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Suckling: a time-constant in the nursing behaviour of the rabbit
Physiology and Behavior, Vol. 13, pp. 711--714. Brain Research Publications Inc., 1974. Printed in the U.S.A.
BRIEF COMMUNICATION Suckling: A Time-Constant in the Nursing Behaviour of the Rabbit D. W. L I N C O L N
Department o f Anatomy, The Medical School, University Walk, Bristol. BS8 ITD, England
(Received 30 April 1974) LINCOLN, D. W. Suckling: a time.constant in the nursing behaviour of the rabbit. PHYSIOL. BEHAV. 13(5) 711-714, 1974. - The milk yield and the daily duration of suckling of rabbits on a once per day nursing routine were recorded, and the effects (1) of mating these animals postpartum and (2) of depleting the neurohypophysial stores of oxytocin by electrical stimulation were investigated. Milk yields peaked at about 190 g/rabbit/day between Days 15-20 of lactation. Thereafter yields declined and lactation terminated in pregnant animals between Days 25-27, and in nonpregnant animals at some time after Day 50. Suckling lasted for only 2 - 4 min each day, and this period declined as a linear function as lactation progressed. The suckling period was characteristically constant for the individual animal and was unrelated to milk yield or the fullness of the mammary glands. Thus, pregnant rabbits in which milk yield had declined to less than 10 g on Day 26 and electrically-stimulated rabbits in which 42% of the stored milk was not ejected terminated their daily period of suckling at a time identical to that of the controls (i.e., 3 min). Suckling
Milk ejection
Maternal behaviour
Rabbit
analysis of the suckling period and the milk yield of pregnant rabbits mated i m m e d i a t e l y after parturition. Study 2 involved an analysis of these functions after the electrical stimulation of the h y p o t h a l a m o - n e u r o h y p o p h y s i a l tract i m m e d i a t e l y prior to nursing, thereby depleting the neurohypophysial stores of readily releasable o x y t o c i n . A similar dissociation b e t w e e n m a m m a r y engorgement and nursing behaviour has been observed by Findlay and R o t h [ 3 ] . In their studies rabbits were found to display normal nursing behaviour after the m a m m a r y glands had been emptied during a brief period of anaesthesia.
T H E milk-ejection reflex of the rabbit, and the maternal behaviour associated with this event, have been the subject of several studies [2, 3, 6 ] , yet the remarkable precision with which the rabbit regulates her nursing appears always to have been overlooked. The lactating rabbit nurses i n f r e q u e n t l y and a natural periodicity of a b o u t once a day has been suggested [ 2 ] . Certainly, rabbit pups thrive if only allowed access to the m o t h e r for one daily feed. Suckling lasts from 2 - 4 min and the y o u n g obtain 1 5 0 - 2 0 0 g of milk. This is a very short period w h e n related to the 24 hr day and the f o o d intake is e n o r m o u s , for a y o u n g litter it can represent 25% of their b o d y weight. The behaviour of the nursing rabbit is most stereotyped. F o r the first 2 - 3 min she crouches motionless over the feeding young, her expression is r e m o t e and she appears oblivious to peripheral events. A second stage follows in which she assidiously attends to the g r o o m i n g of her young, frequently licking and palpating their genital regions. T h e n suddenly, nursing is terminated by her abrupt departure from the nest. The y o u n g are still keen to feed and will pursue her eagerly, but further access to the nipples is strongly resisted. In two separate studies of lactating rabbits, p e r f o r m e d some three years apart, the period of suckling has been observed to be remarkably constant and virtually indep e n d e n t of the m o s t obvious determining factor - the fullness of the m a m m a r y glands. Study 1 involved an
METHOD
Animals Adult rabbits of the Californian strain were used, weight 2 . 5 - 3 . 5 g. Mating was controlled and gestation timed from the exposure of the oestrous rabbit to 3 males within a period of 5 min. Litters were reduced where necessary to 5 y o u n g within 2 days of birth. A pelleted rabbit f o o d and water were available to the m o t h e r and y o u n g at all times.
