Physiology&Behavior.Vol. 52, pp. 17-20, 1992
0031-9384/92 $5.00 + .00 Copyright © 1992PergamonPress Ltd.
Printed in the USA.
Effect of Group Size on Social Organization in Rats with Restricted Access to Food F. T H U L L I E R , .1 D. D E S O R , * J. M O S ¢ A N D B. K R A F F T *
*Laboratoire de Biologie du Comportement et Physiologie, URA C.N.R.S., 1293, Universit6 de Nancy I, B.P. 239, 54506 Vandoeuvre les Nancy Cedex, France, and i'Solvay Duphar B. V., Department of Pharmacology, P.O. Box 900, 1380 DA Weesp, Holland R e c e i v e d 5 S e p t e m b e r 1991 THULLIER, F., D. DESOR, J. MOS AND B. KRAFFT. Effect of group size on socialorganization in rats with restrictedaccess to food. PHYSIOL BEHAV 52(1) 17-20, 1992.--In groups of rats subjected to a experimental design in which access to the feeder was made difficult, a behavioural differentiation developed. Some rats brought back food pellets to the cage (carrier rats) while the others stayed in the home cage (noncarrier rats). We compared the social organization underlying the adoption of these roles in groups of three and six rats. Reducing group size increased the incidence of carrier rats, leading to a smaller number of differentiated groups. However, differentiated groups seemed to adapt to the situation in the same way as the groups of six rats. In both cases, carriers could be distinguished from noncarriers by their way of obtaining food and by how food possession stopped. Moreover, behavioural variables generally exhibited better stability in small groups and noncarrier/carrier rat relations were more extensive, resulting in more typical organization. The potential use of such groups for pharmacological studies is discussed. Rat
Social organization
Social behaviour
Group size
METHOD
COLIN and Desor (3) and Desor et al. (4) recently described the development of behavioural differentiation in groups of rats where food accessibility was made difficult by progressively immersing the only way of access with water. During the course of the test two main categories of rats appeared: carrier rats which dive and bring food back to the home cage and noncarrier rats which never dive, but steal food from other members of the group. This experimental situation discloses several behaviours also described in the wild rat such as diving and carrying and stealing food (8,10). This differentiation regularly happens, and observations clearly indicate the stability for up to at least several months (5). The rats have similar weight curves as laboratoryhoused animals and remain healthy, demonstrating a proper adaptation to the conditions. This experimental situation, in which rats exhibit differentiated behavioural profiles, seems interesting in the field of psychoactive drug research. Previous studies have always been done with groups of six rats. However, the behavioural analysis is time-consuming and the emerging social structure is complex. We, thus, studied the possibility of using groups of three rats, which is far more convenient for behaviour scoring. We determined whether reducing group size could modify expressions of behavioural profiles or global health of the rats. Since group size can influence social organization (1), groups of three rats were directly compared to ones of six, especially with respect to similarities in social organization.
Animals" One hundred and eight Wistar males (IFFA Credo, France), weighing 250-300 g at the start of the experiment, were used. Twelve groups of six rats and twelve groups of three rats were randomly constituted and housed in wire mesh cages (50 X 40 X 30 cm).
Experimental Setup Each cage was linked to an aquarium (120 X 20 X 30 cm) by a tunnel (Fig. 1). A sliding door controled access to the tunnel at the end of which a feeder was placed, distributing one food pellet at a time. The m a x i m u m water level was fixed by a perspex cover over the aquarium, forcing the rats to swim underwater.
Procedures From day l of the experiment, access to the feeder was limited to a daily 3-hour period. The aquarium was filled according to the following time schedule: on days l and 2 there was no water; on days 3 to 7 the way of access was progressively immersed until the m a x i m u m water level was attained. The first day of m a x i m u m level was called M W L day l, and rats had to dive to reach the feeder. The whole experiment lasted 2 months, and was run in two successive series, each comprising six groups of six rats and six
Requests for reprints should be addressed to F. Thullier.
