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Temporal boundary of the sensitive period for hormonal organization of social play in juvenile rats
Physiology & Behavior, Vol. 26, pp. 241-243. Pergamon Press and Brain Research Publ., 1981. Printed in the U.S.A.
Temporal Boundary of the Sensitive Period for Hormonal Organization of Social Play in Juvenile Rats ' WILLIAM
W. BEATTY, ANTHONY
M. D O D G E , K A T H Y L . T R A Y L O R
Department of Psychology, North Dakota State University, Fargo, ND 58105 AND MICHAEL
J. M E A N E Y
Department of Psychology, Concordia University, Montreal, Quebec, Canada R e c e i v e d 7 A u g u s t 1980 BEATTY, W. W., A. M. DODGE, K. L. TRAYLOR AND M. J. MEANEY. Temporal boundary of the sensitive period for hormonal organization of social play in juvenile rats. PHYSIOL. BEHAV. 26(2) 241-243, 1981.--The play-fighting of juvenile rats is a sexually dimorphic behavior which is influenced by the organizational actions of testosterone and dihydrotestosterone but is not dependent on activational influences of these testicular hormones. To delimit more precisely the temporal period in early development when these organizational effects are expressed, male rats were castrated 1, 6, I0 or 20 days after birth and their play behavior was compared to that of intact males and females. Daily observations were made of groups of 6-7 animals from 31-40 days of age. Intact males engaged in rough play more frequently than females and also initiated more play bouts. Castration at day 10 or 20 did not affect play in males, but castration on day 1 or 6 reduced male play to levels that were not reliably different from females. The development of social play depends on the presence of testicular androgens for at least 6 days after birth.
Play Social development Social behavior Gonadectomy Gonadal hormones Hormonal organization Sensitive period Critical period Sex differences V A R I O U S play behaviors are observed among juvenile mammals when they are studied in a social setting. Certain of these behaviors, notably responses that are variously termed rough and tumble play or play-fighting, are performed more frequently by males than by females, at least in some species. Such sex differences in play have been well documented in both rhesus monkeys and Norway rats [9, 10, 14, 15]. In both of these species sexually dimorphic play patterns apparently differentiate early in life under the organizational influence of testosterone or its 5a reduced metabolite, dihydrotestosterone [9,14]. Exposing females to T or DHT early in development (prenatally in monkeys, neonatally in rats) masculinizes their juvenile play behavior. Likewise castrating male rats within 24 hr after birth reduces the frequency of play-fighting. By contrast, castrating male rats at 23 days of age or male monkeys shortly after birth does not alter sexually dimorphic play; animals so treated play like intact males [9,15]. These findings imply that organizational but not activational effects of gonadal hormones are important regulators of play-fighting and further suggest that the organiz-
ing actions of testicular hormones may occur during a temporally limited period early in development as is true of other sexually dimorphic responses such as reproductive and aggressive behavior (see e.g. [2,10] for reviews). In the present experiment we sought to delimit more precisely, the sensitive period for the organizational effect o f testicular hormones on social play by studying play behaviors in males that were castrated at various ages between birth and 20 days. Although serum levels of testosterone and other androgens decline after birth in both sexes [5, 12, 19], males have higher serum androgen levels from birth to 25 days of age [5]. Hence, castration was performed at several different ages. METHOD
Animals
The animals were albino rats of both sexes born in the laboratory to dams obtained early in pregnancy from the Holtzman Co., Madison, WI. Upon arrival the pregnant females were placed into 24×46×15 cm plastic maternity cages with free access to food and water at all times. The
1Supported by Grant HD 12620-01 from NICHHD to William W. Beatty. Michael J. Meaney was supported by a predoctoral fellowship from the Ministere de l'Education, Province de Quebec. Address reprint requests to the first author.
C o p y r i g h t © 1981 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/81/020241-03502.00/0
BEATTY ET AL.
242 colony room was maintained on a 12L:12D cycle with the fluorescent lights on at 0600 hr. Within 24 hr of birth the animals were sexed and randomly assigned to litters of no more than 10 pups each.
