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Effect of the post-weaning environment on the climbing behaviour of wild and domestic Norway rats
,4him.Behav.,1976,24, 3~4--371
EFFECT OF THE, POST-WEANING ENVIRONMENT O N THE CLIMBING BEHAVIOUR OF WILD AND DOMESTIC NORWAY RATS BY U. WILLIAM H U C K & E. O. PRICE
Department of Zoology, State University of New York College of Environmental Science and Forestry, Syracuse~ N.Y. 13210 Abstract. The climbing behaviour of wild and domestic Norway rats (Rattus norvegicus) was compared after early rearing in three post-weaning environments offering different climbing experiences. Wild rats climbed in the test apparatus even when denied early climbing experience; male domestic rats did not. Ear!y climbing experience increased the climbing scores of both stocks but influenced the climbing proficiency of wild rats only. Treatment differences in climbing behaviour may be related to zpecific motor experiences:gained during development and the effect of early experience on the response to a novel environment (test apparatus). Stock differences in climbing behaviour may reflect a general reduction in motor activity among domestic rats and their reduced sensitivity to stimulus change or novelty. Most investigations comparing the behaviour of Norway rats (Rattus norvegicus)given enriched or impoverished post-weaning experience have concentrated on differences in emotionality (Denenberg 1969; Koch & Arnold 1972; Smith 1972; Archer 1973; Hughes 1975), learning (Hymovitch 1952; Cooper & Zubeck 1958; Nyman 1967; Morgan 1973), social and sexual behaviour (Denenberg & Greta 1964; Duffy & Hendricks 1973; Spevak et al. 1973) and brain anatomy and chemistry (Krech, Rosenzweig & Bennett 1962; Bennett et al. 1964; Rosenzweig 1971). Unfortunately, studies dealing with the effect of environmental enrichment on motor behaviour have been relatively uncommon. One notable exception is a recent study by Morgan (1973). In that study, domestic Norway rats reared in three different post-weaning environments were trained to perform a variety of learning tasks. Subsequent performance of the learned responses was then made contingent ~n the performance of 'complex' motor tasks such as climbing a ladder to reach a door leading to a food compartment. In general, deprived rats were slower than enriched subjects in the transfer phase of the above tests, although not in original acquisition. Such investigations emphasize the need to examine the development of motor skills, per se, in studies dealing with environmental enrichment. Strain or stock differences in motor behaviour are often related to the evolutionary forces or pressures to which animal populations are exposed. In addition, variability in motor performance may be related to past motor experience or the specific testing procedures employed.
Price (1973) reported that wild Norway rats are more prone to jump from a raised platform than their domestic counterparts, yet, previous iumping experience will influence the propensity of wild rats to jump from any given height. Similarly, stock x treatment differences exist between wild and domestic rats in both gnawing (Price 1973) and open-field behaviour (Huck & Price 1975; Hughes 1975). The climbing behaviour of rodent species is generally related to their habitat requirements (Horner 1954; King, Price & Weber 1968; Layne 1970). The wild Norway rat is considered a burrowing species although it is still quite adept at climbing (Barnett 1963, p. 2). It can be assumed that the climbing skills of the free-living rat are sometimes important in obtaining food and shelter and avoiding predators (particularly man, himself). However, the laboratory environment offers little or no opportunity for climbing experience or motor activity in general. Therefore, it is hypothesized that selective pressures related to climbing and other motor skills have been relaxed in the laboratory where the laboratory rat has evolved over the last 150 years (Robinson 1965). If such is the case, domestic Norway rats might show a reduction in climbing proficiency or willingness to climb relative to their wild ancestors or both. The objectives of the following experiments were to determine: (I) effects of early experience on the adult climbing behaviour of the Norway rat and (2) effects of domestication on the climbing behaviour of this species. Particular attention was placed on genotype x environment interactions in the development of climbing ~64
HUCK & PRICE: EARLYEXPERIENCEEFFECTSON CLIMBINGIN WILD & DOMESTICRATS behaviour and their relationship to the different selective pressures impinging on free-living and captive populations of this species. Experiment 1 The objective of this experiment was to compare the climbing behaviour of wild and domestic Norway rats reared in standard laboratory cages. Methods Nineteen wild and twenty domestic (LongEvans) rats, divided equally by sex, were used in the present study. Age and body weight data are presented in Table I. Wild subjects were conceived in the laboratory and represented the offspring of fifteen pairs of field-trapped adults. The domestic stock represented eleven different litters. All subjects were weaned at 25-4-1 days of age and maintained in 36 x 36 x 18-cm metal cages in unisexual groups (two to three rats) until testing. Rats were given Charles River Rat and Mouse Diet and water ad libitum. The climbing apparatus consisted of hardware cloth (188 mesh) shaped into a cylindrical enclosure approximately two metres high and one metre in diameter. Masking tape was used to divide the wall of the cylinder into a grid of 25 X 25-cm sectors. A rope was suspended vertically from the plywood top of the cylinder to the plywood base. In addition, a 1-m x I88 threaded rod was placed horizontally through the centre of the apparatus, approximately fifteen centimetres off the floor. The apparatus was located in a 10 x 3'4 x 2.4-m room maintained at 22 • I~ and 43 to 57 per cent relative humidity. Light intensity was approximately 375-1ux and approximately 75 dB of white noise (re 2 x 10-5 N/m2) was played continuously. All data recording apparatus (e.g. event recorder, counters, timers, e t c . ) w a s located outside the test room and observations were made through a one-way mirror. Subjects were placed in individual cages 8 days prior to testing and were given a 15-min openfield test on days 2 to 6 of this period (Price & Huck 1975). Rats were transferred to the climbing apparatus in a 15 x 10 x 15-cm wooden handling box with a removable metal sliding door. The handling box was placed in the centre of the floor of the climbing apparatus with the sliding door facing downward. The door (bottom) was removed and after a 2-min acclimation period the box was raised from the subject by means of a rope-and pulley system,
365
thus starting the trial. Tests were conducted during the light phase of a 1 2 : 1 2 LD cycle commencing at 07.00 hours. Each subject was given a single 15-min test during which the following data were recorded: (1) the frequency and duration of climbing on the cylinder wall, (2) the number of grid sectors entered while climbing and (3) number of bouts and time on the raised, horizontal rod. None of the subjects attempted to climb the rope. Results Wild rats climbed more frequently (MannWhitney U : 23.5, P < 0.001) and for longer duration (U = 23.0, P < 0.001) than domestic rats (Table I). Whereas all wild subjects spent some time climbing, more than half of the domestic rats never left the floor of the apparatus. Wild rats also entered more grid sectors while climbing (U = 14.0, P < 0.001). Female domestic rats participated in more climbing bouts than domestic male subjects (U ----- 17.5, P < 0-02) whereas the frequency of climbing for male and female wild rats was very similar (t = 0.51, df= 17). Females of both stocks spent more time climbing than males (Wild: t = 2"3, df= 17, P < 0.02; Domestic: U---- 14, P < 0-01). Sex differences in number of sectors entered while climbing were not Statistically significant for either stock (Wild: t = 1.5, df= 17; Domestic: U = 40). Of the wild subjects only two females and two males failed to climb to the top of the cylinder wall at least once. All eight wild males and two of seven wild females fell to the floor below, at least once while climbing. Only one domestic rat (a female) climbed to the top of the apparatus. Only four wild rats and two domestic subjects mounted the raised horizontal rod (Table I). Since only one of these six subjects spent more than 3-6 s on the rod, the data were not analysed statistically.
