~ 38.7" _

ON OFF ON OFF

i I J f f I I i

T IME (10 m in i n t e r va l s )

FIG. 13. Sample epochs of maternal core and ventral temperatures and nest temperatures during and between nesting bouts of mothers with Cool (22.5°C) or Control (unmanipulated temperature of 33- 35°C) Pups. Bout duration is indicated by the heavy black line. A thin line is drawn from the temperatures at the onset of first bout to facilitate a visual comparison of the ongoing temperature changes.

the core and ventral temperatures at bout initiation and bout termination of dams in the cool pup, control and warm pup conditions.

There was a significant decrease in both core and ventral temperatures of cool pup dams during nest bouts (p 's<0.05 and 0.01, respectively). Control dams experienced a rise in core temperature that did not reach statistical significance (p>0.05), but they had a highly significant rise in ventral temperature during their nest bouts (p <0.01). Warm pup dams had significant increases in both core and ventral temperatures during nesting (p's<0.01).

Nest temperatures invariably rose during, and fell between nest bouts in all three groups. Cool pups had a mean increase of 3.55°C and declined 3.62°C during the inter-bout interval. Warm pup temperature rose 0.83°C during bouts and dropped 0.77°C between bouts. Control pups rose 1.65°C above their temperature at bout onset, and fell 1.77°C between bouts.


3 8 , 5 -

=o

38.0

o o

Z 37.5

37.0

@

r--1

B O U T I N I T I A T I O N

B O U T T E R M I N A T I O N

-I

COOL CONTROL PUPS

WARM PUPS

FIG. 14. Mean core temperatures at bout onset and termination for mothers with Cool (22.5°C), Control (unmanipulated temperature of

33-35°C) and Warm (39*(3) Pups.

Discussion

These data are consistent with the suggestion that increases in maternal temperature during a nesting bout limit the length of encounters with the young. Maternal temperatures did not rise when dams were with cool pups, and these mothers had long bouts. Control dam temperature increased during nest bouts, and they had shorter bouts than dams with cool pups. Mothers with warmed pups had a sharp temperature rise and terminated their bouts even sooner than controls. The inverse relationship between maternal temperature rise during nest bouts and the duration of those bouts suggests that maternal temperature rise is a factor in limiting mother-young contact.

Pup temperature rose reliably during bouts in all three

MJ R"

I-- <~

I,M

gJ I-- , - I <¢ o r I - Z I,u >

z o r I,M I - <Z

38.5

38.0

37.5

37.0

36.5

36.0

[ ] BOUT INITIATION [ ] BOUT TERMINATION~

.,.......... i ..... ~ iiiiiiiiiiii ..,........ . : . . . . . ,.,.......

~.......,.. ..........,

:::::::::::: ......,.... ,.,.,... ,.. ~:,:.:+:.:

iiiiiiiii ........... +: . : , : . : . -:.:-:-:.:.

: : : ' : T . . . . , . .

ii~i~i~ ~̧ iiiiiiiiiiii

ii~iiiiii iiiiililili

i!iiiii!i!i iii!iii!i .......... iiiii!i!iiii i : i : i : i : i :!:!:~:!:~:

i : i : i : i : i i i i i i i i i i~i l

+ : . :+ : . : :.:,:.:.:.:~ + :+ : . : . : :.:.:.:.:.:~ .:.:+:.:,:

:ii i:~:i:i:?:i: ......... i!i]i!~i~i!i ;i[!ii~i!ill

COOL CONTROL PUPS

WARM PUPS

FIG. 15. Mean ventral temperatures at bout onset and termination for mothers with Cool (22.5°C), Control (unmanipulated temperature

of 33-35°C) or Warm (39°C) Pups.

groups. If dams stayed on the pups to warm them, they should have warmed the cool and control pups but not the warm pups, since the warmed young were already above the temperature at which they are normally kept. These data therefore do not support the proposal that dams remain with the pups for sufficient time to warm the young.

EXPERIMENT 10

In Experiment 9, we presented data that is consistent with the proposal that maternal nest bout duration is limited by thermal constraints on the mothers. We now seek to provide direct evidence to determine the validity of this proposal.