Procedure F r o m Day 9 of lactation the m o t h e r s and y o u n g were placed on a once-a-day nursing routine. The m o t h e r s were removed to a separate r o o m and the y o u n g brought to t h e m 711
712
LINCOLN
for their daily feed. The period of suckling, defined as the period of time that the pups were attached to the nipple, was recorded to the second. This was simple to measure for the y o u n g attached themselves to the nipple within 5 30 sec of entering their m o t h e r ' s cage, and suckling was terminated by the abrupt departure of the m o t h e r from her suckling young. The y o u n g were then removed and the weight gain of the young over this suckling period of 2 4 min was recorded as the milk yield. Weight losses during this brief period in urine and feces were negligible. The second study was c o n d u c t e d on rabbits with stimulation electrodes chronically implanted in the h y p o t h a l a m o neurohypophysial tract, in the region of the infundibulum. These animals were previously used for studies on the electrical induction of labour, and the procedure for electrode implantation has already been published [5]. These electrodes were so placed that stimulation with pulse trains above 30 Hz released large a m o u n t s of o x y t o c i n from the neurohypophysis.
RESULT Normal lactation (control). Figure 1 illustrates the lactation curve in milk yield/day for the rabbit from Days 1 2 40 of lactation. Peak milk p r o d u c t i o n occurred b e t w e e n Days 15--20, and ranged from 1 0 0 - 2 8 0 g/day (mean 190). A natural weaning of the y o u n g t o o k place on the night of either Day 22 or 23, i.e., the y o u n g t o o k solid food or water for the first time and gained weight overnight. Lactation continued for m a n y m o r e weeks and the rabbits were still prepared to feed their young (or any other litter of young) at 50 days of lactation. By this time, however, the 5 young weighed twice that of the m o t h e r and nursing became a rather violent exercise. In the days prior to weaning the weight loss of the pups from one day to the next provided, when related to the milk consumed, an index of feed conversion. Thus, about 2.2 g of milk were required by the pups for each gram of body weight gain. The period of suckling ranged from 2 - 5 min, and declined progressively t h r o u g h o u t lactation (Figs. 1 and 2). This decline was linear and c o n f o r m e d to a regression function expressed by the equation y = 2.14x + 233.13 (where y is the suckling period measured in seconds and x is the day of lactation). This regression was significant at p = 0.001. The decline in milk yield and the period of suckling were not parallel, and the time spent by the individual doe in the feeding of her young was not related to whether she had a high or a low milk yield (Fig. 2). The period of suckling was remarkable in another respect for whilst some variation was observed between individual animals the suckling period of the individual animal never varied more than 10% from the individuals o w n regression line. Lactation o f the pregnant rabbit. The milk yields of pregnant rabbits, m a t e d post-partum, were identical with controls until Day 18 of lactation. Thereafter, yields declined sharply and lactation terminated on Day 25, 26 or 27 (Fig. 1). By contrast, the duration of suckling followed precisely the linear regression observed in controls. Thus, one observed the u n e x p e c t e d situation of an animal at Day 26 of lactation suckling for the full period o f about 3 rain and yielding less than 10 g of milk. Had the animal not been pregnant the milk yield in this same period would have exceeded 140 g. T e r m i n a t i o n of lactation was characterized by the fact that on one day the rabbit displayed
W
T
p
2oo
is0
~ ';, -* ~
\
100
'i\ xx'OO]
50
*"~*-*
0 ,
,
,
i%
, ,
,
250
r
==
0
y :, -- 2.14x + 233.13
o
o%;.z,_..&._
f
*-.....,...__,*
0
•~
o
,
150
0~
1'5
2AO
25
Day
of
3'0
3`5
4()
Lactation
FIG. 1. Milk yields (upper graph) and suckling times (lower graph) of pregnant (stars) and non-pregnant (asterisks) rabbits of the Californian type. W, weaning; T, termination of lactation in pregnant does; P, parturition of the same. There were 8 does in each group and 5 young in each litter. Weaning was defined as the day on which the pups first took solid food. This occurred at approximately the same time in the litters of both the pregnant and non-pregnant does.
240 I j
.