Experiment 2 The preceding experiment demonstrated that wild rats are more prone to climbing than their domestic counterparts when reared in~ small laboratory cages (offering little or no climbing experience). The objectives of the following experiment were to determine: (1) whether the climbing behaviour of wild and domestic Norway rats could be modified by increasing the climbing experiences offered in the early rearing environment and (2) whether early climbing experience would affect the climbing behaviour of the two stocks differently.
ANIMAL
366
BEHAVIOUR,
Table I. Mean Climbing Statistics for Cage-Reared Wild and Domestic Rats (Experiment 1)
Wild
Domestic
Male Female No. of'subjects
Male Female
10
9
10
10
Age (days)
213
220
222
241
Bodyweight (g)
326
214
480
327
Cylinder wall: No. which climbed 10 9 No. climbing bouts 3.8 3-3 Climbing duration (s) 248-4 524.4 No. sectors entered 31"9 52-8
1 0-2
8 1-1
0.7 0.1
27-4 1.6
Horizontal rod: No. mounting rod NO. bouts Time on rod (s)
1 0.3 1"1
1 0.1 0.4
3 0-7 1.1
1 0.2 0"4
Methods Thirty-two wild andthirty-two domestic rats, divided equally by sex, were assigned at weaning (25 -4- 1 days of age) to either unenriched or enriched post-weaning environments where they remained until testing in unisexual groups of three to five animals. In most cases litters were split between the two treatments. Wild rats were conceived in the laboratory to five pairs of fieldtrapped parents (six litters represented). Domestic rats were the offspring of eight pairs of LongEvans hooded rats (eight litters represented). Food and water were available ad libitum. The unenriched environment consisted of an empty 0.61 • 0.61 • 0.91-m cage with l t - c m mesh hardware cloth enclosing three sides, ceiling and floor. Masonite or wood was placed on the outside o f the cage (sides only) in order to limit visual experience. The construction o f the enriched environments was the same except that the sides of the cage were exposed and the interior was partitioned into three horizontal tiers connected by wooden ramps. In addition, the following items were provided: a 35-cm diameter activity wheel, a 0.61 -m • 1-25-cm horizontal threaded rod, two 15-era-long vertical threaded rods, a 0.61-m-long climbing chain and several other items (e.g. nest-boxes, maze, mirror, manipulative objects) described in another paper (Huck & Price 1975). The climbing apparatus and test conditions were identical to that described for experiment 1.
24,
2
Subjects were placed in individual 36 • 36 x 18-era metal cages at 95 :[: 5 days of age and climbing tests were begun 8 days later. (Openfield tests were administered on days 2 to 6 of this 8-day period; Huck & Price 1975). Each subject received a 15-rain climbing test on each of five consecutive days. Testing pro: cedure and dependent variables were the same as for experiment 1, with the addition of climbing latency scores and an analysis of changes in behaviour, both within and between trials. Within-trial changes in behaviour were analysed by subdividing each trial into five 3-rain periods and recording the number of 2-s intervals during each 3-mirt period in which a given behaviour pattern was observed. StudentNeumans-Keuls tests at the one-tailed 0.05 alpha level (Sokal & Rohlf 1969) were used to conduct a posteriori multiple comparisons for both within- and between- trial data. Results In general, climbing activity was enhanced by rearing rats under conditions providing climbing experience. Except for activity on the horizontal rod by unenriched rats, all climbing scores increased relative to rats reared in standard laboratory cages (experiment 1). The results of the statistical analyses are presented in Table II. Climbing latency. Unenriched rats had shorter latencies than their enriched counterparts (Fig. l(a)). Wild rats had shorter latencies than domestic subjects and females had shorter latencies than males. Treatment had a greater influence on the raw scores of domestic rats but the per cent change in behaviour was greater for wild rats (Table III). Males were influenced more by treatment than females. Latencies tended to increase over trials (all subjects) but these changes were not statistically significant. Climbing bouts. Sex was the only significant main effect; females climbed more often than males. Enriched wild rats climbed more frequently than unenriched wild subjects while the reverse was true for domestic rats. Enriched females climbed more frequently than unenriched females, while the reverse was true for male rats. Climbing duration. Unenriched subjects spent more time on the cylinder wall than enriched subjects (Fig. l(b)). Wild rats climbed longer than domestic subjects and females climbed longer than males. Enriched and unenriched
I-IUCK & PRICE: EARLY EXPERIENCE EFFECTS ON C L I M B I N G I N WILD & DOMESTIC RATS
367
Table 1/. Partial Summary of Results of Statistical Analysis of Climbing Test Data (Experiment 2)
Variable
Source of variation
Climbing latency
Treatment (A)
36"2t***
Stock (B)
205-0***
Sex (C)
99.0***
Trial (D)
2"1
Climbing bouts 0"7t 1.8 43-0***
Sectors entered while climbing
Climbing duration 66"7t***
l'0t
Bouts on horizontal rod
Time on horizontal rod
P < 0.001++ P < 0-001.*
308.6***
78.8***
51.4"**
143.6"**
NS
NS
1 "4
16.0"**
--
--
5"2*
33"0***
--
--
2.0
P < 0.05
P < 0.05
A x B
17"9"**
40"5***
A • C
12"8"**
8"9***
0"2
0"1
--
--
0.6
2"7*
1"6
--
--
0"1
--
--
A • D
3.1"
B • C
54"9***
1"8
21"4"**
B • D
1"0
1"6
1"4
C • D
1-2
0.7
0.2
19"1"**
--
1.2
--
--
*P < 0"05. **P < 0"01. ***P < 0.001. f F-Va! ue. ~Average result of four modified Mann-Whitney tests.