17
18
IHI I III-:R VI
C,,
D......... , ~
:
F
FIG. 1. Experimental setup: A, aquarium: C, cage; D, door: F, feeder: T, tunnel).
three-rat groups. Data from both series were pooled for the analysis. Behavioural Scoring
Because most food carrying occurs at the beginning of the session (2), behavioural analyse was conducted by video-recording the first hour of the session on M W L days 4, 10, and 16. For technical reasons, the sessions were recorded in daylight (0700-1900). Behavioural data were directly entered into a computer. We only took analysed behavioural items related to feeding in this context. In this situation, a food possession period (Fig. 2), i.e., when a rat touches food, may start in one of the three following ways: by carrying food, by stealing food, or by picking up food from the ground. It may end in one of the four following ways: the rat eats up the food pellet, the pellet is stolen by an other rat, the pellet is lost (during an antagonistic encounter), or the pellet is left on the ground. The owner of the food may be attacked by other rats. Stealing attempts start by an approach to which the owner may respond by fleeing or kicking (13). Some approaches end by full attack, some of which evolve to simultaneous biting of the pellet for at least 1 second. The attacks may or may not lead to appropriation of the food. RESULTS
\I
16. no signiticant diflierence was observed between the categories of rats ••(4) .... 5.358, NS. 1-o compare feeding behaviour in the rats, we divided the number of pellets brought back during the first hour of the session by the n u m b e r of rats living in the cage (Table la). -Ihe number of pellets available per rat increased in the groups of six (Friedman's test: X~(2) -- 15.16, p < 0.001: Wilcoxon's test: T !1. p < 0.05 in MWI, day 10/16 comparison). Fhe n u m b e r of tood pellets available was stable in the groups of three, [x~(2) - 0.273, NS in G3: X~(2) : 1.444, NS in ND G3]. Compared with diiL ferentialed groups, nondifferentiated ones obtained more food on d a y s 4 a n d 1011/(2): 7.221, p < 0 . 0 5 , L' : 6.5, p < 0 . 0 5 ; 11(2) 5.355, 0.05 < p < 0.10, ~: = 5.5, p < (t.03]. However, the different types of groups had equivalent feeding possibilities [t1(2) - 3.978, NS] at the end of the experiment. The n u m b e r of pellets brought back by each carrier rat represents its workload (Table lb). It slightly increased in the groups ofsix [X~(2) 7.875. p < 0.02: - - 3.1 t, p < 0.01 in MWL day 10/16 comparison], and remained stable in the groups of three [X~(2) '-: 0.059, NS; x~;(2) :,: 0.368, NS, in G3 and ND G3 respectively]. However, at days 4 and 16 the carriers in the differentiated groups of six or three rats showed no significant difference in carrying activity [MWL day 4: H(2) - 7.361, p < 0.(15, = - 1.692, NS; MWL day 10: t1(2) 3.218, NS: MWL day 16: H(2) - 10.444, p < 0.01, _- - 0.256, NS, respectivelyl. The total n u m b e r of attacks observed in the groups (data in Table lc) was very, low and stable throughout the experiment in the nondifferentiated groups [x](2) - 1.077, NS]. The mean n u m b e r of attacks tended to decrease in G3 [×~(2) = 5.333. p < 0.10], and was stable in G6 [X~(2) = 1.911, NSI. However, on a day-to-day basis no differences in aggression were found in both types of groups (U .... 32, NS: U 36. NS: ~' ~ 26, NS on days 4, 10, and 16). CARRYING FOOD
STEALING PICKINGUP FOOD FOOD
FLIGHT OF THE OWNER \
/
,mr., ~
APPROACH
KICKING
/
/
/ /
In all six-rat groups a behavioural differentiation occurred at all days tested (called G6). By contrast, the differentiation was lower in the three-rat groups: at the end of the experiment, only half the groups showed behavioural differentiation (called G3) and the other six ones (called N D G3) were composed only of three carriers (number of differentiated three-rat groups: 7/12 on day 4, 7/12 on day 10, 6/12 on day 16). In the groups of six rats, the most frequent pattern of differentiation was three carrier rats on day 4, and four carriers and two noncarriers from day 10 onwards (Fig. 3). In the groups of three rats, the n u m b e r of all-carder rat groups slightly increased and the differentiated groups were equally divided into one-carder groups and two-carder groups. Weight change is an indicator of rat adaptation in the experimental situation. During the first week of m a x i m u m water level, all the rats experienced a weight loss, irrespective of the group size or their role (carrier or noncarrier) [Kruskal-Wallis' test: H(4) = 2.938, NS]. This initial phase was followed by compensation, and from day 10 the rats' weight increased. On day
THEFT OF FOOD 13, ct~ 1.,
FAILURE IN
/ATTACK
STEALING
t
z 0
B
I I
,s a._
THEFT OF FOOD
8 8
f FAILUREIN STEALINGFOOD
BOTHRATSBITE /...~ THEPELL~...-'~/ ~ " ~ END OF FEEDING
FOOD STOLEN
LEAVING
LOOSING THE PELLET THE PELLET
FIG. 2. The behavioural events during a food possession period.