*
200
.~
. * * * *
160
*
*
*
. **
*
*
*
~ *. x-
"~ n~
* * 80'
i 4ot
-- 15
2'0
25 Day
30
35
of
Lactation
40
45
50
FIG. 2. Individual suckling times for two does studied from the 15th to the 50th day of lactation. The linear regressions are significant at p = 0.0l. The milk yields of these two animals were inversely related to the period of suckling.
NURSING B E HAVI O U R OF RABBITS
713 TABLE 1
MILK YIELDS AND SUCKLING TIMES OF THE RABBIT AFTER ELECTRICAL STIMULATION OF THE INFUNDIBULUM AND MEDIAN EMINENCE
Milk yield (g) n =8 Suckling time (s) n =8
Control (Day 1)
Stimulation (Day 2)
Control (Day 3)
167.0 ± 16.1 (S.E.M.) 100%
96.3 ± 8.1 58%
156.9 ± 17.3 94%
185.8 ± 11.0 100%
182.0 ± 14.9 98%
191.5 ± 16.5 103%
The observations were made on three consecutive days between the 20th and 35th days of lactation. Stimulation was applied for 40 sec at 30, 20 and 10 min prior to suckling. Stimulation parameters: 2 msec biphasic square-waves, 50 Hz, 1 mA peak-to-peak. normal maternal behaviour and suckled for the full duration, the next day she refused the young access to the nipples and displayed no maternal behaviour. Effect of electrical stimulation. In anaesthetized rabbits electrical stimulation of the hypothalamo-neurohypophysial tract for 20 sec at 50 Hz (2 msec biphasic square-wave pulses of 1 mA peak-to-peak current) released upwards of 50 mU of oxytocin, as judged by the pressure recorded from a cannulated teat duct [5]. No measurements were made of the amount of oxytocin released by stimulation in the conscious doe, but the amounts released were unlikely to have been less than those observed during anaesthesia. F o r the most part, electrical stimulation of the infundibulum before or during the suckling of the young had no effect on either the behaviour of the mother or the amount of milk obtained by the young. Only when stimulation was prolonged to 40 sec and repeated at 30, 20 and 10 min prior to the introduction of the young were changes observed. Such treatment caused a 42% reduction (p = 0.05) in the amount of milk obtained by the young (Table 1). However, the period of suckling was not reduced or extended and nursing terminated despite the large amount of milk remaining in the mammary gland. Stimulation caused no immediate vegetative or m o t o r response from the rabbit, though grooming of the abdominal region was commonly seen shortly after stimulation. This was probably related to the strong mammary contraction promoted by the experimental release of oxytocin. DISCUSSION These observations of maternal behaviour clearly indicate that milk ejection involves much more than a simple reflex discharge of oxytocin. The fact that laboratory rabbits nurse infrequently and for such a short period is presumably an evolutionary relic which has persisted from the wild where infrequent visits to the young served to protect them from predators or allowed the rabbit to forage more widely for food. Likewise one could look on the maternal desire to nurse as a factor which would have favoured survival by ensuring that the mother returned to the nest at intervals, and this one would have assumed was largely determined by the fullness of the mammary glands. Clearly these studies and the earlier studies of Findlay and Roth [3] in which rabbits were found to display normal
nursing behaviour after the mammary glands had been emptied during a prior period of anaesthesia, show that there is almost no connection between the degree of mammary engorgement and the nursing routine. Pregnant rabbits at Day 26 of lactation suckled for the full period and gave almost no milk, whilst rabbits subjected to electrical stimulation prior to suckling terminated their suckling period at the scheduled time with mammary glands that were far from empty. There are two possible explanations for the failure of the stimulated animals to release all their milk. Stimulation may have reduced the amount of oxytocin available for release in the reflex response to the suckling of the young. Alternatively, the responsiveness of the mammary gland to oxytocin may have been reduced by the large experimental releases of oxytocin in the thirty minutes prior to nursing. The lactation of the rabbit mated post-partum is remarkably well organized to support both the suckling pups and the young in utero. Weaning occurred on Days 2 2 - 2 3 , lactation and nursing terminated on Days 2 5 - 2 7 and parturition followed on Days 3 2 - 3 3 . Had lactation terminated earlier the suckling young would have been subjected to starvation, but had lactation continued to Days 2 9 - 3 0 the oxytocin released by suckling might well have precipitated labour [ 1,4, 5 ]. The duration of the suckling period was remarkable constant, and would appear to reflect the timecourse of a complex series of neurological changes associated with the milk-ejection reflex. This is evident from several facets of the nursing routine. For example, the doe expresses a compelling desire to nurse after a period of separation from her young and this is independent of mammary engorgement, though suckling can be a most harassing experience when the young reach 4 0 - 5 0 days of age. Once the 2 - 4 rain suckling period has been experienced the doe is prepared to physically resist the advances of the young. We have no information as to what neurological mechanisms control this desire to nurse, nor do we understand what cognitive satisfaction is achieved by the act of nursing. It is clear, however, that the milk-ejection reflex involves either directly or indirectly many parts of the brain, and it is perhaps naive to assume that the afferent pathway for the milk-ejection reflex runs directly from the mammary glands to the neurosecretory nuclei of the hypothalamus.