Table HI. Differences in Climbing Behaviour Due to Post-weaning Experience in an Enriched Environment ( Sexes Combined)
Absolute difference* Variable
Wild
Domestic
Per cent difference* Wild
Domestic
Climbing latency (s)
+ 57.9
+330.0
+ 239
+
94
Climbing bouts
+
--
+ 120
--
40
Climbing duration (s)
--502"5
--283"6
--
31
--
42
Sectors entered while climbing
+ 45"8
-- 65.1
+
29
--
52
Bouts on horizontal rod
+
2.0
+
6.2
+1000
+ 886
Time on horizontal rod (s)
+
7.2
+ 15-3
+1800
+1530
6"7
5.1
*Differences based on deviation from scores of unenriched subjects. wild rats differed more than enriched and unenriched domestic subjects, but the per cent change in behaviour was greater for domestic r a t s ( T a b l e III). M a l e a n d f e m a l e d o m e s t i c r a t s differed more than male and female wild rats. T h e r e w a s little c h a n g e i n c l i m b i n g d u r a t i o n scores over trials but scores were significantly l o w e r d u r i n g t h e first 3 - m i n p e r i o d o f e a c h t r i a l
than during remaining periods for unenriched subjects and domestic rats. S e c t o r s entered w h i l e climbing. E n r i c h e d w i l d rats scored higher than unenriched counterparts in distance traversed while climbing, whereas the reverse was true for domestic rats (Fig. l(c)). W i l d s u b j e c t s w e r e m o r e a c t i v e climbers than domestic rats, and females :entered
368
ANIMAL
W~LD
BEHAVIOUR,
DOMESTIC
I imm
I ~ C ~ t o Ftu~,[s ~CHE~ FE~[I E~i~IED MKLES UNF.t~Ir IdAI.IEI
24,
2
WILD
B
DOMESTIC
m m [NracaEa FEU~IttI tT;z3 t l ,tS~lCNtD F E ~ t I
ozo~
17a
| t
WILD
Dr
WILD
DQMESTIC
Fig. I. Climbing behaviour of wild and domestic Norway rats reared in enriched and unenriched artificial environments: (a) latency to climb, (b) climbing duration, (c)number of grid sectors crossed while climbing, (d) time on horizontal rod. more sectors than males. Treatment had a greater influence on the scores of domestic rats than wild subjects (Table III). Betweentrial comparisons indicated that wild rats entered twice as many sectors during trial 1 as during any of the remaining four trials (which did not differ), whereas the behaviour o f domestic rats did not change over trials. Within-trial
comparisons indicated that wild rats entered significantly more sectors during the first 3-min of each trial than thereafter, while domestic rats scored lowest during this same time period. Bouts on horizontal rod. Enriched subjects mounted the horizontal rod more frequently than enriched counterparts. Domestic rats
HUCK & PRICE: EARLYEXPERIENCEEFFECTSON CLIMBINGIN WILD & DOMESTICRATS exhibited more bouts than wild subjects while sexes did not differ. Time on horizontal rod. Enriched subjects spent more time on the horizontal rod than unenriched rats (Fig. l(d)). Domestic rats spent more time on the rod than wild subjects. Sexes did not differ. In general, the rats spent little time on the horizontal rod relative to time spent on the cylinder wall. Miscellaneous. While domestic subjects and enriched wild rats frequently fell from the cylinder wall while climbing, unenriched wild subjects were never observed to fall. None of the subjects attempted to climb the rope present in the climbing apparatus. Discussion The results of the above experiments clearly demonstrate the influence of early climbing experience on adult climbing behaviour. Wild rats develop a propensity to climb even if reared in an environment where climbing is not possible. In addition early climbing experience improves the performance of wild rats in both a quantitative and qualitative manner. Domestic rats, particularly males, develop little propensity to climb if denied early climbing experience. However, climbing behaviour improves (at least quantitatively), as a result of climbing experiences provided in the post-weaning environment. Although climbing was virtually impossible in standard laboratory cages (experiment 1), the enriched and unenriched rearing environments employed in experiment 2 undoubtedly provided different opportunities for climbing experience. In the empty, unenriched environments, climbing was one of few motor activities that could be performed. In the enriched enclosures, however, climbing was one of many possible motor activities. The presence of two upper horizontal tiers in this environment precluded climbing the cage wall from top to bottom. Movement from one tier to another could be accomplished only by climbing narrow wooden ramps. The presence of vertical chains and rods, horizontal rods, activity wheels, etc. could have provided motor experiences related to climbing. Because of the eomplexity of the enriched environment, attempts to quantify the climbing experiences of the enriched subjects proved impractical. In general, unenriched wild rats were more agile climbers than the other rats when in the test apparatus. Only these subjects were able
369
to climb downward with their bodies oriented in that direction. Domestic rats and enriched wild subjects would slowly descend the wall backward or in an oblique (spiral) path. In addition, falling was frequently observed among domestic and enriched wild rats but never for unenriched wild subjects. Therefore, differences in climbing proficiency provide one explanation for the greater climbing duration scores of unenriched subjects and possibly the greater number of grid sectors entered by unenriched subjects of the domestic stock. Shorter latencies to climb provide a second explanation for the longer duration scores of the unenriched subjects. It was noted that during routine maintenance o f the post-weaning environments, unenriched subjects (particularly" the wild stock) would frequently climb the enclosure walls when disturbed (e.g. approached by experimenter), whereas enriched subjects would retreat to their respective nest boxes. Since the climbing apparatus used in testing resembled the unenriched post-weaning environments in being an empty arena with wire-mesh sides, the shorter climbing latencies of the unenriched subjects in experiment 2 could be based on a simple response learned in the rearing environment. The greater mobility of the enriched wild rats while climbing may have also accentuated the differences between enriched and unenriched wild rats in regard to time spent climbing. Whereas unenriched wild rats would begin each trial with a short burst of climbing activity and then cling to the wire in a given position for the remainder of the trial, enriched wild rats took longer tO initiate climbing and were more likely to return to the floor of the apparatus during the course of testing. The greater mobility of enriched wild rats while climbing may represent the same kind of increased mobility displayed by this group when tested in the open-field (Huck & Price 1975). The specificity of the effects of early climbing experience was seen in response to the horizontal rod. Enriched subjects were the only :rats to show any appreciable use of the rod. Early experience effects provide the most logical explanation for this difference in that rods were present in the rearing environments of the enriched group only. It could also be argued that unenriched subjects spent less time on the rod because they spent more time on the cylinder wall. Aside from the fact that the time element involved is very small ( < 10 s) this explanation
370
ANIMAL BEHAVIOUR,
is unlikely since cage-reared rats used the rod no more than unenriched subjects even though they spent considerably less time on the cylinder wall during testing. There are at least three explanations for the greater propensity of wild rats to climb in the test apparatus employed: (1) differences in the selective advantage of climbing (or motor skills in general) in field versus laboratory environments, (2) differences between wild and domestic rats in response to a novel environment, and (3) the heavier body weights of domestic rats. In general, motor skills are considered more important for the survival and reproduction of free-living animals than their captive counterparts (see introduction). The domestic Norway rat is no exception in this regard, since, aside from the motor skills associated with agonistic and sexual behaviour, little is required of the captive rodent to achieve maximum fitness. It could also be argued that, in captivity, selection for reduced motor potential (both in a quantitative and qualitative sense) would facilitate adaptation to an environment where the motor activities of animals are severely limited (Price 1973). Climbing behaviour in a novel environment could be a manifestation of escape, exploration or some form of conflict experience (e.g. displacement activity). Wild rats are initially more active than domestic rats when placed in a totally unfamiliar environment (Barnett 1958; Price & Loomis 1973; Price & Huck 1976). It appears that being placed in a novel environment is a more aversive experience for wild rats (Price & Huck 1976) and that the heightened activity of wild rats under these circumstances may be a manifestation of natural escape or avoidance tendencies. The climbing apparatus used in the present study was initially unfamiliar t o the test subjects. The fact that activity by wild rats while climbing on the cylinder wall was greatest during trial 1 (between-trial comparison) and at the beginning of each trial (within-trial comparison), whereas activity by climbing domestic rats was more evenly distributed within and between trials, suggests that wild rats were more responsive to novelty of the test conditions employed. The observation that unenriched wild rats were more likely than their domestic counterparts to climb the wall of their rearing environment when approached by the experimenter points to the fact that the climbing behaviour of wild rats may be more easily triggered by stimulus change or novelty.