Procedure Males were randomly assigned to groups of animals that were castrated on Day 1 (N=13), Day 6 ( N = l l ) , Day l0 (N = 10), Day 20 (N = 12) or left unoperated (N = 14). The day of birth was considered Day 0. Females (N=31) were left untreated. Hypothermia was used as the anesthetic for 1 and 6 day-old animals; ether was employed for the 10 day-old pups while Chloropent was employed for the 20 day-old rats. The animals were weaned at 20 days of age, but continued to live in the maternity cages until 25 days of age. Then they were assigned to one of 14 groups of 6 or 7 animals each and marked on the tail for identification with colored felt-tip pens. Once formed the groups remained intact for the duration of the study. Each group contained at least one intact male and two females and was housed in observation cages (38x61 × 38 cm high) made of galvanized metal. The back of the cage was made of hardware cloth and the front of clear plastic. Illumination was provided continuously by a white 7.5 W bulb mounted in the ceiling. These cages were located in the colony room. Food and water were freely available throughout the study. The animals were not disturbed except for normal maintenance and to renew tail markings every 3-4 days. Between 31 and 40 days of age daily observations of social play were obtained. Observations began at the onset of the dark phase of the L:D cycle and continued until each group had been observed for 105, 20 sec-long periods each day (typically 4-6 hr). Within the group every animal that participated in play during that period was given a score of " l " . Likewise every animal that initiated play during an observation period was given a score of " 1 " in the play initiation category. Hence, individual scores could range from 0-105 for each day on each category. The observed range of daily scores was 0-32 for social play and 0-20 for play initiation. The behavioral rating system has been described in detail elsewhere [13,15]. In brief, play-fighting in juvenile rats includes the following behaviors: pouncing--one rat lunges at another with its forepaws extended; wrestling--rats roll and tumble about; boxing--rats stand erect pawing at one another with their forepaws; pinning--(on-top, on-bottom postures) one rat stands on another while the rat on the bottom struggles to escape; play-biting--one rat bites another usually on the tail or leg. Play-biting may involve vigorous pulling, but the biting is inhibited and we never observed a wound. Pouncing, tail and leg pulling were scored as play initiation behaviors. RESULTS Table 1 depicts mean frequency of social play and play initiation for the various treatment groups. As expected from earlier work [13, 14, 15] males engaged in more rough play than females and were more likely to initiate play bouts. Castrating males on day 1 reduced levels of play on both measures, eliminating the sex difference. Castration at day 10 or 20 was without effect on either measure. Males castrated on day 6 engated in about as much play as females and were somewhat more likely to initiate play bouts. Statistical analysis confirmed these apparent trends by
TABLE 1 MEAN FREQUENCY (_+ SEM)
Treatment
Social Play
Intact Males Day 20 Castrates Day 10 Castrates Day 6 Castrates Day 1 Castrates Intact Females
132.6 _+ 6.0 122.5 _ 7.0 121.1 _+ 7.7 100.0 _+ 8.2 90.5 _+ 6.5 99.5 _+ 4.9
Play Initiation 62.5 55.6 53.9 43.4 35.3 35.6
_+ 4.5 _+ 5.2 -+ 3.3 _+ 5.5 _+ 3.0 - 2.9
revealing reliable differences among treatment groups on both measures, F(5,85)=6.01 for social play and 8.40 for play initiation, both p<0.001. Subsequent analysis with t-tests demonstrated that the intact males and the males castrated on Day 10 or Day 20 played more frequently and initiated more play bouts than either the Day 1 castrates or the intact females (t~>2.37, all p<0.02). There were no significant differences on either measure among the Day 10 castrates, the Day 20 castrates and the intact males (t~<1.32) or between the Day 1 castrates and the females (t~2.11) and the difference between the Day 6 and the Day 10 castrate groups approached significance (t = 1.94). On the play initiation measure Day 6 castrates differed from intact males (t =3.03), but differences between the Day 6 castrates and the Day 10 or Day 20 castrates were not significant. There were no reliable differences between the Day 6 castrates and the females or the Day 1 castrates on either measure (t~<1.42). Insofar as we could determine play frequency was not systematically related to differences in the composition of the social groups. However, since 13 of the 14 groups contained from 2-4 males that were intact or had been castrated at Day 10 or 20, the absence of an influence of group composition is not surprising. DISCUSSION The