If a dam terminates nest bouts as a consequence of in-


creased heat retention while huddling, then bout duration should depend on the rate at which she is able to dissipate heat to her surroundings. Heat loss of huddle members var- ies with, among other factors, the thermal gradient between huddle members and the environment. The huddle, and therefore mothers, should retain heat less efficiently at low ambient temperatures. The temperatures of these dams in a cool environment should not increase rapidly during an encounter with the young, and such dams should have relatively long bouts. Conversely, mothers in a warm environment have a low thermal gradient with their surroundings and should experience a temperature rise relatively quickly following nest bout initiation. The nesting bouts of dams in a warm ambience should therefore be shorter than in a cool ambience. To determine whether the rate of increase of maternal temperature during a bout influences the duration of mother-young contact, we allowed dams to raise pups in a cool or warm environment and monitored both number of bouts and daily nest time for two weeks. We also recorded maternal temperature continuously for a single day to determine whether the temperature of dams nesting in a warm environment rises during a bout, and whether the maternal temperature rise is prevented in a cool environment.

To provide additional evidence relevant to the proposal that dams monitor pup temperature and terminate bouts when the pups reach a particular thermal range, we recorded pup temperature daily during the test period in cool, control, and warm environments. Unlike dams whose milk delivery has been suppressed, or mothers whose pups are experi- mentally maintained at high temperatures, these dams have both normal milk delivery and undisturbed pups. If intact mothers monitor pup temperature and remain nesting to warm the young, then we would expect dams in the different thermal environments to keep normal pups within the same temperature range. We therefore recorded the temperature of pups kept with mothers in cool, warm or control ambient conditions each day for two weeks postpartum. In addition, we monitored pup temperature continuously for a single day under the three ambient temperatures to determine whether pup temperature increased during nesting bouts in these conditions.

Method

Ten mothers were assigned to each of 4 groups on the day of parturition. One group raised 8 pups in a 26°C room (warm room) and their behavior was compared to dams raising their young in a 22°C room (control). A third group was in an 18°C room (cool room) and their behavior was compared to another control group in a 22°C room. All room temperature deviations were kept within I°C. Maternal nesting behavior and pup skin temperature were recorded during the 14 day test period and analyzed as previously described.

Continuous recordings of maternal nesting behavior, maternal core and maternal ventral and pup temperatures were obtained as previously described for 5 dams kept in an 18°C ambience and for 5 dams at a 26°C ambience on Day 7. The data obtained from these females were compared with that of the controls of the previous experiment. Statistical comparisons were made with the Mann-Whitney U test.

Results

The main results of Experiment 10 are presented in Figs. 16 and 17, which shows the mean daily nest time of dams in

1500

z :z

:E l -

Z

1300

1100

900

700

SO0

300

100

"]'--+-,.+_..~,,

! i - - W A R M R O O M

. . . . C O N T R O L

I I I I I I I I ] I I I I 1 2 3 4 5 6 7 8 9 10 11 12 13 14

DAYS POSTPARTUM

FIG. 16. Mean daily nest times of mothers in a Warm Room (26°C), or a Control Room (22°C).

1500

z

uJ :E l-

tkl Z

1300

1100

9OO

7OO

5OO

3OO

100 I

1

+ COOL ROOM

. . . . . CONTROL

I I I I I I I I I I I I I 2 3 4 S 6 7 8 9 10 11 12 13 14

DAYS POSTPARTUM

FIG. 17. Mean daily nest times of mothers in a Cool Room (18°C) or a Control Room (22°C).

warm (26°), cool (18°C) or control (22°C) environments. Mothers in the warm environment remained with their pups for considerably less time than controls during the 14 day period (p<0.01), dams in a cool environment spent much more time on the pups than did their controls (p<0.01). The mean daily number of nesting bouts initiated by dams in warm and cool environments was no different from the number initiated by controls in the two weeks postpartum (p 's>0.05, Table lk, 11). Nesting time therefore increased when the thermal gradient between the mother and her surroundings was elevated.