24, 2
The body weights of the test subjects when removed from their respective rearing environments are presented in Table I (experiment 1) and in another paper (Huck & Price 1975). Domestic rats were heavier than wild rats in both experiments 1 and 2. This could have biased the results in favour of the wild subjects by making climbing more difficult for domestic rats. On the other hand, wild males climbed more (duration scores) than domestic females in both experiments even though their bodyweights were very similar and even though females of both stocks climbed more than males. A subsequent study conducted in a more natural setting confirmed the stock differences in climbing behaviour discussed above. Six female wild and six female domestic rats (cage-reared) were released in pairs (one of each stock) in a circular, wire out-door enclosure measuring 10.9 m in diameter and 4.6 m high. During the first hour following release wild rats averaged 478 s of climbing time as opposed to only 0.4 s for their domestic counterparts (Mann-Whitney U = 1, P < 0.002). Hence, although nearly 35 m2 of horizontal (ground) space was available, wild rats, as a group, spent over 13 per cent of their time climbing the enclosure wall. The present study provides data relative to the hypothesis that wild rats are more sensitive than their domestic counterparts to the effects of early experience (Huck & Price 1975). Although the response to early experience was significantly greater for wild rats for only two of six quantitative variables, Climbing performance (qualitative measures) was improved in wild rats only. In terms of adapting to the natural environment an improvement in climbing skill could be just as important as changes in pro, pensity to climb. In conclusion, the preceding experiments have demonstrated: (1) differences in climbing behaviour of wild and domestic Norway rats when reared in identical environments (the domestication problem) and (2) differences in climbing behaviour based on the nature of the postweaning rearing environment. Further studies are needed to determine the motivational factors underlying climbing behaviour in this species and how specific experiences gained in the postweaning environment influence climbing propensity and the development of climbing (motor) skills. Acknowledgments This study was supported by N.I,M.H. Grant MH21467-01 to E. O. Price. The authors would
H U C K & PRICE: EARLY EXPERIENCE EFFECTS ON CLIMBING I N WILD & DOMESTIC RATS
like to thank J. Moreau for his assistance in constructing the apparatus used in this study. The authors are also indebted to J. Bacon for recording the climbing behaviour of wild and domestic rats in an outdoor enclosure (described in the discussion). REFERENCES Archer, J. (1973). Tests for emotionality in rats and mice: A review. Anim. Behav., 21, 205-235. Barnett, S. A. (1958). Exploratory behavior. Br. J. PsychoL, 49, 289-310. Barnett, S. A. (1963). The Rat. Chicago: Aldine. Bennett, E.L., Diamond, M.C., Krech, D. & Rosenzweig, M. R. (1964). Chemical and anatomical plasticity of brain. Science, N.Y., 146, 610-619. Cooper, R. M. & Zubek, J. P. (1958). Effects of enriched and restricted early environments on the learning ability of bright and dull rats. Can. J. Psychol., 12, 159-164. Denenberg, V. H. (1969). The effects of early experience. In: The Behaviour of Domestic Animals (Ed. by E. S. E. F/afez), 2nd edn., pp. 95-130. Baltimore: Williams & Wilkins. Denenberg, V. H. & Grota, L. J. (1964). Social-seeking behavior as a function of differential rearing histories. J. abnorm, soc. PsychoL, 4, 453-456. Duffy, J. A. & Hendricks, S. E. (1973). Influences of social isolation during development on sexual behavior of the rat. Anita. Learn. & Behav., 1, 223-227. Homer, B. (1954). Arboreal adaptations of Peromyscus, with special reference to use of the tail. Contrib. Lab. Vert. BioL, Univ. Mich. 61, 1-85. Huck, U. W. & Price, E. O. (1975). Differential effects of environmental enrichment on the open-field of wild and domestic Norway rats. J. comp. physioL PsychoL, 89, 892-898. Hughes, C. W. (1975). Early experience in domestication. J. comp. physioL PsychoL, 88, 407--417. Hymovitch, B. (1952). The effects of experimental variations on problem solving in the rat. J. comp. physioL PsychoL, 45, 313-320. King, J. A., Price, E. O. & Weber, P. G. (1968). Behavioral comparisons within the genus Peromyscus. Papers Mich. Acad. Sci. Arts Letters, 53, 113-136.
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Koch, M. D. ~ Arnold, W. J. (1972). Effects of early social deprivation on emotionality in rats. J. comp. physioL PsychoL, 78, 391-399. Kreeh, D., Rosenzweig, M. R. & Bennett, E. L. (1962). Relations between brain chemistry and problemsolving among rats raised in enriched and impoverished environments. J. comp. physioL PsychoL, 55, 801"807. Layne, J. (1970). Climbing behavior of Peromyscus floridanus and Peromyscus gossypinus. J. MammaL, 51, 580--591. Morgan, M. J. (1973). Effects of post-weaning environment on learning in the rat. Anim. Behav., 21, 429--442. Nyman, A. J. (1967). Problem solving in rats as a function of experience at different ages. J. genet. PsychoL, 110, 31-39. Price, E. O. (1973). Some behavioral differences between wild and domestic Norway rats: Gnawing and platform jumping. Anita. Learn. & Behav., 1, 312-316. Price, E. O. & Loomis, S. (1973). Maternal influence on the response of wild and domestic Norway rats to a novel environment. Develop. PsychobioL, 6, 203-208. Price, E. O. & Huck, U. W. (1976). Open-field behavior of wild and domestic Norway rats. Anita. Learn. & Behav., Robinson, R. (1965). Genetics of the Norway Rat. Oxford: Pergamon Press. Rosenzweig, M. R. (1971). Effects of environment on development of brain and of behaviour. In: Biopsychology of Development (Ed. by E. Tobach, L. Aronson & E. Shaw), pp. 303-342. New York: Academic Press. Sokal, R. R. & Rohlf, F. J. (1969). Biometry. San Francisco: Freeman. Smith, F/. V. (1972). Effects of environmental enrichment on open-field activity and F/ebb-WiUiams problem solving in rats. J. comp. physioL PsychoL, 80, 163-168. Spevak, A. M., Quadagno, D. M., Knoeppel, D. & Poggio, J. P. (1973). The effects of isolation on sexual and social behavior in the rat. Behav. BioL, 8, 63-73.
