Selective Pup Retrieving by Mother Rats: Sex and Early Development Characteristiis as Discri&ination Factors DOMINIQUE DEVITERNE DIDIER DESOR Laboratoire de Biologie du Comportement U.R.A. 1293 C . N . R . S . Uniuersitt de Nancy I B.P. 239 54506 Vandoeuure-12s-Nancy, Cedex, France
Discrimination between own-litter pups by mother rats was studied over 14 litters, in a standardized situation eliciting maternal pup-retrieving activity. Results showed some consistency in the order in which pups of a litter were retrieved by the mother in the 4-day and 9-day tests and that this order was related to certain characteristics of the pups: 1) on Day 4 and 9, the best-developed pups of the litter (in terms of body weight and neuromotor behaviors) were first retrieved; 2) on Day 9, sex of pups became an additional discrimination factor, as males were retrieved before their female littermates. Variables included in these global discrimination factors and possible consequences of such differential mother-pups interactions are discussed.
Introduction In mammals, and particularly in rodents, mothers give birth to very immature young that are completely dependent on parental care during the nursing period. Using such species and experimentally enhanced interindividual differences, many works have demonstrated relations between the quality of displayed maternal care and pup development. Yet, they did not reach clear conclusions as to the direction of these relations and even their real existence in normal ranges of variations (Hofer, 1981). In a previous study using rats (Deviterne, Desor, & Krafft, 1990), we specified these relations for two components of maternal behavior (time spent with young Reprint requests should be sent to Dominique Deviterne, Laboratoire de Biologie du Comportement-URA 1293 CNRS, Universitt de Nancy I, BP 239, 54506, Vandoeuvre-18s-Nancy, France. Received for publication 22 May 1989 Revised for publication 30 January 1990 Accepted at Wiley 20 March 1990 Developmental Psychobiology 23(4):361-368 (1990) 0 1990 by John Wiley & Sons, Inc.
CCC 0012- 1630/90/040361-8$04.00
362
DEVITERNE AND DESOR
and rapidity of retrieving). It was found that some criteria of the development of the pups (in terms of weight gain and neuromotor behaviors) were positively related to the time the mother spent with young but not to the rapidity of retrieving. This latter maternal characteristic was, in contrast, positively linked to the physical state of pups (in terms of body weight) on the test day. These results vere obtained by comparison of litters considered as statistical units relative to pup characteristics. The underlying assumption was that maternal care was equally shared between all the pups of the litter. Yet, considering that in each litter the range of variation in pup development is often large, a large range of variation in the care the mother will administer to variously developed pups of its litter may be assumed. This would imply that the mother is able to discriminate between its pups on the basis of some of their characteristics. Such an hypothesis is gaining adherence as several works have demonstrated that a pup’s sex determines the way the mother licks its anogenital area (Moore & Morelli, 1979; Moore, 1981; 1982; 1985; Moore & Chadwick-Dias, 1986; Richmond & Sachs, 1984). The aim of the present study is to prove the existence of such maternal discrimination and to analyze its underlying factors in the retrieving activity.
Method Subjects All animals were albino Wistar rats, bred in the laboratory from stock originally obtained from Iffa Credo Breeding Laboratory. Temperature in the colony room was kept at 22”C, and a 12-h light/12-h dark cycle, with lights off at 1100 h, was maintained. Water and standard laboratory diet were available ad libitum. Virgin females homogeneous in age (80-100 days) and weight (200-250 g) were bred by placing them with males until sperm was found in a daily vaginal lavage. Pregnant females were confined to individual polycarbonate cages (50 x 30 x 20 cm). As of two days before expected parturition, females were daily checked for pups. The day when pups were found was designated Day 1. The total number of pups was 169, divided over 14 natural litters, of size varying between 10 and 14 pups (2-4 litters of each size), for which the gestation period was normal (21 days ? 12 h), with a relatively balanced sex ratio (maximum 65% for either sex), and without postpartum mortality (during the first 10 postnatal days).