Sample traces of maternal core, maternal ventral and pup temperatures during nesting in the cool, control and warm environments are presented in Fig. 18. Analyses of these temperature records are summarized in Figs. 18 and 19 which show the mean maternal core and ventral temperatures, respectively, at bout initiation and termination under warm, cool and control conditions. Dams in the cool room


3~.~ 1 COOL RO01~

~8 -

36.7o] ~ - - - ~ ~ - "

Z

ON OFF ON OFF

, , . . . . , , , . . . . ] , , , ,

CONTROL ~ 38.5, ,~8 37'7"]" ~ - ' ~ - ~ ~

i Z

ON OFF ON OFF

WARM ROOM 3 8 , 3 *

' ~ 3 8 . 8 *

~; 37.2 ° ] . . . .

t~ Z

O N O F F Ot~ O F F

, . . . . . . . . f . . . . , , , , , , , ,

TIME 110 rain intervals)

FIG. 18. Sample epochs of maternal core and ventral temperatures and nest temperatures during and between nesting of bouts of mothers in Cool (18°C), Warm (26°C) or Control (22"C) Rooms. Bout duration is indicated by a heavy black line. A thin line is drawn from the temperature at the onset of the trwst bout to facilitate a visual

comparison of the ongoing temperature changes.

had significantly lower core and ventral temperatures than controls at both bout initiation and termination 6o's<0.01). Neither core nor ventral temperatures of cool room dams rose during bouts 6o's>0.05). In the warm room, maternal core temperature at bout initiation was higher than that of controls, but this difference did not reach statistical significance 6O >0.05). Mean core temperature of warm room dams at bout termination was significantly higher than that of controls 6O <0.05). Ventral temperatures of dams in the warm environment were lower than controls at bout initiation and termination 6o's<0.01). Core temperature of mothers in the warm room rose during bouts, but this elevation did not reach statistical significance 6O>0.05). However, the ventral temperatures of these dams rose significantly over the course of a nesting bout 6O <0.01). Core and ventral temperatures of control dams rose during bouts, but only the elevation in ventral temperature was statistically significant 6O>0.05 and p<0.01), respectively.

3 8 , 5 --

O O

ttl ¢I: k - < Ug O.

uJ

kg

O o .=J < z r~ Il l

<

38.0

37.5

37.0

D BOUT INITIATION

BOUT TERMINATION T

I

:::~.

COOL R O O M

C O N T R O L W A R M R O O M

FIG. 19. Mean core temperatures at bout onset and termination for mothers in a Cool (18°C), Control (22°C) or Warm (26°C) Room.

The mean nesting times of these dams in the warm, cool, and control environments on Day 7 are shown in Fig. 21. In the continuous recording situation, cool room dams spent more time with pups than controls 6O <0.01), who spent more time with litters than warm room dams 6o<0.01). Mean nest bout number did not differ between cool room dams (Y(= 18.0) and contr_ols (X=23.6), nor did it differ between warm room dams (X=28.6) and the controls (p>0.05).

Continuous records of cool room pups revealed that nest temperatures rose by an average of 3.17°C during bouts and fell an average of 2.96°C between bouts. Warm room pups rose 1.64°C during bouts and fell 1.60°C between bouts. Con- trol pups rose 1.65°C during bouts and fell 1.77°C between bouts. Maternal presence clearly warmed the pups. The 14 day mean pup temperature was lower for cool room pups (X=35.30°C) than their controls (X=35.90°C; p<0.01). Pup


o 1,4J cc

cc ~J o.

u.i I - i I < r~ I,- Z ug > . . I < z cc i l l

38.5

38.0

37.5

37.0

36.5

36.0

D BOUT INITIATION

[ ] BOUT TERMINATION[_~...

: : :5 : : : : : : : : : !:!:!:!:!:!:!: ......-.-..... : : : : :5 : : : : : : : . . . . . . . . . . . • . . . . . . - . . . : : : : : : : : : : : : : : . : . : . : . : . : . : . : • . . . . . . . . . . . • . . . . . . . . . . .

+ : + : . : . : - :

: i : i :~$1 : i : ~:~:~:[:~:~:~:[~

:~:[:~:~:~$

iiii::i::i::iiii iiii::iiii~ii~ !i!iiiii iiii . . . , . . . . . . . . : + : . : . : + : :::::::::::::::::::::::::::::

~:~:~:~:~:~:~:~:~ }i~i~i~::{~i! : : : : : : : : : : : : : : : : a :[:[:~:[:[:[:~

:::::::::::::::::::::::: i~::!::;i!i~i~::! :::::::::::::::::::::::::::: [iiiiii::~:::{:: iiii!iii![iiii[i] iiiiiiiiiiiiii

: . : - : . : . : . : . : ................., . . . . . . .

i:[:~:~:~:~:[:~:[1 : ' : ' : ' : ' : : : . . . . . . .