(Received 10 June 1974; revised 10 June 1975; MS. number: A1575)
EFFECT OF THE, POST-WEANING ENVIRONMENT O N THE CLIMBING BEHAVIOUR OF WILD AND DOMESTIC NORWAY RATS BY U. WILLIAM H U C K & E. O. PRICE
Department of Zoology, State University of New York College of Environmental Science and Forestry, Syracuse~ N.Y. 13210 Abstract. The climbing behaviour of wild and domestic Norway rats (Rattus norvegicus) was compared after early rearing in three post-weaning environments offering different climbing experiences. Wild rats climbed in the test apparatus even when denied early climbing experience; male domestic rats did not. Ear!y climbing experience increased the climbing scores of both stocks but influenced the climbing proficiency of wild rats only. Treatment differences in climbing behaviour may be related to zpecific motor experiences:gained during development and the effect of early experience on the response to a novel environment (test apparatus). Stock differences in climbing behaviour may reflect a general reduction in motor activity among domestic rats and their reduced sensitivity to stimulus change or novelty. Most investigations comparing the behaviour of Norway rats (Rattus norvegicus)given enriched or impoverished post-weaning experience have concentrated on differences in emotionality (Denenberg 1969; Koch & Arnold 1972; Smith 1972; Archer 1973; Hughes 1975), learning (Hymovitch 1952; Cooper & Zubeck 1958; Nyman 1967; Morgan 1973), social and sexual behaviour (Denenberg & Greta 1964; Duffy & Hendricks 1973; Spevak et al. 1973) and brain anatomy and chemistry (Krech, Rosenzweig & Bennett 1962; Bennett et al. 1964; Rosenzweig 1971). Unfortunately, studies dealing with the effect of environmental enrichment on motor behaviour have been relatively uncommon. One notable exception is a recent study by Morgan (1973). In that study, domestic Norway rats reared in three different post-weaning environments were trained to perform a variety of learning tasks. Subsequent performance of the learned responses was then made contingent ~n the performance of 'complex' motor tasks such as climbing a ladder to reach a door leading to a food compartment. In general, deprived rats were slower than enriched subjects in the transfer phase of the above tests, although not in original acquisition. Such investigations emphasize the need to examine the development of motor skills, per se, in studies dealing with environmental enrichment. Strain or stock differences in motor behaviour are often related to the evolutionary forces or pressures to which animal populations are exposed. In addition, variability in motor performance may be related to past motor experience or the specific testing procedures employed.
Price (1973) reported that wild Norway rats are more prone to jump from a raised platform than their domestic counterparts, yet, previous iumping experience will influence the propensity of wild rats to jump from any given height. Similarly, stock x treatment differences exist between wild and domestic rats in both gnawing (Price 1973) and open-field behaviour (Huck & Price 1975; Hughes 1975). The climbing behaviour of rodent species is generally related to their habitat requirements (Horner 1954; King, Price & Weber 1968; Layne 1970). The wild Norway rat is considered a burrowing species although it is still quite adept at climbing (Barnett 1963, p. 2). It can be assumed that the climbing skills of the free-living rat are sometimes important in obtaining food and shelter and avoiding predators (particularly man, himself). However, the laboratory environment offers little or no opportunity for climbing experience or motor activity in general. Therefore, it is hypothesized that selective pressures related to climbing and other motor skills have been relaxed in the laboratory where the laboratory rat has evolved over the last 150 years (Robinson 1965). If such is the case, domestic Norway rats might show a reduction in climbing proficiency or willingness to climb relative to their wild ancestors or both. The objectives of the following experiments were to determine: (I) effects of early experience on the adult climbing behaviour of the Norway rat and (2) effects of domestication on the climbing behaviour of this species. Particular attention was placed on genotype x environment interactions in the development of climbing ~64
HUCK & PRICE: EARLYEXPERIENCEEFFECTSON CLIMBINGIN WILD & DOMESTICRATS behaviour and their relationship to the different selective pressures impinging on free-living and captive populations of this species. Experiment 1 The objective of this experiment was to compare the climbing behaviour of wild and domestic Norway rats reared in standard laboratory cages. Methods Nineteen wild and twenty domestic (LongEvans) rats, divided equally by sex, were used in the present study. Age and body weight data are presented in Table I. Wild subjects were conceived in the laboratory and represented the offspring of fifteen pairs of field-trapped adults. The domestic stock represented eleven different litters. All subjects were weaned at 25-4-1 days of age and maintained in 36 x 36 x 18-cm metal cages in unisexual groups (two to three rats) until testing. Rats were given Charles River Rat and Mouse Diet and water ad libitum. The climbing apparatus consisted of hardware cloth (188 mesh) shaped into a cylindrical enclosure approximately two metres high and one metre in diameter. Masking tape was used to divide the wall of the cylinder into a grid of 25 X 25-cm sectors. A rope was suspended vertically from the plywood top of the cylinder to the plywood base. In addition, a 1-m x I88 threaded rod was placed horizontally through the centre of the apparatus, approximately fifteen centimetres off the floor. The apparatus was located in a 10 x 3'4 x 2.4-m room maintained at 22 • I~ and 43 to 57 per cent relative humidity. Light intensity was approximately 375-1ux and approximately 75 dB of white noise (re 2 x 10-5 N/m2) was played continuously. All data recording apparatus (e.g. event recorder, counters, timers, e t c . ) w a s located outside the test room and observations were made through a one-way mirror. Subjects were placed in individual cages 8 days prior to testing and were given a 15-min openfield test on days 2 to 6 of this period (Price & Huck 1975). Rats were transferred to the climbing apparatus in a 15 x 10 x 15-cm wooden handling box with a removable metal sliding door. The handling box was placed in the centre of the floor of the climbing apparatus with the sliding door facing downward. The door (bottom) was removed and after a 2-min acclimation period the box was raised from the subject by means of a rope-and pulley system,
365
thus starting the trial. Tests were conducted during the light phase of a 1 2 : 1 2 LD cycle commencing at 07.00 hours. Each subject was given a single 15-min test during which the following data were recorded: (1) the frequency and duration of climbing on the cylinder wall, (2) the number of grid sectors entered while climbing and (3) number of bouts and time on the raised, horizontal rod. None of the subjects attempted to climb the rope. Results Wild rats climbed more frequently (MannWhitney U : 23.5, P < 0.001) and for longer duration (U = 23.0, P < 0.001) than domestic rats (Table I). Whereas all wild subjects spent some time climbing, more than half of the domestic rats never left the floor of the apparatus. Wild rats also entered more grid sectors while climbing (U = 14.0, P < 0.001). Female domestic rats participated in more climbing bouts than domestic male subjects (U ----- 17.5, P < 0-02) whereas the frequency of climbing for male and female wild rats was very similar (t = 0.51, df= 17). Females of both stocks spent more time climbing than males (Wild: t = 2"3, df= 17, P < 0.02; Domestic: U---- 14, P < 0-01). Sex differences in number of sectors entered while climbing were not Statistically significant for either stock (Wild: t = 1.5, df= 17; Domestic: U = 40). Of the wild subjects only two females and two males failed to climb to the top of the cylinder wall at least once. All eight wild males and two of seven wild females fell to the floor below, at least once while climbing. Only one domestic rat (a female) climbed to the top of the apparatus. Only four wild rats and two domestic subjects mounted the raised horizontal rod (Table I). Since only one of these six subjects spent more than 3-6 s on the rod, the data were not analysed statistically.
