Influence of Age and Sex on the Behavior of Rats Deprived of the Rearing Response LESLEY A. SYME Department of Psychology University of Canterbury Christchurch, New Zealand
Rearing o n the hind legs, one component of “general activity,” was restricted in male and female rats both before (25 days) and after (50 days) the development of this response. In contrast to previous restriction studies the animals were kept in groups and allowed free locomotor activity in the cage environment. After 8 weeks, open-field testing showed that selective restriction decreased the frequency of rearing behavior, particularly unsupported rearing which requires physical coordination and balance. Effects were gxeater for feinales (which usually rear more than males) and for animals restricted at weaning (25 days).
Restriction studies using laboratory rodents provide little experimental evidence concerning specific kinds of activity that are eliminated by a given confinement procedure, sex differences in response to confinement, and importance of the age at which confinement is imposed. The rearing response is particularly suitable for examination of the effects of such restriction in that the experimental procedure (lowering the cage roof) allows the subjects to remain with their cagemates, thus avoiding the habitual confounding of social isolation and confinement (Lore, 1968). Restriction can also be imposed before and after development of the rearing response, permitting the investigation of 2 hypotheses proposed by Bronfenbrenner (1 968): (1 ) early stimulus deprivation of a particular modality leads t o functional impairment in that modality later in life; and (2) the later in life the stimulus deprivation occurs, the less Severe is the impairment of function. This study with rats of both sexes examined the effects of restricting the rearing response at 25 or 50 days of age and the potential of this selective manipulation as a research technique.
Method The subjects were 28 male and 26 female hooded rats ( R Q ~ ~norvegicus) US of the New Zealand Black and White Strain derived from stock originally obtained from the Otago Received for publication 19 November 1973 Revised for publication 9 May 1974 Developmental Psycliobiology, 8(1): 35-39 (1974) @ 1975 by John Wiley & Sons, Inc.
35
36
SYME
University Animal Breeding Centre. Experimental housing conditions were imposed at 50 days of age for 1 4 males and 1 4 females (7 nonlittermate rats per cage for each condition). The other 12 females and 14 males were housed selectively (6 or 7 nonlitterniate rats per cage for each condition) when weaned at 25 days of age. The animals were maintained on a reversed light schedule (dark from 0700 to 1900 hours) with food and water ad lib. Control subjects were housed in cages measuring 75 x 45 x 33 cm high, whereas the restricted-rearing cages measured 75 x 4 5 x 8 cm high. All were constructed of wood with a wre-mesh front wall and roof. The open field apparatus was painted black and measured 60 x 60 x 15 cm. The floor was divided by white painted lines into squares, 15 x 15 cm. Illumination was provided by a 22-W fluorescent lamp suspended 75 cm above the center of the field. White masking noise of about 40dB was used throughout testing. The subjects were all kept the same length of time in their cage conditions and tested at different ages (rather than keeping them in their respective cage conditions for differing lengths of time and testing at the same age) given the uniformity of open-field behavior in rats between 80 and 1 10 days of age (Candland & Nagy, 1969). Thus, after 8 weeks in their different cage conditions the subjects (aged either 81 or 106 days) were observed for 10 min in the open field. Each rat was placed in a corner of the field; at every 5th sec the observer noted its position in a corner, wall, or inner square as well as whether it was ambulating (time spent walking), rearing up on the hind legs (with or without support from the walls of the apparatus), grooming, or remaining immobile. The total numbers of lines crossed by the hind legs of the subject over the 10-min period was also recorded. A random order of testing between conditions was adopted over 2 days. Tested subjects were lightly marked with dye and returned to their home cage.
Results Because of the unequal group sizes (faulty sexing resulted in 3 of the young restricted males being discarded from the experiment within 2 days of caging) a 2 x 2 x 2 unweighted-means analysis of variance miner, 1962) was performed on each behavioral category and on the position-preference measures. Significant main effects were found only on the grooming measure (p < .05), whereas 9 of the 10 1st- and 2nd-order interaction terms (except squares entered) were significant beyond the .01 level. The large number of significant 2nd-order interactions reflects the interdependence of the 3 factors: Age at Caging, Sex, and Housing. Subsequently, t-tests were used to provide an overall comparison between means, Table 1 shows the calculated t-test values and reveals that the majority of significant t values occurred on 4 measures: inner-square occupancy, squares entered, total rearing, and unsupported rearing. These effects are represented in Figures 1 t o 4.