present findings confirm earlier observations regarding the differential effects on play of castrating neonatal and weanling male rats [1 I]. In addition the present data suggest that the development of typically masculine levels of social play requires exposure to testicular hormones for more than 6 days following birth and that the organizing action of androgens is normally completed by I0 days of age. Thus, the temporal boundaries for hormonal organization of play and male sexual behavior [11] in the rat are quite similar. Because of the obvious morphological similarity of the responses made in play-fighting by juveniles to the agonistic behaviors of adults it is of interest to compare hormonal influences on play and aggression. Gonadal hormones acting during the neonatal period influence adult levels of aggression in rats [1], but the boundary of the sensitive period has not been delineated in this species. However, the sensitive period for hormonal o~'ganization of play in the rat is similar to the comparable temporal boundary for organizational effects of gonadal hormones on fighting by adult mice [6,16]. At the same time it is important to recognize that hormonal control of adult aggressive behavior in both rats and mice differs from hormonal regulation of play in several important respects: (1) The display of typically masculine levels of ag-
EARLY CASTRATION AND PLAY
243
gression by adult rats and mice depends on both the organizational and activational actions of androgens [2, 3, 6] while masculine levels of play are independent of circulating androgens at the time of testing [15]. (2) Estradiol exerts a significant organizational influence on aggressive behavior in mice [4,8], but is without effect on play-fighting in rats [15] and (3) prenatal exposure to androgens, such as can occur in utero, exerts a masculinizing effect on aggression in female mice [18]. At present nothing is known of possible prenatal influences of gonadal hormones on play in young rats. Such influences, if they exist at all, would be expected to be quite modest inasmuch as castrating males at birth completely eliminates the sex difference in play-fighting. Perhaps it is not surprising that hormonal control of juvenile play-fighting
and adult aggression are different since social play is clearly not essential for the development of aggressive behavior in adults. Mice, for example, exhibit little or no social play [17]. Finally, although the present results indicate that hormonal organization of structures important to play is normally completed by 10 days of age, this in no way implies that the relevant mechanisms lose their ability to respond to hormonal stimuli introduced at a later age. Again aggressive behavior in mice provides a potentially instructive perspective. Although hormonal organization of this sexually dimorphic behavior is normally completed within the first 10 days of postnatal life, prolonged treatment with testosterone can masculinize aggressive behavior of female mice, even if the treatment is not initiated until 30 days of age [7].
REFERENCES 1. Barr, G. A., J. L. Gibbons and K. E. Moyer. Male-female differences and the influence of neonatal and adult testosterone on intraspecies aggression in rats. J. comp. physiol. Psychol. 90: 1169-1183, 1976. 2. Beatty, W. W. Gonadal hormones and sex differences in nonreproductive behaviors in rodents: Organizational and activational influences. Hormones Behav. 12:112-163, 1979. 3. Beeman, E. A. The effect of male hormone on aggressive behavior in mice. Physiol. Zool. 14: 323-340, 1947. 4. Bronson, F. H. and C. H. Desjardins. Aggression in adult mice: Modification by neonatal injections of gonadal hormones. Science 161: 705-706, 1968. 5. Dohler, K. D. and W. Wuttke. Changes with age of serum gonadotrophins, prolactin, and gonadal steroids in prepubertal male and female rats. Endocrinology 97: 898-907, 1975. 6. Edwards, D. A. Early androgen stimulation and aggressive behavior in male and female mice. Physiol. Behav. 4: 333-338, 1969. 7. Edwards, D. A. Post-neonatal androgenization and adult aggressive behavior in female mice. Physiol. Behav. 5: 465-467, 1970. 8. Edwards, D. A. and J. Herudon. Neonatal estrogen stimulation and aggressive behavior in female mice. Physiol. Behav. 5: 993-995, 1970. 9. Goy, R. W. Development of play and mounting behaviour in female rhesus virilized prenatally with esters of testosterone or dihydrotestosterone. In: Recent Advances in Primatology, Vol. ! Behaviour, edited by D. J. Chivers and J. Herbert. London: Academic Press, 1978, pp. 449-462. 10. Goy, R. W. and D. A. Goldfoot. Hormonal influences on sexually dimorphic behavior. In: Handbook of Physiology, Sect. 7, Vol. 2, Part l, edited by R. O. Greep. Baltimore: Williams and Wilkins, 1973, pp. 169-186.
