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Differences in locomotor activity between rats and gerbils in response to novelty
BEHAVIORAL BIOLOGY 19, 548--553 (1977), Abstract No. 6192
BRIEF REPORT Differences in Locomotor Activity between Rats and Gerbils in Response to Novelty GuY L.
OSBORNE 1
Department of Psychology, University of North Carolina at Charlotte, Charlotte, North Carolina 28223 Locomotor activity of rats and gerbils was assessed in two tasks which permitted escape and exploratory behaviors to be distinguished. In each task, the animal chose between being in a novel portion of the test apparatus or its home cage. Rats avoided the novel area in both tasks either by running to or remaining in their home cage. Gerbils, in contrast, approached the novel area in both tasks. These results suggest that rats and gerbils differ with regard to the motivational processes which control locomotor activity in novel settings. Locomotor activity of rats in novel settings represents defensive reactions such as freezing or attempts at escape, whereas the locomotor activity of gerbils represents exploration.
The Mongolian gerbil exhibits a different pattern of locomotor responding in novel experimental settings than does the laboratory rat. Gerbils are more active (Powell and Peck, 1969; Thompson and Lippman, 1972) and less thigmotactic (Nauman, 1968) than rats. In avoidance tasks, gerbils maintain a greater amount of locomotor responding than rats (Lippman, Galosy, and Thompson, 1970; Galvani, RiddeU, and Foster, 1975) which apparently determines the differences which occur in task performance between the species (Osborne, Caul, and Vanstrum, 1976). Also, rats reduce their locomotor activity in an open field when allowed access to a small side compartment, whereas gerbils do not (Glickman and Hartz, 1964). A frequent interpretation of these species differences has been that gerbils are more exploratory than rats (e.g., Lippman et al., 1970; Thompson and Lippman, 1972; Galvani et al., 1975). According to this view, gerbils are observed to be more active than rats since they investigate the experimental apparatus at a higher rate. Thus, relative performance by the species on a given task depends upon whether the demands of the task are compatible with the high rate of exploration in gerbils versus the lower rate seen in rats (cf., Galvani et al., 1975). Present address: Department of Psychology, Wesley Hall, Vanderbilt University, Nashville, Tennessee 37240. The author thanks Dr. William F. Caul for his comments on this paper and W. Reade Baker for his help in behavioral testing. 548 Copyright © 1977 by Academic Press, Inc. All rights of reproduction in any form reserved.
ISSN 0091-6773
LOCOMOTOR ACTIVITY IN RATS AND GERBILS
549
Implicit in this interpretation is the assumption that rats, even though less exploratory than gerbils, nevertheless do engage in some exploration of the novel experimental apparatus. Recent data reported by Blanchard, Kelly, and Blanchard (1974) suggest, however, that locomotor activity of rats in novel settings represents defensive reactions rather than exploration. Rats readily entered a novel maze only after initial fear responses were reduced by forced exposure to the apparatus. Furthermore, rats withdrew from a novel open field when entry into the home cage was permitted, even though they had to withstand shock to do so. Such findings are not compatible with the assumption that rats are merely less exploratory than gerbils. Rather, they suggest that the motivational processes which control locomotor activity in novel settings may be different in these species. Locomotor activity of rats in response to novelty may represent defensive reactions such as freezing or attempts at escape as suggested by Blanchard et al. (1974), while locomotor activity of gerbils may indeed represent exploration. To make this inference, however, data are needed from test situations which permit these classes of activity to be clearly distinguished. The purpose of the present study was to assess differences in locomotor activity between rats and gerbils in two free-exploration tasks similar to those of Blanchard et al. (1974). In a free-exploration task, escape and exploratory behaviors can be distinguished since the animal is presented a choice between being in a novel portion of the test apparatus or in an adjoining area, such as the animal's home cage (Welker, 1957). Predictions concerning the latency and frequency of movement between these areas depend on how the animal is thought to respond to novelty. For example, if novelty elicits defensive reactions in rats, latencies for these animals should be long for entry into the novel area but short for entry into the home cage. Conversely, if novelty elicits exploration in gerbils, they should be observed to behave in the opposite manner and frequent the novel area of the apparatus. The subjects were 12 male Sprague-Dawley rats from Zivic-Miller Laboratories, Glenshaw, Pennsylvania, and 