Experiment 2 The preceding experiment demonstrated that wild rats are more prone to climbing than their domestic counterparts when reared in~ small laboratory cages (offering little or no climbing experience). The objectives of the following experiment were to determine: (1) whether the climbing behaviour of wild and domestic Norway rats could be modified by increasing the climbing experiences offered in the early rearing environment and (2) whether early climbing experience would affect the climbing behaviour of the two stocks differently.
ANIMAL
366
BEHAVIOUR,
Table I. Mean Climbing Statistics for Cage-Reared Wild and Domestic Rats (Experiment 1)
Wild
Domestic
Male Female No. of'subjects
Male Female
10
9
10
10
Age (days)
213
220
222
241
Bodyweight (g)
326
214
480
327
Cylinder wall: No. which climbed 10 9 No. climbing bouts 3.8 3-3 Climbing duration (s) 248-4 524.4 No. sectors entered 31"9 52-8
1 0-2
8 1-1
0.7 0.1
27-4 1.6
Horizontal rod: No. mounting rod NO. bouts Time on rod (s)
1 0.3 1"1
1 0.1 0.4
3 0-7 1.1
1 0.2 0"4
Methods Thirty-two wild andthirty-two domestic rats, divided equally by sex, were assigned at weaning (25 -4- 1 days of age) to either unenriched or enriched post-weaning environments where they remained until testing in unisexual groups of three to five animals. In most cases litters were split between the two treatments. Wild rats were conceived in the laboratory to five pairs of fieldtrapped parents (six litters represented). Domestic rats were the offspring of eight pairs of LongEvans hooded rats (eight litters represented). Food and water were available ad libitum. The unenriched environment consisted of an empty 0.61 • 0.61 • 0.91-m cage with l t - c m mesh hardware cloth enclosing three sides, ceiling and floor. Masonite or wood was placed on the outside o f the cage (sides only) in order to limit visual experience. The construction o f the enriched environments was the same except that the sides of the cage were exposed and the interior was partitioned into three horizontal tiers connected by wooden ramps. In addition, the following items were provided: a 35-cm diameter activity wheel, a 0.61 -m • 1-25-cm horizontal threaded rod, two 15-era-long vertical threaded rods, a 0.61-m-long climbing chain and several other items (e.g. nest-boxes, maze, mirror, manipulative objects) described in another paper (Huck & Price 1975). The climbing apparatus and test conditions were identical to that described for experiment 1.
24,
2
Subjects were placed in individual 36 • 36 x 18-era metal cages at 95 :[: 5 days of age and climbing tests were begun 8 days later. (Openfield tests were administered on days 2 to 6 of this 8-day period; Huck & Price 1975). Each subject received a 15-rain climbing test on each of five consecutive days. Testing pro: cedure and dependent variables were the same as for experiment 1, with the addition of climbing latency scores and an analysis of changes in behaviour, both within and between trials. Within-trial changes in behaviour were analysed by subdividing each trial into five 3-rain periods and recording the number of 2-s intervals during each 3-mirt period in which a given behaviour pattern was observed. StudentNeumans-Keuls tests at the one-tailed 0.05 alpha level (Sokal & Rohlf 1969) were used to conduct a posteriori multiple comparisons for both within- and between- trial data. Results In general, climbing activity was enhanced by rearing rats under conditions providing climbing experience. Except for activity on the horizontal rod by unenriched rats, all climbing scores increased relative to rats reared in standard laboratory cages (experiment 1). The results of the statistical analyses are presented in Table II. Climbing latency. Unenriched rats had shorter latencies than their enriched counterparts (Fig. l(a)). Wild rats had shorter latencies than domestic subjects and females had shorter latencies than males. Treatment had a greater influence on the raw scores of domestic rats but the per cent change in behaviour was greater for wild rats (Table III). Males were influenced more by treatment than females. Latencies tended to increase over trials (all subjects) but these changes were not statistically significant. Climbing bouts. Sex was the only significant main effect; females climbed more often than males. Enriched wild rats climbed more frequently than unenriched wild subjects while the reverse was true for domestic rats. Enriched females climbed more frequently than unenriched females, while the reverse was true for male rats. Climbing duration. Unenriched subjects spent more time on the cylinder wall than enriched subjects (Fig. l(b)). Wild rats climbed longer than domestic subjects and females climbed longer than males. Enriched and unenriched
I-IUCK & PRICE: EARLY EXPERIENCE EFFECTS ON C L I M B I N G I N WILD & DOMESTIC RATS
367
Table 1/. Partial Summary of Results of Statistical Analysis of Climbing Test Data (Experiment 2)
Variable
Source of variation
Climbing latency
Treatment (A)
36"2t***
Stock (B)
205-0***
Sex (C)
99.0***
Trial (D)
2"1
Climbing bouts 0"7t 1.8 43-0***
Sectors entered while climbing
Climbing duration 66"7t***
l'0t
Bouts on horizontal rod
Time on horizontal rod
P < 0.001++ P < 0-001.*
308.6***
78.8***
51.4"**
143.6"**
NS
NS
1 "4
16.0"**
--
--
5"2*
33"0***
--
--
2.0
P < 0.05
P < 0.05
A x B
17"9"**
40"5***
A • C
12"8"**
8"9***
0"2
0"1
--
--
0.6
2"7*
1"6
--
--
0"1
--
--
A • D
3.1"
B • C
54"9***
1"8
21"4"**
B • D
1"0
1"6
1"4
C • D
1-2
0.7
0.2
19"1"**
--
1.2
--
--
*P < 0"05. **P < 0"01. ***P < 0.001. f F-Va! ue. ~Average result of four modified Mann-Whitney tests.