Procedure Selective Retrieving Test Each mother was subjected to three retrieving tests using a device (Fig. 1) constituted with a circular platform (70 cm in diameter) placed on the top of the rearing cage. The mother could gain access to this platform through a central hole. Its access was made easier by a small, suspended, circular platform. On each test, all the pups of the litter were simultaneously presented to the mother by randomly placing them in the required number of small plastic containers which were regularly disposed all around the large platform.
SELECTIVE PUP RETRIEVING BY MOTHER RATS
363
Fig. 1. Device used for the selective retrieving test. Example for a 1Zpups litter.
The first retrieving test, on Day 1, was considered as a mother habituation to the device. During the two other tests, on Days 4 and 9, we noted the order in which the mother retrieved its pups (Ro 4 and Ro 9).
Developmental Evaluation of Pups On Day 1 , each pup was marked with a subcutaneous injection of a physiological dye to permit identification throughout testing. Its sex was also determined. On Days 4 and 9, it was characterized for its body weight (Wt 4, Wt 9) and motor development level, evaluated by standardized scores (varying from 1: “worst” score to 9: “best” score) at classic items extracted from a Neuromotor Development Scale (Deviterne et al., 1990). On Day 4, individual neuromotor score (Mot 4) was obtained by summing the two standard scores on: -Item 1: Righting reflex. The young rat was placed on its back on a horizontal board which was covered with felt. Time spent to return to the normal dorso-ventral position determined the standard score.
364
DEVITERNE AND DESOR -Item 2: Grasping rejex. It was evaluated using a wire-netting board fixed onto the axis of an electric motor (1 revolution per min). The young rat was placed on the board when stopped at the horizontal position. During the rotation, to avoid falling, the rat had to cling onto the wire-mesh. A pointer was used to measure the angle of rotation reached when the animal let go. The angle-value reached (usually between 45"and 90") determined the standard score.
On Day 9, individual neuromotor score (Mot 9) was obtained by summing the two standard scores on: -Itern 3: Negative geotaxis. The rat was placed head down on an inclined plane at an angle of 20" from the horizontal position. The time spent to make a 180" turn back to the normal position with head upward determined the standard score. --Item 4: Suspension. On postnatal Day 9, the forefeet of the rat were made to touch a horizontal thread (0.5mm thick, 25 cm long, horizontally extended between two poles 30 cm high) which immediately triggered a grab-on reaction. Once this reaction was well established, the rat was gently loosened. The time during which it remained hanging determined the standard score. Pup ranks in their litters were regrouped in five classes for retrieving order, for body weight, and for motor development level (C1 for the first pup to C5 for the last pup). In order to permit parametric analysis of the data (Zar, 1974), the frequency distributions in these classes were chosen to fit the theorical frequencies of the Normal distribution. In order to account for the effect of pup sex, a "dummy variable" (Zar, 1974) was introduced in computation (X = 0 for males and X = 1 for females).
Results Within each litter, 4-day and 9-day body weight gradings (Wt 4 and Wt 9) were significantly correlated (Table 1). This was not the case between 4-day and 9-day neuromotor gradings (Mot 4 and Mot 9), which otherwise were not found to be significantly related to body weight grading (Table 1). Retrieving orders (Ro 4 and Ro 9) were significantly correlated between Day 4 and Day 9 (Table 1). On Days 4 and on Day 9, the retrieving order of the pups was significantly related to their body weight- and neuromotor-gradings on the test day (Table 1): heaviest pups were retrieved before lightest ones (Fig. 2) and more developed pups, in terms of neuromotor behaviors, tended to be retrieved sooner than less developed pups. The order of retrieving was not significantly related to the sex of pups on Day 4. On Day 9, however, this relation was significant (Table 1): young males were more often retrieved before their female siblings (Fig. 3). This latter relation did not result from the mere effect of weight, despite the
SELECTIVE PUP RETRIEVING BY MOTHER RATS
365
Table 1 Correlation Matrixfor the Variables Studied (Weight, Neuromotor, and Retrieving Order Gradings and Sex of Pups) on Postnatal Days 4 and 9. Ro 4
Wt 4
Mot 4
Ro 9
Wt 9
Wt 4
,258
Mot 4
,247
,048
Ro 9
.306
,565
.I13
Wt 9
.237
,663
,005
.518
Mot 9
.I29
,247
.043
.231
.199
Sex
.090
.350
.023
.350
,244
Mot 9
**
*
**
**
**
*
*
**
** *
**
*
.170
* p < .01. ** p
Discrimination between own-litter pups by mother rats was studied over 14 litters, in a standardized situation eliciting maternal pup-retrieving activity. Results showed some consistency in the order in which pups of a litter were retrieved by the mother in the 4-day and 9-day tests and that this order was related to certain characteristics of the pups: 1) on Day 4 and 9, the best-developed pups of the litter (in terms of body weight and neuromotor behaviors) were first retrieved; 2) on Day 9, sex of pups became an additional discrimination factor, as males were retrieved before their female littermates. Variables included in these global discrimination factors and possible consequences of such differential mother-pups interactions are discussed.