714
LINCOLN
REFERENCES
1. Cross, B.A. On the mechanism of labour in the rabbit. J. Endocr. 16: 261-276, 1958. 2. Cross, B. A. and G. W. Harris. The role of the neurohypophysis in the milk-ejection reflex. J. Endocr. 8: 148-161, 1952. 3. Findlay, A . L . R . and L.L. Roth. Long-term dissocation of nursing behavior and the condition of the mammary gland in the rabbit. J. cornp, physioL Psychol. 72: 341-344, 1970.
4. Fuchs, A.R. Oxytocin and the onset of labour in rabbits. J. Endocr. 30: 217-224, 1964. 5. Lincoln, D. W. Labour in the rabbit: effect of electrical stimulation applied to the infundibulum and median eminence. J. Endocr. 50: 607-618, 1971. 6. Zarrow, M. X., V. H. Denenberg and C. O. Anderson. Rabbit: frequency of sucking in the pup. Science 150: 1835-1836, 1965.
BRIEF COMMUNICATION Suckling: A Time-Constant in the Nursing Behaviour of the Rabbit D. W. L I N C O L N
Department o f Anatomy, The Medical School, University Walk, Bristol. BS8 ITD, England
(Received 30 April 1974) LINCOLN, D. W. Suckling: a time.constant in the nursing behaviour of the rabbit. PHYSIOL. BEHAV. 13(5) 711-714, 1974. - The milk yield and the daily duration of suckling of rabbits on a once per day nursing routine were recorded, and the effects (1) of mating these animals postpartum and (2) of depleting the neurohypophysial stores of oxytocin by electrical stimulation were investigated. Milk yields peaked at about 190 g/rabbit/day between Days 15-20 of lactation. Thereafter yields declined and lactation terminated in pregnant animals between Days 25-27, and in nonpregnant animals at some time after Day 50. Suckling lasted for only 2 - 4 min each day, and this period declined as a linear function as lactation progressed. The suckling period was characteristically constant for the individual animal and was unrelated to milk yield or the fullness of the mammary glands. Thus, pregnant rabbits in which milk yield had declined to less than 10 g on Day 26 and electrically-stimulated rabbits in which 42% of the stored milk was not ejected terminated their daily period of suckling at a time identical to that of the controls (i.e., 3 min). Suckling
Milk ejection
Maternal behaviour
Rabbit
analysis of the suckling period and the milk yield of pregnant rabbits mated i m m e d i a t e l y after parturition. Study 2 involved an analysis of these functions after the electrical stimulation of the h y p o t h a l a m o - n e u r o h y p o p h y s i a l tract i m m e d i a t e l y prior to nursing, thereby depleting the neurohypophysial stores of readily releasable o x y t o c i n . A similar dissociation b e t w e e n m a m m a r y engorgement and nursing behaviour has been observed by Findlay and R o t h [ 3 ] . In their studies rabbits were found to display normal nursing behaviour after the m a m m a r y glands had been emptied during a brief period of anaesthesia.