Table HI. Differences in Climbing Behaviour Due to Post-weaning Experience in an Enriched Environment ( Sexes Combined)
Absolute difference* Variable
Wild
Domestic
Per cent difference* Wild
Domestic
Climbing latency (s)
+ 57.9
+330.0
+ 239
+
94
Climbing bouts
+
--
+ 120
--
40
Climbing duration (s)
--502"5
--283"6
--
31
--
42
Sectors entered while climbing
+ 45"8
-- 65.1
+
29
--
52
Bouts on horizontal rod
+
2.0
+
6.2
+1000
+ 886
Time on horizontal rod (s)
+
7.2
+ 15-3
+1800
+1530
6"7
5.1
*Differences based on deviation from scores of unenriched subjects. wild rats differed more than enriched and unenriched domestic subjects, but the per cent change in behaviour was greater for domestic r a t s ( T a b l e III). M a l e a n d f e m a l e d o m e s t i c r a t s differed more than male and female wild rats. T h e r e w a s little c h a n g e i n c l i m b i n g d u r a t i o n scores over trials but scores were significantly l o w e r d u r i n g t h e first 3 - m i n p e r i o d o f e a c h t r i a l
than during remaining periods for unenriched subjects and domestic rats. S e c t o r s entered w h i l e climbing. E n r i c h e d w i l d rats scored higher than unenriched counterparts in distance traversed while climbing, whereas the reverse was true for domestic rats (Fig. l(c)). W i l d s u b j e c t s w e r e m o r e a c t i v e climbers than domestic rats, and females :entered
368
ANIMAL
W~LD
BEHAVIOUR,
DOMESTIC
I imm
I ~ C ~ t o Ftu~,[s ~CHE~ FE~[I E~i~IED MKLES UNF.t~Ir IdAI.IEI
24,
2
WILD
B
DOMESTIC
m m [NracaEa FEU~IttI tT;z3 t l ,tS~lCNtD F E ~ t I
ozo~
17a
| t
WILD
Dr
WILD
DQMESTIC
Fig. I. Climbing behaviour of wild and domestic Norway rats reared in enriched and unenriched artificial environments: (a) latency to climb, (b) climbing duration, (c)number of grid sectors crossed while climbing, (d) time on horizontal rod. more sectors than males. Treatment had a greater influence on the scores of domestic rats than wild subjects (Table III). Betweentrial comparisons indicated that wild rats entered twice as many sectors during trial 1 as during any of the remaining four trials (which did not differ), whereas the behaviour o f domestic rats did not change over trials. Within-trial
comparisons indicated that wild rats entered significantly more sectors during the first 3-min of each trial than thereafter, while domestic rats scored lowest during this same time period. Bouts on horizontal rod. Enriched subjects mounted the horizontal rod more frequently than enriched counterparts. Domestic rats
HUCK & PRICE: EARLYEXPERIENCEEFFECTSON CLIMBINGIN WILD & DOMESTICRATS exhibited more bouts than wild subjects while sexes did not differ. Time on horizontal rod. Enriched subjects spent more time on the horizontal rod than unenriched rats (Fig. l(d)). Domestic rats spent more time on the rod than wild subjects. Sexes did not differ. In general, the rats spent little time on the horizontal rod relative to time spent on the cylinder wall. Miscellaneous. While domestic subjects and enriched wild rats frequently fell from the cylinder wall while climbing, unenriched wild subjects were never observed to fall. None of the subjects attempted to climb the rope present in the climbing apparatus. Discussion The results of the above experiments clearly demonstrate the influence of early climbing experience on adult climbing behaviour. Wild rats develop a propensity to climb even if reared in an environment where climbing is not possible. In addition early climbing experience improves the performance of wild rats in both a quantitative and qualitative manner. Domestic rats, particularly males, develop little propensity to climb if denied early climbing experience. However, climbing behaviour improves (at least quantitatively), as a result of climbing experiences provided in the post-weaning environment. Although climbing was virtually impossible in standard laboratory cages (experiment 1), the enriched and unenriched rearing environments employed in experiment 2 undoubtedly provided different opportunities for climbing experience. In the empty, unenriched environments, climbing was one of few motor activities that could be performed. In the enriched enclosures, however, climbing was one of many possible motor activities. The presence of two upper horizontal tiers in this environment precluded climbing the cage wall from top to bottom. Movement from one tier to another could be accomplished only by climbing narrow wooden ramps. The presence of vertical chains and rods, horizontal rods, activity wheels, etc. could have provided motor experiences related to climbing. Because of the eomplexity of the enriched environment, attempts to quantify the climbing experiences of the enriched subjects proved impractical. In general, unenriched wild rats were more agile climbers than the other rats when in the test apparatus. Only these subjects were able
369
to climb downward with their bodies oriented in that direction. Domestic rats and enriched wild subjects would slowly descend the wall backward or in an oblique (spiral) path. In addition, falling was frequently observed among domestic and enriched wild rats but never for unenriched wild subjects. Therefore, differences in climbing proficiency provide one explanation for the greater climbing duration scores of unenriched subjects and possibly the greater number of grid sectors entered by unenriched subjects of the domestic stock. Shorter latencies to climb provide a second explanation for the longer duration scores of the unenriched subjects. It was noted that during routine maintenance o f the post-weaning environments, unenriched subjects (particularly" the wild stock) would frequently climb the enclosure walls when disturbed (e.g. approached by experimenter), whereas enriched subjects would retreat to their respective nest boxes. Since the climbing apparatus used in testing resembled the unenriched post-weaning environments in being an empty arena with wire-mesh sides, the shorter climbing latencies of the unenriched subjects in experiment 2 could be based on a simple response learned in the rearing environment. The greater mobility of the enriched wild rats while climbing may have also accentuated the differences between enriched and unenriched wild rats in regard to time spent climbing. Whereas unenriched wild rats would begin each trial with a short burst of climbing activity and then cling to the wire in a given position for the remainder of the trial, enriched wild rats took longer tO initiate climbing and were more likely to return to the floor of the apparatus during the course of testing. The greater mobility of enriched wild rats while climbing may represent the same kind of increased mobility displayed by this group when tested in the open-field (Huck & Price 1975). The specificity of the effects of early climbing experience was seen in response to the horizontal rod. Enriched subjects were the only :rats to show any appreciable use of the rod. Early experience effects provide the most logical explanation for this difference in that rods were present in the rearing environments of the enriched group only. It could also be argued that unenriched subjects spent less time on the rod because they spent more time on the cylinder wall. Aside from the fact that the time element involved is very small ( < 10 s) this explanation
370
ANIMAL BEHAVIOUR,
is unlikely since cage-reared rats used the rod no more than unenriched subjects even though they spent considerably less time on the cylinder wall during testing. There are at least three explanations for the greater propensity of wild rats to climb in the test apparatus employed: (1) differences in the selective advantage of climbing (or motor skills in general) in field versus laboratory environments, (2) differences between wild and domestic rats in response to a novel environment, and (3) the heavier body weights of domestic rats. In general, motor skills are considered more important for the survival and reproduction of free-living animals than their captive counterparts (see introduction). The domestic Norway rat is no exception in this regard, since, aside from the motor skills associated with agonistic and sexual behaviour, little is required of the captive rodent to achieve maximum fitness. It could also be argued that, in captivity, selection for reduced motor potential (both in a quantitative and qualitative sense) would facilitate adaptation to an environment where the motor activities of animals are severely limited (Price 1973). Climbing behaviour in a novel environment could be a manifestation of escape, exploration or some form of conflict experience (e.g. displacement activity). Wild rats are initially more active than domestic rats when placed in a totally unfamiliar environment (Barnett 1958; Price & Loomis 1973; Price & Huck 1976). It appears that being placed in a novel environment is a more aversive experience for wild rats (Price & Huck 1976) and that the heightened activity of wild rats under these circumstances may be a manifestation of natural escape or avoidance tendencies. The climbing apparatus used in the present study was initially unfamiliar t o the test subjects. The fact that activity by wild rats while climbing on the cylinder wall was greatest during trial 1 (between-trial comparison) and at the beginning of each trial (within-trial comparison), whereas activity by climbing domestic rats was more evenly distributed within and between trials, suggests that wild rats were more responsive to novelty of the test conditions employed. The observation that unenriched wild rats were more likely than their domestic counterparts to climb the wall of their rearing environment when approached by the experimenter points to the fact that the climbing behaviour of wild rats may be more easily triggered by stimulus change or novelty.