11. Grady, K. L., C. H. Phoenix and W. C. Young. Role of the developing rat testis in differentiation of the neural tissues mediating mating behavior. J. comp. physiol. Psychol. 59: 176182, 1965. 12. Lee, V. W. K., D. M. De Kretser, B. Hudson and C. Wang. Variations in serum FSH, LH and testosterone levels in male rats from birth to sexual maturity. J. Reprod. Fert. 42: 121-126, 1975. 13. Meaney, M. J., A. M. Dodge and W. W. Beatty. Sex-dependent effects of amygdaloid lesions on the social play of prepubertal rats. Submitted for publication. 14. Meaney, M. J. and J. Stewart. A descriptive study of social development in the rat (Rattus norvegicus). Anim. Behav. in press. 15. Meaney, M. J. and J. Stewart. Neonatal androgens influence the social play of prepubescent rats. Submitted for publication. 16. Peters, P. J., F. H. Bronson and J. M. Whitsett. Neonatal castration and intermale aggression in mice. Physiol. Behav. 8: 265-268, 1972. 17. Poole, T. B. and J. Fish. An investigation of playful behaviour in Rattus norvegieus and Mus musculus (Mammalia). J. Zool., Lond. 175: 61-71, 1975. 18. Vom Saal, F. S. and F. H. Bronson. In utero proximity of female mouse fetuses to males: Effects on reproductive performance in later life. Biol. Reprod. 19: 842-853, 1978. 19. Weisz, J. and I. L. Ward. Plasma testosterone and progesterone titers of pregnant rats, their male and female fetuses, and neonatal offspring. Endocrinology 1116:306-316, 1980.
Temporal Boundary of the Sensitive Period for Hormonal Organization of Social Play in Juvenile Rats ' WILLIAM
W. BEATTY, ANTHONY
M. D O D G E , K A T H Y L . T R A Y L O R
Department of Psychology, North Dakota State University, Fargo, ND 58105 AND MICHAEL
J. M E A N E Y
Department of Psychology, Concordia University, Montreal, Quebec, Canada R e c e i v e d 7 A u g u s t 1980 BEATTY, W. W., A. M. DODGE, K. L. TRAYLOR AND M. J. MEANEY. Temporal boundary of the sensitive period for hormonal organization of social play in juvenile rats. PHYSIOL. BEHAV. 26(2) 241-243, 1981.--The play-fighting of juvenile rats is a sexually dimorphic behavior which is influenced by the organizational actions of testosterone and dihydrotestosterone but is not dependent on activational influences of these testicular hormones. To delimit more precisely the temporal period in early development when these organizational effects are expressed, male rats were castrated 1, 6, I0 or 20 days after birth and their play behavior was compared to that of intact males and females. Daily observations were made of groups of 6-7 animals from 31-40 days of age. Intact males engaged in rough play more frequently than females and also initiated more play bouts. Castration at day 10 or 20 did not affect play in males, but castration on day 1 or 6 reduced male play to levels that were not reliably different from females. The development of social play depends on the presence of testicular androgens for at least 6 days after birth.
Play Social development Social behavior Gonadectomy Gonadal hormones Hormonal organization Sensitive period Critical period Sex differences V A R I O U S play behaviors are observed among juvenile mammals when they are studied in a social setting. Certain of these behaviors, notably responses that are variously termed rough and tumble play or play-fighting, are performed more frequently by males than by females, at least in some species. Such sex differences in play have been well documented in both rhesus monkeys and Norway rats [9, 10, 14, 15]. In both of these species sexually dimorphic play patterns apparently differentiate early in life under the organizational influence of testosterone or its 5a reduced metabolite, dihydrotestosterone [9,14]. Exposing females to T or DHT early in development (prenatally in monkeys, neonatally in rats) masculinizes their juvenile play behavior. Likewise castrating male rats within 24 hr after birth reduces the frequency of play-fighting. By contrast, castrating male rats at 23 days of age or male monkeys shortly after birth does not alter sexually dimorphic play; animals so treated play like intact males [9,15]. These findings imply that organizational but not activational effects of gonadal hormones are important regulators of play-fighting and further suggest that the organiz-
ing actions of testicular hormones may occur during a temporally limited period early in development as is true of other sexually dimorphic responses such as reproductive and aggressive behavior (see e.g. [2,10] for reviews). In the present experiment we sought to delimit more precisely, the sensitive period for the organizational effect o f testicular hormones on social play by studying play behaviors in males that were castrated at various ages between birth and 20 days. Although serum levels of testosterone and other androgens decline after birth in both sexes [5, 12, 19], males have higher serum androgen levels from birth to 25 days of age [5]. Hence, castration was performed at several different ages. METHOD
Animals
The animals were albino rats of both sexes born in the laboratory to dams obtained early in pregnancy from the Holtzman Co., Madison, WI. Upon arrival the pregnant females were placed into 24×46×15 cm plastic maternity cages with free access to food and water at all times. The
1Supported by Grant HD 12620-01 from NICHHD to William W. Beatty. Michael J. Meaney was supported by a predoctoral fellowship from the Ministere de l'Education, Province de Quebec. Address reprint requests to the first author.