T H E milk-ejection reflex of the rabbit, and the maternal behaviour associated with this event, have been the subject of several studies [2, 3, 6 ] , yet the remarkable precision with which the rabbit regulates her nursing appears always to have been overlooked. The lactating rabbit nurses i n f r e q u e n t l y and a natural periodicity of a b o u t once a day has been suggested [ 2 ] . Certainly, rabbit pups thrive if only allowed access to the m o t h e r for one daily feed. Suckling lasts from 2 - 4 min and the y o u n g obtain 1 5 0 - 2 0 0 g of milk. This is a very short period w h e n related to the 24 hr day and the f o o d intake is e n o r m o u s , for a y o u n g litter it can represent 25% of their b o d y weight. The behaviour of the nursing rabbit is most stereotyped. F o r the first 2 - 3 min she crouches motionless over the feeding young, her expression is r e m o t e and she appears oblivious to peripheral events. A second stage follows in which she assidiously attends to the g r o o m i n g of her young, frequently licking and palpating their genital regions. T h e n suddenly, nursing is terminated by her abrupt departure from the nest. The y o u n g are still keen to feed and will pursue her eagerly, but further access to the nipples is strongly resisted. In two separate studies of lactating rabbits, p e r f o r m e d some three years apart, the period of suckling has been observed to be remarkably constant and virtually indep e n d e n t of the m o s t obvious determining factor - the fullness of the m a m m a r y glands. Study 1 involved an
METHOD
Animals Adult rabbits of the Californian strain were used, weight 2 . 5 - 3 . 5 g. Mating was controlled and gestation timed from the exposure of the oestrous rabbit to 3 males within a period of 5 min. Litters were reduced where necessary to 5 y o u n g within 2 days of birth. A pelleted rabbit f o o d and water were available to the m o t h e r and y o u n g at all times.
Procedure F r o m Day 9 of lactation the m o t h e r s and y o u n g were placed on a once-a-day nursing routine. The m o t h e r s were removed to a separate r o o m and the y o u n g brought to t h e m 711
712
LINCOLN
for their daily feed. The period of suckling, defined as the period of time that the pups were attached to the nipple, was recorded to the second. This was simple to measure for the y o u n g attached themselves to the nipple within 5 30 sec of entering their m o t h e r ' s cage, and suckling was terminated by the abrupt departure of the m o t h e r from her suckling young. The y o u n g were then removed and the weight gain of the young over this suckling period of 2 4 min was recorded as the milk yield. Weight losses during this brief period in urine and feces were negligible. The second study was c o n d u c t e d on rabbits with stimulation electrodes chronically implanted in the h y p o t h a l a m o neurohypophysial tract, in the region of the infundibulum. These animals were previously used for studies on the electrical induction of labour, and the procedure for electrode implantation has already been published [5]. These electrodes were so placed that stimulation with pulse trains above 30 Hz released large a m o u n t s of o x y t o c i n from the neurohypophysis.