Introduction In mammals, and particularly in rodents, mothers give birth to very immature young that are completely dependent on parental care during the nursing period. Using such species and experimentally enhanced interindividual differences, many works have demonstrated relations between the quality of displayed maternal care and pup development. Yet, they did not reach clear conclusions as to the direction of these relations and even their real existence in normal ranges of variations (Hofer, 1981). In a previous study using rats (Deviterne, Desor, & Krafft, 1990), we specified these relations for two components of maternal behavior (time spent with young Reprint requests should be sent to Dominique Deviterne, Laboratoire de Biologie du Comportement-URA 1293 CNRS, Universitt de Nancy I, BP 239, 54506, Vandoeuvre-18s-Nancy, France. Received for publication 22 May 1989 Revised for publication 30 January 1990 Accepted at Wiley 20 March 1990 Developmental Psychobiology 23(4):361-368 (1990) 0 1990 by John Wiley & Sons, Inc.
CCC 0012- 1630/90/040361-8$04.00
362
DEVITERNE AND DESOR
and rapidity of retrieving). It was found that some criteria of the development of the pups (in terms of weight gain and neuromotor behaviors) were positively related to the time the mother spent with young but not to the rapidity of retrieving. This latter maternal characteristic was, in contrast, positively linked to the physical state of pups (in terms of body weight) on the test day. These results vere obtained by comparison of litters considered as statistical units relative to pup characteristics. The underlying assumption was that maternal care was equally shared between all the pups of the litter. Yet, considering that in each litter the range of variation in pup development is often large, a large range of variation in the care the mother will administer to variously developed pups of its litter may be assumed. This would imply that the mother is able to discriminate between its pups on the basis of some of their characteristics. Such an hypothesis is gaining adherence as several works have demonstrated that a pup’s sex determines the way the mother licks its anogenital area (Moore & Morelli, 1979; Moore, 1981; 1982; 1985; Moore & Chadwick-Dias, 1986; Richmond & Sachs, 1984). The aim of the present study is to prove the existence of such maternal discrimination and to analyze its underlying factors in the retrieving activity.
Method Subjects All animals were albino Wistar rats, bred in the laboratory from stock originally obtained from Iffa Credo Breeding Laboratory. Temperature in the colony room was kept at 22”C, and a 12-h light/12-h dark cycle, with lights off at 1100 h, was maintained. Water and standard laboratory diet were available ad libitum. Virgin females homogeneous in age (80-100 days) and weight (200-250 g) were bred by placing them with males until sperm was found in a daily vaginal lavage. Pregnant females were confined to individual polycarbonate cages (50 x 30 x 20 cm). As of two days before expected parturition, females were daily checked for pups. The day when pups were found was designated Day 1. The total number of pups was 169, divided over 14 natural litters, of size varying between 10 and 14 pups (2-4 litters of each size), for which the gestation period was normal (21 days ? 12 h), with a relatively balanced sex ratio (maximum 65% for either sex), and without postpartum mortality (during the first 10 postnatal days).
Procedure Selective Retrieving Test Each mother was subjected to three retrieving tests using a device (Fig. 1) constituted with a circular platform (70 cm in diameter) placed on the top of the rearing cage. The mother could gain access to this platform through a central hole. Its access was made easier by a small, suspended, circular platform. On each test, all the pups of the litter were simultaneously presented to the mother by randomly placing them in the required number of small plastic containers which were regularly disposed all around the large platform.