12 male Mongolian gerbils (Meriones unguiculatus) from the colony maintained by the University of North Carolina at Charlotte Department of Psychology. The ages of the animals were 6-9 months for rats and 6-18 months for gerbils. All animals were experimentally naive and were not handled prior to the present experimental procedures. Animals were individually housed and maintained on ad lib. food and water. Like previous comparisons of these species (cf., Thompson and Lippman, 1972; Galvani et al., 1975), testing occurred during the light phase of the light-dark cycle. The first apparatus was a block U maze constructed of plywood painted flat black with a top of hardware cloth. Each of the three arms of the maze had a width of 16.33 cm and a depth of 19.68 cm. The lengths of the left
550
GUY L. OSBORNE
arm, the middle arm, and the fight arm were 76.2, 97.79, and 103.5 cm, respectively. The animal's home cage was attached to the left, shorter arm of the maze. The floor of the maze was divided into 12 segments of equal size by lines drawn perpendicular to the walls. The second apparatus was a square open field 91.76 cm on each side and 19.68 cm deep. It was constructed of unpainted plywood with a hardware cloth cover. An opening in the center of one of the walls permitted access to a connecting chamber which measured 29.21 cm long x 11,75 cm wide x 17.46 cm deep. Depending on the experimental condition, either the animal's home cage or a second open field with the same dimensions and construction as described earlier was attached to the connecting chamber. Shock was supplied to the half of the grid floor of the connecting chamber closest to the attached unit by a Model SG-903 B R S / L V E shock generator. The home cage of each animal had a rectangular opening in the front which was 8.89 cm wide x 10.16 cm high, the bottom of which was flush with the floor of the cage. The opening was covered by a door made of hardware cloth. Four 40-W fluorescent tubes (two tubes in two parallel rows spaced 10.16 cm apart) were centered 187.96 cm above each apparatus. Illumination in the center of each arm of the U maze was approximately 30 ft-c as measured by a Model 214 General Electric candlefoot meter. Illumination in the center of the first open field was 60 ft-c. The procedure for each animal involved two phases. Phase 1 was intended to assess how quickly the animal m o v e d from the familiar home cage into the novel U maze and its subsequent activity within the maze. The purpose of Phase 2 was to assess how frequently the animal moved from the open field across the electrified floor of the connecting chamber into the attached unit, which was either the home cage or the second open field. To start Phase 1, the home cage was attached to the U maze, and the door of the cage was raised. L a t e n c y to leave the home cage was recorded when the animal placed all four feet inside the U maze. The animal was then observed for 15 rain, during which the number of lines crossed and the time spent in the home cage were recorded. It was also noted whether the animal entered only the first, the first and second, or all three arms of the maze. If no entry into the maze occurred within 15 rain after the door was raised, testing was concluded for that animal. The U maze was cleaned after each animal was tested. The same animals were tested in Phase 2 4-6 hr after the conclusion of Phase 1. For six gerbils and six rats, the unit that was attached to the connecting chamber of the first open field was the animal's home cage. For the other six animals of each species, the attached unit was the second open field. A trial began when an animal was placed into the first
L O C O M O T O R A C T I V I T Y I N RATS A N D G E R B I L S
551
open field with its nose 12.1 cm from the opening to the connecting chamber. The trial ended either when the animal placed all four feet inside the attached unit or after 3 min if no entry had occurred. The dependent variable was the number of trials on which the animal entered the attached unit within the 3-min time limit. No shock was present on the grid floor of the connecting chamber on the initial triM. If an entrY into the attached unit occurred, 0.2-mA shock was supplied to the grid floor for the second trial. If another entry occurred, the shock was increased to 0.4 mA for the third trial. This procedure continued for the remaining trials in that shock level was increased following each entry into the attached unit. The other shock intensities were 0.8, 1.5, and 3.0 mA. If the animal did not enter the attached unit within 3 min on a given trial, it was replaced at the start position for another trial at the same shock intensity. Testing was concluded if the animal again failed to enter the unit on this trial. If the animal entered the attached unit at each level of shock and hence did not have two consecutive 3-min trials, testing was concluded following entry at the 3.0 mA intensity. As in Phase 1, the apparatus was cleaned after each animal was tested. Table 1 shows that gerbils and rats differed markedly in free exploration of the novel U maze and open field. In Phase 1, only 5 of the 12 rats