C o p y r i g h t © 1981 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/81/020241-03502.00/0
BEATTY ET AL.
242 colony room was maintained on a 12L:12D cycle with the fluorescent lights on at 0600 hr. Within 24 hr of birth the animals were sexed and randomly assigned to litters of no more than 10 pups each.
Procedure Males were randomly assigned to groups of animals that were castrated on Day 1 (N=13), Day 6 ( N = l l ) , Day l0 (N = 10), Day 20 (N = 12) or left unoperated (N = 14). The day of birth was considered Day 0. Females (N=31) were left untreated. Hypothermia was used as the anesthetic for 1 and 6 day-old animals; ether was employed for the 10 day-old pups while Chloropent was employed for the 20 day-old rats. The animals were weaned at 20 days of age, but continued to live in the maternity cages until 25 days of age. Then they were assigned to one of 14 groups of 6 or 7 animals each and marked on the tail for identification with colored felt-tip pens. Once formed the groups remained intact for the duration of the study. Each group contained at least one intact male and two females and was housed in observation cages (38x61 × 38 cm high) made of galvanized metal. The back of the cage was made of hardware cloth and the front of clear plastic. Illumination was provided continuously by a white 7.5 W bulb mounted in the ceiling. These cages were located in the colony room. Food and water were freely available throughout the study. The animals were not disturbed except for normal maintenance and to renew tail markings every 3-4 days. Between 31 and 40 days of age daily observations of social play were obtained. Observations began at the onset of the dark phase of the L:D cycle and continued until each group had been observed for 105, 20 sec-long periods each day (typically 4-6 hr). Within the group every animal that participated in play during that period was given a score of " l " . Likewise every animal that initiated play during an observation period was given a score of " 1 " in the play initiation category. Hence, individual scores could range from 0-105 for each day on each category. The observed range of daily scores was 0-32 for social play and 0-20 for play initiation. The behavioral rating system has been described in detail elsewhere [13,15]. In brief, play-fighting in juvenile rats includes the following behaviors: pouncing--one rat lunges at another with its forepaws extended; wrestling--rats roll and tumble about; boxing--rats stand erect pawing at one another with their forepaws; pinning--(on-top, on-bottom postures) one rat stands on another while the rat on the bottom struggles to escape; play-biting--one rat bites another usually on the tail or leg. Play-biting may involve vigorous pulling, but the biting is inhibited and we never observed a wound. Pouncing, tail and leg pulling were scored as play initiation behaviors. RESULTS Table 1 depicts mean frequency of social play and play initiation for the various treatment groups. As expected from earlier work [13, 14, 15] males engaged in more rough play than females and were more likely to initiate play bouts. Castrating males on day 1 reduced levels of play on both measures, eliminating the sex difference. Castration at day 10 or 20 was without effect on either measure. Males castrated on day 6 engated in about as much play as females and were somewhat more likely to initiate play bouts. Statistical analysis confirmed these apparent trends by
TABLE 1 MEAN FREQUENCY (_+ SEM)
Treatment
Social Play
Intact Males Day 20 Castrates Day 10 Castrates Day 6 Castrates Day 1 Castrates Intact Females
132.6 _+ 6.0 122.5 _ 7.0 121.1 _+ 7.7 100.0 _+ 8.2 90.5 _+ 6.5 99.5 _+ 4.9
Play Initiation 62.5 55.6 53.9 43.4 35.3 35.6
_+ 4.5 _+ 5.2 -+ 3.3 _+ 5.5 _+ 3.0 - 2.9
revealing reliable differences among treatment groups on both measures, F(5,85)=6.01 for social play and 8.40 for play initiation, both p<0.001. Subsequent analysis with t-tests demonstrated that the intact males and the males castrated on Day 10 or Day 20 played more frequently and initiated more play bouts than either the Day 1 castrates or the intact females (t~>2.37, all p<0.02). There were no significant differences on either measure among the Day 10 castrates, the Day 20 castrates and the intact males (t~<1.32) or between the Day 1 castrates and the females (t~
EARLY CASTRATION AND PLAY
243
gression by adult rats and mice depends on both the organizational and activational actions of androgens [2, 3, 6] while masculine levels of play are independent of circulating androgens at the time of testing [15]. (2) Estradiol exerts a significant organizational influence on aggressive behavior in mice [4,8], but is without effect on play-fighting in rats [15] and (3) prenatal exposure to androgens, such as can occur in utero, exerts a masculinizing effect on aggression in female mice [18]. At present nothing is known of possible prenatal influences of gonadal hormones on play in young rats. Such influences, if they exist at all, would be expected to be quite modest inasmuch as castrating males at birth completely eliminates the sex difference in play-fighting. Perhaps it is not surprising that hormonal control of juvenile play-fighting
and adult aggression are different since social play is clearly not essential for the development of aggressive behavior in adults. Mice, for example, exhibit little or no social play [17]. Finally, although the present results indicate that hormonal organization of structures important to play is normally completed by 10 days of age, this in no way implies that the relevant mechanisms lose their ability to respond to hormonal stimuli introduced at a later age. Again aggressive behavior in mice provides a potentially instructive perspective. Although hormonal organization of this sexually dimorphic behavior is normally completed within the first 10 days of postnatal life, prolonged treatment with testosterone can masculinize aggressive behavior of female mice, even if the treatment is not initiated until 30 days of age [7].