SELECTIVE PUP RETRIEVING BY MOTHER RATS
363
Fig. 1. Device used for the selective retrieving test. Example for a 1Zpups litter.
The first retrieving test, on Day 1, was considered as a mother habituation to the device. During the two other tests, on Days 4 and 9, we noted the order in which the mother retrieved its pups (Ro 4 and Ro 9).
Developmental Evaluation of Pups On Day 1 , each pup was marked with a subcutaneous injection of a physiological dye to permit identification throughout testing. Its sex was also determined. On Days 4 and 9, it was characterized for its body weight (Wt 4, Wt 9) and motor development level, evaluated by standardized scores (varying from 1: “worst” score to 9: “best” score) at classic items extracted from a Neuromotor Development Scale (Deviterne et al., 1990). On Day 4, individual neuromotor score (Mot 4) was obtained by summing the two standard scores on: -Item 1: Righting reflex. The young rat was placed on its back on a horizontal board which was covered with felt. Time spent to return to the normal dorso-ventral position determined the standard score.
364
DEVITERNE AND DESOR -Item 2: Grasping rejex. It was evaluated using a wire-netting board fixed onto the axis of an electric motor (1 revolution per min). The young rat was placed on the board when stopped at the horizontal position. During the rotation, to avoid falling, the rat had to cling onto the wire-mesh. A pointer was used to measure the angle of rotation reached when the animal let go. The angle-value reached (usually between 45"and 90") determined the standard score.
On Day 9, individual neuromotor score (Mot 9) was obtained by summing the two standard scores on: -Itern 3: Negative geotaxis. The rat was placed head down on an inclined plane at an angle of 20" from the horizontal position. The time spent to make a 180" turn back to the normal position with head upward determined the standard score. --Item 4: Suspension. On postnatal Day 9, the forefeet of the rat were made to touch a horizontal thread (0.5mm thick, 25 cm long, horizontally extended between two poles 30 cm high) which immediately triggered a grab-on reaction. Once this reaction was well established, the rat was gently loosened. The time during which it remained hanging determined the standard score. Pup ranks in their litters were regrouped in five classes for retrieving order, for body weight, and for motor development level (C1 for the first pup to C5 for the last pup). In order to permit parametric analysis of the data (Zar, 1974), the frequency distributions in these classes were chosen to fit the theorical frequencies of the Normal distribution. In order to account for the effect of pup sex, a "dummy variable" (Zar, 1974) was introduced in computation (X = 0 for males and X = 1 for females).
Results Within each litter, 4-day and 9-day body weight gradings (Wt 4 and Wt 9) were significantly correlated (Table 1). This was not the case between 4-day and 9-day neuromotor gradings (Mot 4 and Mot 9), which otherwise were not found to be significantly related to body weight grading (Table 1). Retrieving orders (Ro 4 and Ro 9) were significantly correlated between Day 4 and Day 9 (Table 1). On Days 4 and on Day 9, the retrieving order of the pups was significantly related to their body weight- and neuromotor-gradings on the test day (Table 1): heaviest pups were retrieved before lightest ones (Fig. 2) and more developed pups, in terms of neuromotor behaviors, tended to be retrieved sooner than less developed pups. The order of retrieving was not significantly related to the sex of pups on Day 4. On Day 9, however, this relation was significant (Table 1): young males were more often retrieved before their female siblings (Fig. 3). This latter relation did not result from the mere effect of weight, despite the
SELECTIVE PUP RETRIEVING BY MOTHER RATS
365
Table 1 Correlation Matrixfor the Variables Studied (Weight, Neuromotor, and Retrieving Order Gradings and Sex of Pups) on Postnatal Days 4 and 9. Ro 4
Wt 4
Mot 4
Ro 9
Wt 9
Wt 4
,258
Mot 4
,247
,048
Ro 9
.306
,565
.I13
Wt 9
.237
,663
,005
.518
Mot 9
.I29
,247
.043
.231
.199
Sex
.090
.350
.023
.350
,244
Mot 9
**
*
**
**
**
*
*
**
** *
**
*
.170
* p < .01. ** p