TABLE 1 Differences between Gerbils and Rats in Free Exploration of a U Maze (Phase 1) and of an Open Field (Phase 2) Phase 1
Species
Mean latency to enter U maze (sec) Mean lines crossed after entry into U maze Mean percentage of time in home cage after entry Mean n u m b e r of different maze arms entered Phase 2
= P P P
Gerbils
U
450.0 24.4
9.4 431.1
0"** 0"**
87.8 1.4
3.2 3.0
0"** 6**
Attached unit
Mean number of entries into attached unit from the open field a n * ** ***
Rats ~
5. < < <
Species
Home cage
Rats Gerbils
4.0 2.8
Open field 0.3 4.0
Only 5 of the 12 rats entered the U maze in the 15-min test period. 0.05. 0.01. 0.001.
U
0"** 5*
552
GUY L. OSBORNE
entered the U maze from the home cage within 15 rain after the door was raised, whereas all gerbils entered the maze and were very active during testing. Of the rats which did enter the maze, most confined their activity to the first arm of the maze where the home cage was attached and spent almost all of the test period in the home cage. Gerbils, in contrast, visited all three arms of the maze and spent almost all of the test period outside the home cage. In Phase 2, rats withdrew from the novel open field via the connecting chamber to enter the home cage much more frequently than to enter the second open field, even though they had to withstand shock to do so. This finding, with rats of a random-bred Sprague-Dawley strain, replicates that of Blanchard et al. (1974), who used a similar procedure with a Wistar-derived strain. Conversely, gerbils crossed the chamber more frequently to enter the second open field than to enter the home cage. Time of day for testing may have been an important variable in this study since gerbils and rats do not have similar patterns of diurnal activity. Thus, the present data as well as previous studies reporting differences in task performance or exploration between these species may apply only to the light phase. However, it is important to note that during Phase 2, rats in the home cage condition had exactly the same number of entries into the attached unit as gerbils in the open field condition and actually had more entries than gerbils in the home cage condition (U (6, 6) = 5, P < 0.05). This fact argues against interpreting the results of the present study as reflecting only a species difference in diurnal pattern of activity. Rats did not score less on all measures of locomotor activity. Rather, levels of locomotor activity in both species depended on what response was required in order to enter or remain in the preferred area of the apparatus. Thus, rats were inactive in the vicinity of the home cage in the U maze but were active in withdrawing from the novel open field when escape to the home cage was permitted. In contrast, gerbils were active within the novel area of the U maze and in crossing the connecting chamber to gain access to the second open field, but were relatively inactive in leaving the novel open field to enter the home cage. While rats and gerbils no doubt differ in exploration, these results indicate that additional factors must be considered when analyzing species differences in locomotor activity in novel settings. Consistent with the position of Blanchard et al. (1974), locomotor activity of rats apparently represents defensive reactions in that novelty is strongly avoided by these animals. The fact that this avoidance of novelty was accomplished by low levels of locomotor activity in the U maze in Phase 1 versus higher levels under the home cage condition in Phase 2 supports the argument that fear-eliciting, novel stimuli may produce either freezing or attempts at escape in rats depending on the specific characteristics of the task (Blanchard and Blanchard, 1969; Bolles, 1970). Gerbils, how-
LOCOMOTOR ACTIVITY IN RATS AND GERBILS
553
e v e r , d i s p l a y a s t r o n g p r e f e r e n c e for n o v e l t y . U n l i k e r a t s , l o c o m o t o r a c t i v i t y in n o v e l s e t t i n g s f o r g e r b i l s r e p r e s e n t s e x p l o r a t i o n , a c o n c l u s i o n consistent with past studies showing a strong bias for locomotor activity in g e r b i l s as t h e y a r e i n t r o d u c e d to a v a r i e t y o f t e s t s i t u a t i o n s (e.g., L i p p m a n et al., 1970; T h o m p s o n a n d L i p p m a n , 1972; O s b o r n e et al., 1976). T h u s , d i f f e r e n c e s in l o c o m o t o r a c t i v i t y b e t w e e n r a t s a n d g e r b i l s d o n o t m e r e l y r e f l e c t a q u a n t i t a t i v e d i f f e r e n c e in e x p l o r a t i o n , b u t r a t h e r i n d i c a t e the p r e s e n c e o f d i f f e r e n t s p e c i e s - s p e c i f i c m o t i v a t i o n a l p r o c e s s e s w h i c h f u n c t i o n to d e t e r m i n e a n i n d i v i d u a l ' s r e s p o n s e to a g i v e n t e s t situation.