REFERENCES 1. Barr, G. A., J. L. Gibbons and K. E. Moyer. Male-female differences and the influence of neonatal and adult testosterone on intraspecies aggression in rats. J. comp. physiol. Psychol. 90: 1169-1183, 1976. 2. Beatty, W. W. Gonadal hormones and sex differences in nonreproductive behaviors in rodents: Organizational and activational influences. Hormones Behav. 12:112-163, 1979. 3. Beeman, E. A. The effect of male hormone on aggressive behavior in mice. Physiol. Zool. 14: 323-340, 1947. 4. Bronson, F. H. and C. H. Desjardins. Aggression in adult mice: Modification by neonatal injections of gonadal hormones. Science 161: 705-706, 1968. 5. Dohler, K. D. and W. Wuttke. Changes with age of serum gonadotrophins, prolactin, and gonadal steroids in prepubertal male and female rats. Endocrinology 97: 898-907, 1975. 6. Edwards, D. A. Early androgen stimulation and aggressive behavior in male and female mice. Physiol. Behav. 4: 333-338, 1969. 7. Edwards, D. A. Post-neonatal androgenization and adult aggressive behavior in female mice. Physiol. Behav. 5: 465-467, 1970. 8. Edwards, D. A. and J. Herudon. Neonatal estrogen stimulation and aggressive behavior in female mice. Physiol. Behav. 5: 993-995, 1970. 9. Goy, R. W. Development of play and mounting behaviour in female rhesus virilized prenatally with esters of testosterone or dihydrotestosterone. In: Recent Advances in Primatology, Vol. ! Behaviour, edited by D. J. Chivers and J. Herbert. London: Academic Press, 1978, pp. 449-462. 10. Goy, R. W. and D. A. Goldfoot. Hormonal influences on sexually dimorphic behavior. In: Handbook of Physiology, Sect. 7, Vol. 2, Part l, edited by R. O. Greep. Baltimore: Williams and Wilkins, 1973, pp. 169-186.
11. Grady, K. L., C. H. Phoenix and W. C. Young. Role of the developing rat testis in differentiation of the neural tissues mediating mating behavior. J. comp. physiol. Psychol. 59: 176182, 1965. 12. Lee, V. W. K., D. M. De Kretser, B. Hudson and C. Wang. Variations in serum FSH, LH and testosterone levels in male rats from birth to sexual maturity. J. Reprod. Fert. 42: 121-126, 1975. 13. Meaney, M. J., A. M. Dodge and W. W. Beatty. Sex-dependent effects of amygdaloid lesions on the social play of prepubertal rats. Submitted for publication. 14. Meaney, M. J. and J. Stewart. A descriptive study of social development in the rat (Rattus norvegicus). Anim. Behav. in press. 15. Meaney, M. J. and J. Stewart. Neonatal androgens influence the social play of prepubescent rats. Submitted for publication. 16. Peters, P. J., F. H. Bronson and J. M. Whitsett. Neonatal castration and intermale aggression in mice. Physiol. Behav. 8: 265-268, 1972. 17. Poole, T. B. and J. Fish. An investigation of playful behaviour in Rattus norvegieus and Mus musculus (Mammalia). J. Zool., Lond. 175: 61-71, 1975. 18. Vom Saal, F. S. and F. H. Bronson. In utero proximity of female mouse fetuses to males: Effects on reproductive performance in later life. Biol. Reprod. 19: 842-853, 1978. 19. Weisz, J. and I. L. Ward. Plasma testosterone and progesterone titers of pregnant rats, their male and female fetuses, and neonatal offspring. Endocrinology 1116:306-316, 1980.