24, 2
The body weights of the test subjects when removed from their respective rearing environments are presented in Table I (experiment 1) and in another paper (Huck & Price 1975). Domestic rats were heavier than wild rats in both experiments 1 and 2. This could have biased the results in favour of the wild subjects by making climbing more difficult for domestic rats. On the other hand, wild males climbed more (duration scores) than domestic females in both experiments even though their bodyweights were very similar and even though females of both stocks climbed more than males. A subsequent study conducted in a more natural setting confirmed the stock differences in climbing behaviour discussed above. Six female wild and six female domestic rats (cage-reared) were released in pairs (one of each stock) in a circular, wire out-door enclosure measuring 10.9 m in diameter and 4.6 m high. During the first hour following release wild rats averaged 478 s of climbing time as opposed to only 0.4 s for their domestic counterparts (Mann-Whitney U = 1, P < 0.002). Hence, although nearly 35 m2 of horizontal (ground) space was available, wild rats, as a group, spent over 13 per cent of their time climbing the enclosure wall. The present study provides data relative to the hypothesis that wild rats are more sensitive than their domestic counterparts to the effects of early experience (Huck & Price 1975). Although the response to early experience was significantly greater for wild rats for only two of six quantitative variables, Climbing performance (qualitative measures) was improved in wild rats only. In terms of adapting to the natural environment an improvement in climbing skill could be just as important as changes in pro, pensity to climb. In conclusion, the preceding experiments have demonstrated: (1) differences in climbing behaviour of wild and domestic Norway rats when reared in identical environments (the domestication problem) and (2) differences in climbing behaviour based on the nature of the postweaning rearing environment. Further studies are needed to determine the motivational factors underlying climbing behaviour in this species and how specific experiences gained in the postweaning environment influence climbing propensity and the development of climbing (motor) skills. Acknowledgments This study was supported by N.I,M.H. Grant MH21467-01 to E. O. Price. The authors would
H U C K & PRICE: EARLY EXPERIENCE EFFECTS ON CLIMBING I N WILD & DOMESTIC RATS
like to thank J. Moreau for his assistance in constructing the apparatus used in this study. The authors are also indebted to J. Bacon for recording the climbing behaviour of wild and domestic rats in an outdoor enclosure (described in the discussion). REFERENCES Archer, J. (1973). Tests for emotionality in rats and mice: A review. Anim. Behav., 21, 205-235. Barnett, S. A. (1958). Exploratory behavior. Br. J. PsychoL, 49, 289-310. Barnett, S. A. (1963). The Rat. Chicago: Aldine. Bennett, E.L., Diamond, M.C., Krech, D. & Rosenzweig, M. R. (1964). Chemical and anatomical plasticity of brain. Science, N.Y., 146, 610-619. Cooper, R. M. & Zubek, J. P. (1958). Effects of enriched and restricted early environments on the learning ability of bright and dull rats. Can. J. Psychol., 12, 159-164. Denenberg, V. H. (1969). The effects of early experience. In: The Behaviour of Domestic Animals (Ed. by E. S. E. F/afez), 2nd edn., pp. 95-130. Baltimore: Williams & Wilkins. Denenberg, V. H. & Grota, L. J. (1964). Social-seeking behavior as a function of differential rearing histories. J. abnorm, soc. PsychoL, 4, 453-456. Duffy, J. A. & Hendricks, S. E. (1973). Influences of social isolation during development on sexual behavior of the rat. Anita. Learn. & Behav., 1, 223-227. Homer, B. (1954). Arboreal adaptations of Peromyscus, with special reference to use of the tail. Contrib. Lab. Vert. BioL, Univ. Mich. 61, 1-85. Huck, U. W. & Price, E. O. (1975). Differential effects of environmental enrichment on the open-field of wild and domestic Norway rats. J. comp. physioL PsychoL, 89, 892-898. Hughes, C. W. (1975). Early experience in domestication. J. comp. physioL PsychoL, 88, 407--417. Hymovitch, B. (1952). The effects of experimental variations on problem solving in the rat. J. comp. physioL PsychoL, 45, 313-320. King, J. A., Price, E. O. & Weber, P. G. (1968). Behavioral comparisons within the genus Peromyscus. Papers Mich. Acad. Sci. Arts Letters, 53, 113-136.
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Koch, M. D. ~ Arnold, W. J. (1972). Effects of early social deprivation on emotionality in rats. J. comp. physioL PsychoL, 78, 391-399. Kreeh, D., Rosenzweig, M. R. & Bennett, E. L. (1962). Relations between brain chemistry and problemsolving among rats raised in enriched and impoverished environments. J. comp. physioL PsychoL, 55, 801"807. Layne, J. (1970). Climbing behavior of Peromyscus floridanus and Peromyscus gossypinus. J. MammaL, 51, 580--591. Morgan, M. J. (1973). Effects of post-weaning environment on learning in the rat. Anim. Behav., 21, 429--442. Nyman, A. J. (1967). Problem solving in rats as a function of experience at different ages. J. genet. PsychoL, 110, 31-39. Price, E. O. (1973). Some behavioral differences between wild and domestic Norway rats: Gnawing and platform jumping. Anita. Learn. & Behav., 1, 312-316. Price, E. O. & Loomis, S. (1973). Maternal influence on the response of wild and domestic Norway rats to a novel environment. Develop. PsychobioL, 6, 203-208. Price, E. O. & Huck, U. W. (1976). Open-field behavior of wild and domestic Norway rats. Anita. Learn. & Behav., Robinson, R. (1965). Genetics of the Norway Rat. Oxford: Pergamon Press. Rosenzweig, M. R. (1971). Effects of environment on development of brain and of behaviour. In: Biopsychology of Development (Ed. by E. Tobach, L. Aronson & E. Shaw), pp. 303-342. New York: Academic Press. Sokal, R. R. & Rohlf, F. J. (1969). Biometry. San Francisco: Freeman. Smith, F/. V. (1972). Effects of environmental enrichment on open-field activity and F/ebb-WiUiams problem solving in rats. J. comp. physioL PsychoL, 80, 163-168. Spevak, A. M., Quadagno, D. M., Knoeppel, D. & Poggio, J. P. (1973). The effects of isolation on sexual and social behavior in the rat. Behav. BioL, 8, 63-73.
(Received 10 June 1974; revised 10 June 1975; MS. number: A1575)