::::::::::::::::::::::::: i

C O O L C O N T R O L ROOM

iiiii iii!iiiiiiii ,:.:+:+: :.:.:.:.:.:

iiiiiiiiiili

~:~:~:{:~:~

i:~:~:~:~:~:

:~:~:~:~:~:i

iiilGii +:.:.: . : .~

iill W A R M R O O M

FIG. 20. Mean ventral temperatures at bout onset and termination for mothers in a Cool (18"C), Control (22°C), or Warm (26"C) Room.

temperature was also lower for warm r_oom pups (Y(=34.78°C) compared to their control pups (X=35.26°C; p<0.01).

Discussion

Mothers in the warm environment spent relatively little time with their young over the course of a day, and had relatively short nesting bouts. The core temperature of warm room dams was chronically elevated, and maternal core and ventral temperatures typically increased further during a nesting bout. Together, these data indicate that dams that had a low thermal gradient with their environment did not lose heat efficiently, and the small increase in heat that

1300

1100

A e=

= i

E 900 1.1.1

I

I - I.- 700 1.1.1 Z p,,,.

>- 500 ,< CI

3OO

100

I

+ COOL CONTR()L W A R M ROOM ROOM

FIG. 21. Mean nest time in a 24 hr period in a Cool Room (18°C), Control Room (22°C) or a Warm Room (26°C).

mothers experienced when they huddled with their pups severely limited nest bout duration.

Dams in the cool environment had a large thermal gradient with their environment, and had relatively long nest bouts and elevated dally nesting times. These dams had chronically low temperatures, and this altered maternal state mitigated the consequences of huddling behavior for mother rats. Dams with low temperatures probably decreased heat loss by huddling, thus conserving their need to produce metabolic heat to maintain their own body temperature. Nesting therefore did not result in an increase in the temperature of cool room darns, and their nest bout length was not limited by an acute rise in their temperature when they huddled with their pups.

These data are inconsistent with the suggestion that increased nesting might be a reflection of decreased activity. Using similar temperature parameters, Browman [9] found that female rats increased activity in a cool environment and decreased activity in a warm one. Mothers in the present experiment, however, increased nesting in the cool environment and decreased that behavior in the warm environment. The evidence indicates that our experimental manipulations primarily affect nesting behavior directly, and


that changes in nesting behavior appear not to be mediated by changes in the locomotor activity of mother rats.

Dams in warm, cool and control conditions warmed their pups when nesting, but pup skin temperatures in cool and warm rooms were both depressed below those of control pups. Perhaps pups lost sufficient heat in the cool environment to chronically depress their temperatures, despite the prolonged contact with their dams. The temperatures of warm room pups might have been depressed because those litters experienced prolonged periods without their dam. These data again suggest that maternal thermal state limits nest bout duration.

EXPERIMENT 11

In the previous experiment, we reported that the amount of maternal nesting time shifted in response to a change in the thermal gradient between the mother and her environment. The implication of these data is that any factor that alters the rate of maternal heat loss changes the rate at which maternal temperature rises during a nest bout, and thereby changes the amount of time that passes before the dams reach the temperature at which they terminate bouts. How- ever, it is possible that ambient temperature shifts alter maternal processes other than the efficiency of heat loss by dams. Changes in such processes might then mediate the effects of ambient temperature on maternal nesting time. The thermal changes in mothers induced by shifts in ambient temperature might be correlated with, but not causal to the changes in nesting behaviour brought about by different en- vironmental conditions.

If we could alter the efficiency of maternal heat loss by means other than by shifting ambient temperature, then it would be difficult to argue that other physiological processes determine the duration of maternal nesting bouts and, thereby, daily maternal nesting time. Fortunately, the rate of heat loss of rats can be influenced by factors other than ambient temperature. For example, the tail of the rat is a major window for heat dissipation when heat production exceeds heat loss to the environment [20, 40, 46]. Amputation of the tail removes this avenue for heat loss and enhances the importance of other, including behavioral, alternatives for maintaining thermal homeostasis. We therefore amputated the tails of mother rats either entirely or partially, and compared the nesting behavior of these dams over a two week period to that of sham-operated mothers.