REFERENCES Blanchard, R. J., and Blanchard, D. C. (1969). Passive and active reactions to feareliciting stimuli. J. Comp. Physiol. Psychol. 68, 129-135. Blanchard, R. J., Kelly, M. J., and Blanchard, D. C. (1974). Defensive reactions and exploratory behavior in rats. J. Cornp. Physiol. Psychol. 87, 1129-1133. Bolles, R. C. (1970). Species-specific defense reactions and avoidance learning. Psychol. Rev. 77, 32-48. Galvani, P. F., Riddell, W. I., and Foster, K. M. (1975). Passive avoidance in rats and gerbils as a function of species-specific exploratory tendencies. Behav. Biol. 13, 277290. Glickman, S. E., and Hartz, K. E. (1964). Exploratory behavior in several species of rodents. J. Comp. Physiol. Psychol. 58, 101-104. Lippman, L. G., Galosy, R. A., and Thompson, R. W. (1970). Passive-avoidance learning in gerbils and rats. J. Comp. Physiol. Psychol. 73, 269-273. Nauman, D. J. (1968). Open field behavior of the Mongolian gerbil. Psychon. Sci. 10, 163-164. Osborne, G. L., Caul, W. F., and Vanstrum, R. (1976). Avoidance differences between rats and gerbils. Anita. Learn. Behav. 4, 151-159. Powell, R. W., and Peck, S. (1969). Running-wheel activity and avoidance in the Mongolian gerbil. J. Exp. Anal. Behav. 12, 779-787. Thompson, R. W., and Lippman, G. (1972). Exploration and activity in the gerbil and rat. J. Comp. Physiol. Psychol. 80, 439-448. Welker, W. I. (1957). "Free" versus "forced" exploration of a novel situation by rats. Psychol. Rep. 3, 95-108.
BRIEF REPORT Differences in Locomotor Activity between Rats and Gerbils in Response to Novelty GuY L.
OSBORNE 1
Department of Psychology, University of North Carolina at Charlotte, Charlotte, North Carolina 28223 Locomotor activity of rats and gerbils was assessed in two tasks which permitted escape and exploratory behaviors to be distinguished. In each task, the animal chose between being in a novel portion of the test apparatus or its home cage. Rats avoided the novel area in both tasks either by running to or remaining in their home cage. Gerbils, in contrast, approached the novel area in both tasks. These results suggest that rats and gerbils differ with regard to the motivational processes which control locomotor activity in novel settings. Locomotor activity of rats in novel settings represents defensive reactions such as freezing or attempts at escape, whereas the locomotor activity of gerbils represents exploration.