RESULT Normal lactation (control). Figure 1 illustrates the lactation curve in milk yield/day for the rabbit from Days 1 2 40 of lactation. Peak milk p r o d u c t i o n occurred b e t w e e n Days 15--20, and ranged from 1 0 0 - 2 8 0 g/day (mean 190). A natural weaning of the y o u n g t o o k place on the night of either Day 22 or 23, i.e., the y o u n g t o o k solid food or water for the first time and gained weight overnight. Lactation continued for m a n y m o r e weeks and the rabbits were still prepared to feed their young (or any other litter of young) at 50 days of lactation. By this time, however, the 5 young weighed twice that of the m o t h e r and nursing became a rather violent exercise. In the days prior to weaning the weight loss of the pups from one day to the next provided, when related to the milk consumed, an index of feed conversion. Thus, about 2.2 g of milk were required by the pups for each gram of body weight gain. The period of suckling ranged from 2 - 5 min, and declined progressively t h r o u g h o u t lactation (Figs. 1 and 2). This decline was linear and c o n f o r m e d to a regression function expressed by the equation y = 2.14x + 233.13 (where y is the suckling period measured in seconds and x is the day of lactation). This regression was significant at p = 0.001. The decline in milk yield and the period of suckling were not parallel, and the time spent by the individual doe in the feeding of her young was not related to whether she had a high or a low milk yield (Fig. 2). The period of suckling was remarkable in another respect for whilst some variation was observed between individual animals the suckling period of the individual animal never varied more than 10% from the individuals o w n regression line. Lactation o f the pregnant rabbit. The milk yields of pregnant rabbits, m a t e d post-partum, were identical with controls until Day 18 of lactation. Thereafter, yields declined sharply and lactation terminated on Day 25, 26 or 27 (Fig. 1). By contrast, the duration of suckling followed precisely the linear regression observed in controls. Thus, one observed the u n e x p e c t e d situation of an animal at Day 26 of lactation suckling for the full period o f about 3 rain and yielding less than 10 g of milk. Had the animal not been pregnant the milk yield in this same period would have exceeded 140 g. T e r m i n a t i o n of lactation was characterized by the fact that on one day the rabbit displayed
W
T
p
2oo
is0
~ ';, -* ~
\
100
'i\ xx'OO]
50
*"~*-*
0 ,
,
,
i%
, ,
,
250
r
==
0
y :, -- 2.14x + 233.13
o
o%;.z,_..&._
f
*-.....,...__,*
0
•~
o
,
150
0~
1'5
2AO
25
Day
of
3'0
3`5
4()
Lactation
FIG. 1. Milk yields (upper graph) and suckling times (lower graph) of pregnant (stars) and non-pregnant (asterisks) rabbits of the Californian type. W, weaning; T, termination of lactation in pregnant does; P, parturition of the same. There were 8 does in each group and 5 young in each litter. Weaning was defined as the day on which the pups first took solid food. This occurred at approximately the same time in the litters of both the pregnant and non-pregnant does.
240 I j
.
*
200
.~
. * * * *
160
*
*
*
. **
*
*
*
~ *. x-
"~ n~
* * 80'
i 4ot
-- 15
2'0
25 Day
30
35
of
Lactation
40
45
50
FIG. 2. Individual suckling times for two does studied from the 15th to the 50th day of lactation. The linear regressions are significant at p = 0.0l. The milk yields of these two animals were inversely related to the period of suckling.
NURSING B E HAVI O U R OF RABBITS
713 TABLE 1
MILK YIELDS AND SUCKLING TIMES OF THE RABBIT AFTER ELECTRICAL STIMULATION OF THE INFUNDIBULUM AND MEDIAN EMINENCE
Milk yield (g) n =8 Suckling time (s) n =8
Control (Day 1)
Stimulation (Day 2)
Control (Day 3)
167.0 ± 16.1 (S.E.M.) 100%
96.3 ± 8.1 58%
156.9 ± 17.3 94%
185.8 ± 11.0 100%
182.0 ± 14.9 98%
191.5 ± 16.5 103%
The observations were made on three consecutive days between the 20th and 35th days of lactation. Stimulation was applied for 40 sec at 30, 20 and 10 min prior to suckling. Stimulation parameters: 2 msec biphasic square-waves, 50 Hz, 1 mA peak-to-peak. normal maternal behaviour and suckled for the full duration, the next day she refused the young access to the nipples and displayed no maternal behaviour. Effect of electrical stimulation. In anaesthetized rabbits electrical stimulation of the hypothalamo-neurohypophysial tract for 20 sec at 50 Hz (2 msec biphasic square-wave pulses of 1 mA peak-to-peak current) released upwards of 50 mU of oxytocin, as judged by the pressure recorded from a cannulated teat duct [5]. No measurements were made of the amount of oxytocin released by stimulation in the conscious doe, but the amounts released were unlikely to have been less than those observed during anaesthesia. F o r the most part, electrical stimulation of the