Heat loss is restricted in rats by their insulating fur [26,28]. Removal of the fur would increase heat loss and possibly prevent an increase in maternal temperature during a nesting bout. Bouts of shaven dams should then be prolonged and their daily nest time should be elevated. We therefore removed the ventral fur of mother rats and then monitored their nesting behavior for two weeks.

Method

Study I. Six virgin Wistar females had their tails amputated at the base under deep Nembutal anesthesia after a 2% Xylocaine solution was infused into the tail (No Tail Group). Another group of 6 females each had half of their tail removed (Half Tail Group). Each of the 6 females in a third group was simply anesthetized and returned to its home cage (Control Group). All were impregnated 2 weeks post-surgery. After parturition, their nesting behavior with an 8-pup litter was recorded for 14 days.

Study 2. Eight mothers had their ventral fur shaved and treated with a depilatory (Neet: Whitehall Laboratories) under light ether anesthesia on the day of parturition (Shav- en Group). Eight dams were simply anesthetized and returned to their cages (Furred Group). All females had their nesting behavior recorded for 14 days. The Mann-Whitney U test was used to compare groups in both experiments. To be sure that tail amputation decreased the efficiency of heat loss and that fur removal impaired the ability of dams to maintain thermal homeostasis, experimental and control dams were subjected either to a high (37°C) or low (4°C) ambient temperature at Day 16 and had their rectal temperatures recorded every 15 rain. Their thermal responses clearly indicated that dams without tails lost heat more slowly than Controls and Shaven dams lost heat more quickly than their Furred controls.


The main results of Study 1 are presented in Fig. 22, which depicts the mean daily nest times of No Tail, Half Tail and Intact dams. Both No Tail and Half Tail dams spent significantly less time nesting during the 2 weeks than Intact females (p's<0.01). No Tail and Half Tail dams initiated about the same number of bouts each day as Intact dams (p 's>0.05; Table lm).

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D A Y S P O S T P A R T U M

FIG. 22. Mean daily nest time for mothers with no tail, mothers with half of their tail removed, and sham-operated intact dams.

The main results of Study 2 are presented in Fig. 23 which shows the mean daily nest time of Shaven and Furred dams during the 14 days postpartum. Shaven dams spent more time with pups than Furred dams (p<0.05), and the mean number of bouts in the two weeks did not differ between these two groups (p>0.05; Table ln). The data from both parts of this experiment indicate that direct interference with maternal heat loss and heat retention can change the amount of time dams spend with their young.

These data add further support to the proposal that nesting bouts are limited by an elevation of maternal temperature that is induced by huddling with the pups. It therefore seems probable that a physiological process involving maternal temperature is the mechanism controlling maternal nesting behavior.


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FIG. 23. Mean daily nest time of mothers whose ventral fur was shaved or left furred.

G E N E R A L DISCUSSION

We would like to propose a model for the mechanism that limits maternal nest bout duration; this model is. shown schematically in Fig. 24. When dams initiate a nest bout, they join a huddle, thereby occluding a portion of their ventral area that is available for heat dissipation, while they continue to produce the same amount of heat. The increased heat retention of the entire huddle, brought about by mother-young contact, eventually forces up both maternal and pup temperature, and at some point the rise in maternal temperature induces the mother to terminate the bout. The duration of the bout is then determined by any factor that influences the rate at which huddle temperature, and thereby maternal temperature, rises. Ambient temperature, initial pup temperature, insulation, and the surface area/mass ratio of the huddle are such factors. The last factor would explain why dams spend less time with a large number of pups than with few pups [23], and why heavy body weight dams spend less time with pups than light [14] or malnourished mothers [34,35]. We would expect these differences in nesting behavior because increasing the mass of the huddle that produces heat without a proportional increase in surface area for heat dissipation increases the rate of maternal temperature rise, which should shorten nest bout duration. It also seems entirely possible that the changes in nesting induced by such experimental manipulations might mediate any change in the development of the young.