The Mongolian gerbil exhibits a different pattern of locomotor responding in novel experimental settings than does the laboratory rat. Gerbils are more active (Powell and Peck, 1969; Thompson and Lippman, 1972) and less thigmotactic (Nauman, 1968) than rats. In avoidance tasks, gerbils maintain a greater amount of locomotor responding than rats (Lippman, Galosy, and Thompson, 1970; Galvani, RiddeU, and Foster, 1975) which apparently determines the differences which occur in task performance between the species (Osborne, Caul, and Vanstrum, 1976). Also, rats reduce their locomotor activity in an open field when allowed access to a small side compartment, whereas gerbils do not (Glickman and Hartz, 1964). A frequent interpretation of these species differences has been that gerbils are more exploratory than rats (e.g., Lippman et al., 1970; Thompson and Lippman, 1972; Galvani et al., 1975). According to this view, gerbils are observed to be more active than rats since they investigate the experimental apparatus at a higher rate. Thus, relative performance by the species on a given task depends upon whether the demands of the task are compatible with the high rate of exploration in gerbils versus the lower rate seen in rats (cf., Galvani et al., 1975). Present address: Department of Psychology, Wesley Hall, Vanderbilt University, Nashville, Tennessee 37240. The author thanks Dr. William F. Caul for his comments on this paper and W. Reade Baker for his help in behavioral testing. 548 Copyright © 1977 by Academic Press, Inc. All rights of reproduction in any form reserved.
ISSN 0091-6773
LOCOMOTOR ACTIVITY IN RATS AND GERBILS
549
Implicit in this interpretation is the assumption that rats, even though less exploratory than gerbils, nevertheless do engage in some exploration of the novel experimental apparatus. Recent data reported by Blanchard, Kelly, and Blanchard (1974) suggest, however, that locomotor activity of rats in novel settings represents defensive reactions rather than exploration. Rats readily entered a novel maze only after initial fear responses were reduced by forced exposure to the apparatus. Furthermore, rats withdrew from a novel open field when entry into the home cage was permitted, even though they had to withstand shock to do so. Such findings are not compatible with the assumption that rats are merely less exploratory than gerbils. Rather, they suggest that the motivational processes which control locomotor activity in novel settings may be different in these species. Locomotor activity of rats in response to novelty may represent defensive reactions such as freezing or attempts at escape as suggested by Blanchard et al. (1974), while locomotor activity of gerbils may indeed represent exploration. To make this inference, however, data are needed from test situations which permit these classes of activity to be clearly distinguished. The purpose of the present study was to assess differences in locomotor activity between rats and gerbils in two free-exploration tasks similar to those of Blanchard et al. (1974). In a free-exploration task, escape and exploratory behaviors can be distinguished since the animal is presented a choice between being in a novel portion of the test apparatus or in an adjoining area, such as the animal's home cage (Welker, 1957). Predictions concerning the latency and frequency of movement between these areas depend on how the animal is thought to respond to novelty. For example, if novelty elicits defensive reactions in rats, latencies for these animals should be long for entry into the novel area but short for entry into the home cage. Conversely, if novelty elicits exploration in gerbils, they should be observed to behave in the opposite manner and frequent the novel area of the apparatus. The subjects were 12 male Sprague-Dawley rats from Zivic-Miller Laboratories, Glenshaw, Pennsylvania, and 12 male Mongolian gerbils (Meriones unguiculatus) from the colony maintained by the University of North Carolina at Charlotte Department of Psychology. The ages of the animals were 6-9 months for rats and 6-18 months for gerbils. All animals were experimentally naive and were not handled prior to the present experimental procedures. Animals were individually housed and maintained on ad lib. food and water. Like previous comparisons of these species (cf., Thompson and Lippman, 1972; Galvani et al., 1975), testing occurred during the light phase of the light-dark cycle. The first apparatus was a block U maze constructed of plywood painted flat black with a top of hardware cloth. Each of the three arms of the maze had a width of 16.33 cm and a depth of 19.68 cm. The lengths of the left
550
GUY L. OSBORNE