Discussion These findings provide an interesting illustration of the differences between drive (Baumeister, Hawkins, & Cromwell, 1964; Bronfenbrenner, 1968) and epigenetic (Kuo,
.11 2.7V
5.06+* 1.19 1.40 .49
2.5@ 3.97~ .4 2 2.09 3.7F 1.56 .93 .76
MC25-MR25 FC25-I'R 25 MC5O-MR50 f:C50-I'K50
MR25-FC25 FK 25 -MC25 MR50-I'C50 FR50-MC50 2.7P 1.20 .21 .52
2.3W 1.84 1.47 1.96
1.36 2.45+ 1.50 1.56
1.37 1.32 .83 1.90
Ambulation
Measures
3.60" 2.81+ 4.34* .22
2.79+ 4.09+' 1.64 1.71
1.11
3.31" .97 2.46' 1.91
.88 4.34" 1.76 2.5r
1.42 1.18 1.02 .22
.60 1.1 1 .8 1
.23 .25 2.03 .14
Unsupported Rearing
.2 1 2.09 .87 1.06
Total Rearing
0.46 2.5~ 2.7V 1.04
1.96 I .94 1.94 1.49
2.18 .54 .99 .09
1.71 .81 .36 .I9
Grooming
aF-female,M-niale, R-rzstricted, C-control/unrestricted, 25 or 50-caged selectively at 25 or 50 days ofage respectively. +p < .05,'+p < .01.
3.50" 3.34*
.44 2.4r .76 1.58
.02 1.09 .09 .9 7
MK25-MK50 MC25-MC50 FR25-FR50 ~ ~- 1 2~ 5 50
.84 .92 3.56'+ 2.27+
Squares Entered
.23 .I6 1.22 1.42
Inner Square Occiipancy
FR25-MR25 l'R50-MR50 FC25-MC25 1C50-MC50
Comparisons Between Group Meansa
TABLE I . Values of t-tests Calculated Between Group Means Over the 6 Behavioral Measures,
38
SYME
Fig. 1. Effects of selective. housing conditions imposed pre-rearing ( 2 5 days of age) or post-rearing (50 days of age) on the mean inner square occupancy of restricted (black) and control (white) subjccts;+p < .05. "p < .01, t tCSt.
Fig. 2. Effects of selective housing conditions imposed pre-rearing (25 days of age) or posf-rearing (50 days of age) o n the mean number of squares traversed in the open field hy rcstricted and control subjects. Legend as in Fig. I.
Fig. 3. Effects of selective housing conditions imposed pre-rearing (25 days o f age) o r posf-rearing (50 days of age) on the inem total rearing of restricted and control subjects. Legend as in 1:ig. 1.
Fig. 4. Effects of selective housing conditions imposed pre-rearing (25 days of agc) or post-rearing (50 days of ace) on the mean unsupported rearing of restricted and control subjects. Legend as in Fig. 1.
DEPRIVATION OF REARING RESPONSE
39
1967) theories of behavior development. Both hypotheses previously stated (Bronfenbrenner, 1968) were confirmed. Early deprivation of rearing behavior (25 days) caused later impairment of function; this effect was less marked for animals deprived at 50 days after the response had developed (Figs. 3 & 4). The results also suggest that supported and unsupported rearing should be distinguished in that the latter requires a greater degree of physical coordination and, perhaps, practice. For example, Figure 4 shows that the pre-rearing confined group engaged in unsupported rearing considerably less than the post-rearing group, although this distinction does not appear in the total-rearing scores shown in Figure 3. In terms of Kuo’s concept of ‘‘behavioral potentials” (Kuo, 1967, p. 174) one might conclude that the resultant unsupported rearing is lower in that the original potential was lowered for the pre-rearing confined subjects. For the females, however, the rearing potential was originally higher, in that female rats have been shown to rear more than males in a novel environment (Jilughes & Swanberg, 1970; Masur, 1972), and the “emotional” response to restriction was greater (Fig. 2) when later rearing behavior was decreased (Fig. 4). Drive theories would predict an increased rearing response by females following restriction. After all, these subjects had ample opportunity to exercise in the restricted cage environment and, therefore, should not have been physically deprived in terms of, for example, muscle development.