As the pups develop as homeotherms, the surface area of the huddle for heat dissipation decreases relative to the mass of tissue producing heat. Huddle members, including the mother, probably have an increased rate of temperature rise during bouts as the pups grow, and it seems likely that this is the principal factor producing the progressively shorter bouts that are responsible for the normal, progressive decline in daily nesting time.

The vulnerability of dams to the thermal changes precipitated by contact with the young is due to their chronically elevated body temperature [49]. Adrenal steroids, and the prolactin that pups stimulate their mothers to release [22, 35, 55], are involved in increasing maternal metabolism by

I developing temperature at bout onset

surface area fgr rise determined by : I heat diosipatlon 1. ambient temperature

J l 2. pup temperature at bout onset

31 m ~ n of mother

4, surface a r u / m a N ratio of mother-litler unit

FIG. 24. Schematic summary of the chronic hormonal component and the acute huddling component of a mechanism limiting maternal

nest bout duration.

means of a t r o p h i c or a synergistic (see [30]) relationship. This rise in heat production severely taxes maternal physiological heat loss mechanisms. Eventually, heat production exceeds heat loss and maternal temperature becomes chronically elevated. The additional acute increase in heat retention induced by huddling with the young, stimulates a further increase in maternal temperature that cannot be di- minished by autonomic thermoregulatory mechanisms alone. Rather, the behavior of bout termination halts the rise in maternal temperature.

The model suggests a reason for the differences in absolute nest times among the different control groups in the present series of experiments. Mothers and pups experienced various combinations of weight loss and weight gain, along with altered temperatures of mothers and pups. These factors should have contributed to the determination of different absolute nesting times of control groups. How- ever, the pattern of a gradual, steady decline in nest time over the two week period was invariably observed in the control groups, and each experimental group was paired with its own control group.

As yet, the model that we have proposed for the control of maternal nest bout duration does not account for the termination of nesting bouts by non-thermal factors. We as- sume that dams periodically terminate bouts to eat, drink and eliminate, but it seems clear that thermal factors normally limit the duration of bouts. This model also does not account for the control of nest bout initiations, but the fact that this nesting parameter is subject to experimental manipulation (by warming the pups) suggests that the proximate causation of bout initiation might be amenable to investiga- tion.

We have provided what we consider to be compelling evidence indicating that the duration of maternal nesting bouts is normally limited by the thermal consequences of contact between mother and young. In addition to this specific conclusion, there are two general points that emerge from these experiments: the first concerns the locus of control of the mother-young interaction, and the second point concerns the action of hormones in the control of behavior.

The mother nests with the pups until her own thermal requirements necessitate bout termination. It may be the case that those mothers caring for precocial young do not modulate their maternal care in direct response to acute changes in the pups. Rather, dams may respond to acute changes in their own state, the consequences of which affect the pups.

810 L E O N , C R O S K E R R Y A N D S M I T H

While mothers do not appear to moni tor pup tempera ture , the li t ter is probably normally kept within a fairly narrow thermal range as a result of nest site choice , nest building, milk transfer, and heat conduct ion f rom mother to the young. Pups also contr ibute to their own thermal homeostas is by moving to a preferred tempera ture [19] and by huddling with o ther pups [2,3]. They steadily increase their physiological ability to cope with envi ronmenta l tempera ture changes, and by 15 days they can maintain body tempera ture ove r a broad range of ambient tempera tures , [ 1,12].

The second fact that emerges f rom this research is an indirect route for hormonal invo lvement in behavioral responses. H o r m o n e s have long been known to affect respon- s iveness of animals to specific external stimuli by altering C N S funct ion through direct cellular act ion in specific brain

areas. Nes t bout terminat ion is a hormone dependent behavior that is not media ted by direct neural action. Rather , the hormones work indirectly on ongoing physiological processes which subsequent ly al ter C N S funct ion to influence behavioral responses. It is possible that similar physiological processes commonly mediate endocr ine effects on behavior .

The endocr ine secret ions that pups st imulate in their mothers both sustain lactation and increase maternal heat product ion. The same hormones therefore provide the physiological basis for efficient t ransfer of both milk and heat f rom the mother to the young. It reflects an economy of mechan ism that these hormones and physiological processes have multiple effects that ensure the fine interact ions essen- tial for the success o f the maternal episode.

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M O T H E R - Y O U N G C O N T A C T 811

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Thermal control of mother-young contact in rats

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