arm, the middle arm, and the fight arm were 76.2, 97.79, and 103.5 cm, respectively. The animal's home cage was attached to the left, shorter arm of the maze. The floor of the maze was divided into 12 segments of equal size by lines drawn perpendicular to the walls. The second apparatus was a square open field 91.76 cm on each side and 19.68 cm deep. It was constructed of unpainted plywood with a hardware cloth cover. An opening in the center of one of the walls permitted access to a connecting chamber which measured 29.21 cm long x 11,75 cm wide x 17.46 cm deep. Depending on the experimental condition, either the animal's home cage or a second open field with the same dimensions and construction as described earlier was attached to the connecting chamber. Shock was supplied to the half of the grid floor of the connecting chamber closest to the attached unit by a Model SG-903 B R S / L V E shock generator. The home cage of each animal had a rectangular opening in the front which was 8.89 cm wide x 10.16 cm high, the bottom of which was flush with the floor of the cage. The opening was covered by a door made of hardware cloth. Four 40-W fluorescent tubes (two tubes in two parallel rows spaced 10.16 cm apart) were centered 187.96 cm above each apparatus. Illumination in the center of each arm of the U maze was approximately 30 ft-c as measured by a Model 214 General Electric candlefoot meter. Illumination in the center of the first open field was 60 ft-c. The procedure for each animal involved two phases. Phase 1 was intended to assess how quickly the animal m o v e d from the familiar home cage into the novel U maze and its subsequent activity within the maze. The purpose of Phase 2 was to assess how frequently the animal moved from the open field across the electrified floor of the connecting chamber into the attached unit, which was either the home cage or the second open field. To start Phase 1, the home cage was attached to the U maze, and the door of the cage was raised. L a t e n c y to leave the home cage was recorded when the animal placed all four feet inside the U maze. The animal was then observed for 15 rain, during which the number of lines crossed and the time spent in the home cage were recorded. It was also noted whether the animal entered only the first, the first and second, or all three arms of the maze. If no entry into the maze occurred within 15 rain after the door was raised, testing was concluded for that animal. The U maze was cleaned after each animal was tested. The same animals were tested in Phase 2 4-6 hr after the conclusion of Phase 1. For six gerbils and six rats, the unit that was attached to the connecting chamber of the first open field was the animal's home cage. For the other six animals of each species, the attached unit was the second open field. A trial began when an animal was placed into the first
L O C O M O T O R A C T I V I T Y I N RATS A N D G E R B I L S
551
open field with its nose 12.1 cm from the opening to the connecting chamber. The trial ended either when the animal placed all four feet inside the attached unit or after 3 min if no entry had occurred. The dependent variable was the number of trials on which the animal entered the attached unit within the 3-min time limit. No shock was present on the grid floor of the connecting chamber on the initial triM. If an entrY into the attached unit occurred, 0.2-mA shock was supplied to the grid floor for the second trial. If another entry occurred, the shock was increased to 0.4 mA for the third trial. This procedure continued for the remaining trials in that shock level was increased following each entry into the attached unit. The other shock intensities were 0.8, 1.5, and 3.0 mA. If the animal did not enter the attached unit within 3 min on a given trial, it was replaced at the start position for another trial at the same shock intensity. Testing was concluded if the animal again failed to enter the unit on this trial. If the animal entered the attached unit at each level of shock and hence did not have two consecutive 3-min trials, testing was concluded following entry at the 3.0 mA intensity. As in Phase 1, the apparatus was cleaned after each animal was tested. Table 1 shows that gerbils and rats differed markedly in free exploration of the novel U maze and open field. In Phase 1, only 5 of the 12 rats
TABLE 1 Differences between Gerbils and Rats in Free Exploration of a U Maze (Phase 1) and of an Open Field (Phase 2) Phase 1
Species
Mean latency to enter U maze (sec) Mean lines crossed after entry into U maze Mean percentage of time in home cage after entry Mean n u m b e r of different maze arms entered Phase 2
= P P P
Gerbils
U
450.0 24.4
9.4 431.1
0"** 0"**
87.8 1.4
3.2 3.0
0"** 6**
Attached unit
Mean number of entries into attached unit from the open field a n * ** ***
Rats ~
5. < < <
Species
Home cage
Rats Gerbils
4.0 2.8
Open field 0.3 4.0
Only 5 of the 12 rats entered the U maze in the 15-min test period. 0.05. 0.01. 0.001.