Notes This work is based on a thesis submitted to the University of Canterbury in partial fulfilment of the requirements for the Ph.D. degree. The author would like t o express her gratitude to Dr. R. N. Hughes and Dr. G. J. Syme for their help and encouragement throughout the project. Address reprint requests t o Lesley A. Syme, Department of Psychology, University of Western Australia, Nedlands, W. A. 6009, Australia.
References Baumeister, A., Hawkins, W. F., and Cromwell, R. L. (1964). Need states and activity level. Psychol. Bull., 61 1 438-453. Bronfenbrenner, U. (1968). Early deprivation in mammals: A cross-species analysis. In G. Newton and S. Levine (Eds.), Early Experience and Behavior. Springfield: Thomas. Pp. 627-764, Candland, D. K. and Nagy, Z. M. (1969). The open field: some comparative data. Ann. N. Y. h a d . Sci., 159: 831-85 1. Hughes, R. N. and Swanberg, K. M. (1970). Effects of food deprivation on exploration in deprivationally naive rats. Aust. J. Psychol., 22: 19-84. Kuo, Z-Y. (1967). The Dynamics of Behavior Development. New York: Random House. Lore, R. K. (1968). Activity-drive hypothesis: effects of activity restriction. Psychol. Bull., 70: 566-574. Masur, J. (1972). Sex differences in “emotionality” and behavior of rats in the open field. Behav. Biol., 7 : 149-754. Winer, B. J. (1962). Statistical Principles in Experimental Design. New York: McGraw-Hill.
Rearing o n the hind legs, one component of “general activity,” was restricted in male and female rats both before (25 days) and after (50 days) the development of this response. In contrast to previous restriction studies the animals were kept in groups and allowed free locomotor activity in the cage environment. After 8 weeks, open-field testing showed that selective restriction decreased the frequency of rearing behavior, particularly unsupported rearing which requires physical coordination and balance. Effects were gxeater for feinales (which usually rear more than males) and for animals restricted at weaning (25 days).
Restriction studies using laboratory rodents provide little experimental evidence concerning specific kinds of activity that are eliminated by a given confinement procedure, sex differences in response to confinement, and importance of the age at which confinement is imposed. The rearing response is particularly suitable for examination of the effects of such restriction in that the experimental procedure (lowering the cage roof) allows the subjects to remain with their cagemates, thus avoiding the habitual confounding of social isolation and confinement (Lore, 1968). Restriction can also be imposed before and after development of the rearing response, permitting the investigation of 2 hypotheses proposed by Bronfenbrenner (1 968): (1 ) early stimulus deprivation of a particular modality leads t o functional impairment in that modality later in life; and (2) the later in life the stimulus deprivation occurs, the less Severe is the impairment of function. This study with rats of both sexes examined the effects of restricting the rearing response at 25 or 50 days of age and the potential of this selective manipulation as a research technique.
Method The subjects were 28 male and 26 female hooded rats ( R Q ~ ~norvegicus) US of the New Zealand Black and White Strain derived from stock originally obtained from the Otago Received for publication 19 November 1973 Revised for publication 9 May 1974 Developmental Psycliobiology, 8(1): 35-39 (1974) @ 1975 by John Wiley & Sons, Inc.