infundibulum before or during the suckling of the young had no effect on either the behaviour of the mother or the amount of milk obtained by the young. Only when stimulation was prolonged to 40 sec and repeated at 30, 20 and 10 min prior to the introduction of the young were changes observed. Such treatment caused a 42% reduction (p = 0.05) in the amount of milk obtained by the young (Table 1). However, the period of suckling was not reduced or extended and nursing terminated despite the large amount of milk remaining in the mammary gland. Stimulation caused no immediate vegetative or m o t o r response from the rabbit, though grooming of the abdominal region was commonly seen shortly after stimulation. This was probably related to the strong mammary contraction promoted by the experimental release of oxytocin. DISCUSSION These observations of maternal behaviour clearly indicate that milk ejection involves much more than a simple reflex discharge of oxytocin. The fact that laboratory rabbits nurse infrequently and for such a short period is presumably an evolutionary relic which has persisted from the wild where infrequent visits to the young served to protect them from predators or allowed the rabbit to forage more widely for food. Likewise one could look on the maternal desire to nurse as a factor which would have favoured survival by ensuring that the mother returned to the nest at intervals, and this one would have assumed was largely determined by the fullness of the mammary glands. Clearly these studies and the earlier studies of Findlay and Roth [3] in which rabbits were found to display normal
nursing behaviour after the mammary glands had been emptied during a prior period of anaesthesia, show that there is almost no connection between the degree of mammary engorgement and the nursing routine. Pregnant rabbits at Day 26 of lactation suckled for the full period and gave almost no milk, whilst rabbits subjected to electrical stimulation prior to suckling terminated their suckling period at the scheduled time with mammary glands that were far from empty. There are two possible explanations for the failure of the stimulated animals to release all their milk. Stimulation may have reduced the amount of oxytocin available for release in the reflex response to the suckling of the young. Alternatively, the responsiveness of the mammary gland to oxytocin may have been reduced by the large experimental releases of oxytocin in the thirty minutes prior to nursing. The lactation of the rabbit mated post-partum is remarkably well organized to support both the suckling pups and the young in utero. Weaning occurred on Days 2 2 - 2 3 , lactation and nursing terminated on Days 2 5 - 2 7 and parturition followed on Days 3 2 - 3 3 . Had lactation terminated earlier the suckling young would have been subjected to starvation, but had lactation continued to Days 2 9 - 3 0 the oxytocin released by suckling might well have precipitated labour [ 1,4, 5 ]. The duration of the suckling period was remarkable constant, and would appear to reflect the timecourse of a complex series of neurological changes associated with the milk-ejection reflex. This is evident from several facets of the nursing routine. For example, the doe expresses a compelling desire to nurse after a period of separation from her young and this is independent of mammary engorgement, though suckling can be a most harassing experience when the young reach 4 0 - 5 0 days of age. Once the 2 - 4 rain suckling period has been experienced the doe is prepared to physically resist the advances of the young. We have no information as to what neurological mechanisms control this desire to nurse, nor do we understand what cognitive satisfaction is achieved by the act of nursing. It is clear, however, that the milk-ejection reflex involves either directly or indirectly many parts of the brain, and it is perhaps naive to assume that the afferent pathway for the milk-ejection reflex runs directly from the mammary glands to the neurosecretory nuclei of the hypothalamus.
714
LINCOLN
REFERENCES
1. Cross, B.A. On the mechanism of labour in the rabbit. J. Endocr. 16: 261-276, 1958. 2. Cross, B. A. and G. W. Harris. The role of the neurohypophysis in the milk-ejection reflex. J. Endocr. 8: 148-161, 1952. 3. Findlay, A . L . R . and L.L. Roth. Long-term dissocation of nursing behavior and the condition of the mammary gland in the rabbit. J. cornp, physioL Psychol. 72: 341-344, 1970.
4. Fuchs, A.R. Oxytocin and the onset of labour in rabbits. J. Endocr. 30: 217-224, 1964. 5. Lincoln, D. W. Labour in the rabbit: effect of electrical stimulation applied to the infundibulum and median eminence. J. Endocr. 50: 607-618, 1971. 6. Zarrow, M. X., V. H. Denenberg and C. O. Anderson. Rabbit: frequency of sucking in the pup. Science 150: 1835-1836, 1965.