U
0"** 5*
552
GUY L. OSBORNE
entered the U maze from the home cage within 15 rain after the door was raised, whereas all gerbils entered the maze and were very active during testing. Of the rats which did enter the maze, most confined their activity to the first arm of the maze where the home cage was attached and spent almost all of the test period in the home cage. Gerbils, in contrast, visited all three arms of the maze and spent almost all of the test period outside the home cage. In Phase 2, rats withdrew from the novel open field via the connecting chamber to enter the home cage much more frequently than to enter the second open field, even though they had to withstand shock to do so. This finding, with rats of a random-bred Sprague-Dawley strain, replicates that of Blanchard et al. (1974), who used a similar procedure with a Wistar-derived strain. Conversely, gerbils crossed the chamber more frequently to enter the second open field than to enter the home cage. Time of day for testing may have been an important variable in this study since gerbils and rats do not have similar patterns of diurnal activity. Thus, the present data as well as previous studies reporting differences in task performance or exploration between these species may apply only to the light phase. However, it is important to note that during Phase 2, rats in the home cage condition had exactly the same number of entries into the attached unit as gerbils in the open field condition and actually had more entries than gerbils in the home cage condition (U (6, 6) = 5, P < 0.05). This fact argues against interpreting the results of the present study as reflecting only a species difference in diurnal pattern of activity. Rats did not score less on all measures of locomotor activity. Rather, levels of locomotor activity in both species depended on what response was required in order to enter or remain in the preferred area of the apparatus. Thus, rats were inactive in the vicinity of the home cage in the U maze but were active in withdrawing from the novel open field when escape to the home cage was permitted. In contrast, gerbils were active within the novel area of the U maze and in crossing the connecting chamber to gain access to the second open field, but were relatively inactive in leaving the novel open field to enter the home cage. While rats and gerbils no doubt differ in exploration, these results indicate that additional factors must be considered when analyzing species differences in locomotor activity in novel settings. Consistent with the position of Blanchard et al. (1974), locomotor activity of rats apparently represents defensive reactions in that novelty is strongly avoided by these animals. The fact that this avoidance of novelty was accomplished by low levels of locomotor activity in the U maze in Phase 1 versus higher levels under the home cage condition in Phase 2 supports the argument that fear-eliciting, novel stimuli may produce either freezing or attempts at escape in rats depending on the specific characteristics of the task (Blanchard and Blanchard, 1969; Bolles, 1970). Gerbils, how-
LOCOMOTOR ACTIVITY IN RATS AND GERBILS
553
e v e r , d i s p l a y a s t r o n g p r e f e r e n c e for n o v e l t y . U n l i k e r a t s , l o c o m o t o r a c t i v i t y in n o v e l s e t t i n g s f o r g e r b i l s r e p r e s e n t s e x p l o r a t i o n , a c o n c l u s i o n consistent with past studies showing a strong bias for locomotor activity in g e r b i l s as t h e y a r e i n t r o d u c e d to a v a r i e t y o f t e s t s i t u a t i o n s (e.g., L i p p m a n et al., 1970; T h o m p s o n a n d L i p p m a n , 1972; O s b o r n e et al., 1976). T h u s , d i f f e r e n c e s in l o c o m o t o r a c t i v i t y b e t w e e n r a t s a n d g e r b i l s d o n o t m e r e l y r e f l e c t a q u a n t i t a t i v e d i f f e r e n c e in e x p l o r a t i o n , b u t r a t h e r i n d i c a t e the p r e s e n c e o f d i f f e r e n t s p e c i e s - s p e c i f i c m o t i v a t i o n a l p r o c e s s e s w h i c h f u n c t i o n to d e t e r m i n e a n i n d i v i d u a l ' s r e s p o n s e to a g i v e n t e s t situation.
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