35
36
SYME
University Animal Breeding Centre. Experimental housing conditions were imposed at 50 days of age for 1 4 males and 1 4 females (7 nonlittermate rats per cage for each condition). The other 12 females and 14 males were housed selectively (6 or 7 nonlitterniate rats per cage for each condition) when weaned at 25 days of age. The animals were maintained on a reversed light schedule (dark from 0700 to 1900 hours) with food and water ad lib. Control subjects were housed in cages measuring 75 x 45 x 33 cm high, whereas the restricted-rearing cages measured 75 x 4 5 x 8 cm high. All were constructed of wood with a wre-mesh front wall and roof. The open field apparatus was painted black and measured 60 x 60 x 15 cm. The floor was divided by white painted lines into squares, 15 x 15 cm. Illumination was provided by a 22-W fluorescent lamp suspended 75 cm above the center of the field. White masking noise of about 40dB was used throughout testing. The subjects were all kept the same length of time in their cage conditions and tested at different ages (rather than keeping them in their respective cage conditions for differing lengths of time and testing at the same age) given the uniformity of open-field behavior in rats between 80 and 1 10 days of age (Candland & Nagy, 1969). Thus, after 8 weeks in their different cage conditions the subjects (aged either 81 or 106 days) were observed for 10 min in the open field. Each rat was placed in a corner of the field; at every 5th sec the observer noted its position in a corner, wall, or inner square as well as whether it was ambulating (time spent walking), rearing up on the hind legs (with or without support from the walls of the apparatus), grooming, or remaining immobile. The total numbers of lines crossed by the hind legs of the subject over the 10-min period was also recorded. A random order of testing between conditions was adopted over 2 days. Tested subjects were lightly marked with dye and returned to their home cage.
Results Because of the unequal group sizes (faulty sexing resulted in 3 of the young restricted males being discarded from the experiment within 2 days of caging) a 2 x 2 x 2 unweighted-means analysis of variance miner, 1962) was performed on each behavioral category and on the position-preference measures. Significant main effects were found only on the grooming measure (p < .05), whereas 9 of the 10 1st- and 2nd-order interaction terms (except squares entered) were significant beyond the .01 level. The large number of significant 2nd-order interactions reflects the interdependence of the 3 factors: Age at Caging, Sex, and Housing. Subsequently, t-tests were used to provide an overall comparison between means, Table 1 shows the calculated t-test values and reveals that the majority of significant t values occurred on 4 measures: inner-square occupancy, squares entered, total rearing, and unsupported rearing. These effects are represented in Figures 1 t o 4.
Discussion These findings provide an interesting illustration of the differences between drive (Baumeister, Hawkins, & Cromwell, 1964; Bronfenbrenner, 1968) and epigenetic (Kuo,
.11 2.7V
5.06+* 1.19 1.40 .49
2.5@ 3.97~ .4 2 2.09 3.7F 1.56 .93 .76
MC25-MR25 FC25-I'R 25 MC5O-MR50 f:C50-I'K50
MR25-FC25 FK 25 -MC25 MR50-I'C50 FR50-MC50 2.7P 1.20 .21 .52
2.3W 1.84 1.47 1.96
1.36 2.45+ 1.50 1.56
1.37 1.32 .83 1.90
Ambulation
Measures
3.60" 2.81+ 4.34* .22
2.79+ 4.09+' 1.64 1.71
1.11
3.31" .97 2.46' 1.91
.88 4.34" 1.76 2.5r
1.42 1.18 1.02 .22
.60 1.1 1 .8 1
.23 .25 2.03 .14
Unsupported Rearing
.2 1 2.09 .87 1.06
Total Rearing
0.46 2.5~ 2.7V 1.04
1.96 I .94 1.94 1.49
2.18 .54 .99 .09
1.71 .81 .36 .I9
Grooming
aF-female,M-niale, R-rzstricted, C-control/unrestricted, 25 or 50-caged selectively at 25 or 50 days ofage respectively. +p < .05,'+p < .01.
3.50" 3.34*
.44 2.4r .76 1.58
.02 1.09 .09 .9 7
MK25-MK50 MC25-MC50 FR25-FR50 ~ ~- 1 2~ 5 50
.84 .92 3.56'+ 2.27+
Squares Entered
.23 .I6 1.22 1.42
Inner Square Occiipancy
FR25-MR25 l'R50-MR50 FC25-MC25 1C50-MC50
Comparisons Between Group Meansa
TABLE I . Values of t-tests Calculated Between Group Means Over the 6 Behavioral Measures,
38
SYME
Fig. 1. Effects of selective. housing conditions imposed pre-rearing ( 2 5 days of age) or post-rearing (50 days of age) on the mean inner square occupancy of restricted (black) and control (white) subjccts;+p < .05. "p < .01, t tCSt.
Fig. 2. Effects of selective housing conditions imposed pre-rearing (25 days of age) or posf-rearing (50 days of age) o n the mean number of squares traversed in the open field hy rcstricted and control subjects. Legend as in Fig. I.
Fig. 3. Effects of selective housing conditions imposed pre-rearing (25 days o f age) o r posf-rearing (50 days of age) on the inem total rearing of restricted and control subjects. Legend as in 1:ig. 1.
Fig. 4. Effects of selective housing conditions imposed pre-rearing (25 days of agc) or post-rearing (50 days of ace) on the mean unsupported rearing of restricted and control subjects. Legend as in Fig. 1.
DEPRIVATION OF REARING RESPONSE
39
1967) theories of behavior development. Both hypotheses previously stated (Bronfenbrenner, 1968) were confirmed. Early deprivation of rearing behavior (25 days) caused later impairment of function; this effect was less marked for animals deprived at 50 days after the response had developed (Figs. 3 & 4). The results also suggest that supported and unsupported rearing should be distinguished in that the latter requires a greater degree of physical coordination and, perhaps, practice. For example, Figure 4 shows that the pre-rearing confined group engaged in unsupported rearing considerably less than the post-rearing group, although this distinction does not appear in the total-rearing scores shown in Figure 3. In terms of Kuo’s concept of ‘‘behavioral potentials” (Kuo, 1967, p. 174) one might conclude that the resultant unsupported rearing is lower in that the original potential was lowered for the pre-rearing confined subjects. For the females, however, the rearing potential was originally higher, in that female rats have been shown to rear more than males in a novel environment (Jilughes & Swanberg, 1970; Masur, 1972), and the “emotional” response to restriction was greater (Fig. 2) when later rearing behavior was decreased (Fig. 4). Drive theories would predict an increased rearing response by females following restriction. After all, these subjects had ample opportunity to exercise in the restricted cage environment and, therefore, should not have been physically deprived in terms of, for example, muscle development.
Notes This work is based on a thesis submitted to the University of Canterbury in partial fulfilment of the requirements for the Ph.D. degree. The author would like t o express her gratitude to Dr. R. N. Hughes and Dr. G. J. Syme for their help and encouragement throughout the project. Address reprint requests t o Lesley A. Syme, Department of Psychology, University of Western Australia, Nedlands, W. A. 6009, Australia.
References Baumeister, A., Hawkins, W. F., and Cromwell, R. L. (1964). Need states and activity level. Psychol. Bull., 61 1 438-453. Bronfenbrenner, U. (1968). Early deprivation in mammals: A cross-species analysis. In G. Newton and S. Levine (Eds.), Early Experience and Behavior. Springfield: Thomas. Pp. 627-764, Candland, D. K. and Nagy, Z. M. (1969). The open field: some comparative data. Ann. N. Y. h a d . Sci., 159: 831-85 1. Hughes, R. N. and Swanberg, K. M. (1970). Effects of food deprivation on exploration in deprivationally naive rats. Aust. J. Psychol., 22: 19-84. Kuo, Z-Y. (1967). The Dynamics of Behavior Development. New York: Random House. Lore, R. K. (1968). Activity-drive hypothesis: effects of activity restriction. Psychol. Bull., 70: 566-574. Masur, J. (1972). Sex differences in “emotionality” and behavior of rats in the open field. Behav. Biol., 7 : 149-754. Winer, B. J. (1962). Statistical Principles in Experimental Design. New York: McGraw-Hill.