GROUP SIZE AND BEHAVIOURAL PROFILES
19
o) ' o- - 3
,o- 3
displayed by noncarrier rats are reported in Table ld. G6 noncartier rats significantly increased in number of stolen pellets [X2(2) = 19.22, p < 0.001; z = 2.98, p < 0.01 in the comparison MWL day 10/16], but G3 ones did not [x2(2) = 1.47, NS]. However, statistical analysis did not show any significant difference between rats from G6 or G3 (z = 1.807; z = 0.667; z = 1.620 on days 4, 10, and 16) at any session. At the end of the experiment, food possession period for a carrier rat in G6 generally started with food carrying. The other two ways of obtaining food, especially stealing, were not extensively used (carrying: 82.63%; stealing: 5.53%; picking up: 11.83% of food possession periods). The rats were able to keep part of the food (eaten up: 40.11%, stolen: 42.54%) and few pellets were lost or left (9.89% and 7.46%). Noncarrier rats in G6 often started a food possession period by stealing food (82.48%). It was kept until the end more frequently than with carriers (57.99%). We noted that their food was rarely stolen (10.04% of stolen food; 8.18% of lost food; left food: 23.79%). There do not seem to be major changes in strategy between groups of six and groups of three rats. In G3, carrier rats mainly obtained food by diving to the feeder (carrying: 84.41%; stealing: 1.83%; picking up: 13.76%). On the other hand, noncarriers generally started a feeding period by stealing food (73.42%). The food of carrier rats was often stolen (eaten up: 30.58%; stolen: 48.76%; lost: 11.57%; left: 9.09%), while the noncarriers ate up their food and few pellets were stolen (eat up: 65.82%; stolen: 1.27%; lost: 7.59%; left: 25.32%).
.Q2 Z
-Q2 Z
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B
MWL day 4
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Effective behavioural models for screening psychoactive drugs increasingly call for analysis of social interactions in small groups. The rat is used in different study models for dominance behaviour (9), maternal aggression (6,12) and territorial behaviour in a resident-intruder paradigm (11) and in a colony-intruder model (7). Those models assess alterations in social interactions between individuals with previously assessed behavioural characteristics. Social relations of behaviourally differentiated rats
6
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5 TABLE 1
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.~2 Z 1
Z
MWL Day 4
3 2 1
0
0123456 nb carrier rats
0 1 2 3 nb carrier rats
FIG. 3. Distribution of the six-rat groups (A) and three-rat groups (B) according to the number of carrier rats per group. We compared the relative contribution of the four types of food exchange on day 16 that resulted from attacks. In both group sizes, the main flow was from carrier to noncarrier rats (79.2% versus 95%). Contrary to this, food exchange from noncarriers to carriers was low in G6 and did not occur in G3. Only a small percentage of food exchange was intracategorial in both types of groups, particularly in the small groups where it was practically nonexistent. Survival of noncarrier rats in this experimental design depends on their efficiency in stealing food. The number of food thefts
MWL,Day 10
MWL,Day 16
G6 G3 ND G3
4.7 (3.5-5.8) 5.3 (4.7-5.9) 6.0 (5.9-7.0)
4.9 (3.9-5.9) 4.7 (4.4-6.1) 6.7 (5.7-7.7)
6.3 (5.2-6.8) 5.2 (3.3-6.3) 6.5 (6.3-8.3)
G6 G3 ND G3
7.0 (6.0-10.0) 10.0 (7.0-13.2) 6.0 (6.0-7.0)
8.5 (6.0-10.0) 8.0 (7.0-12.5) 6.0 (5.2-8.2)
9.0 (8.0-12.0) 10.0 (7.7-12.2) 6.0 (5.0-8.0)
G6 G3 ND G3
7.3 (5.4-13.2) 13.0 (6.3-14.9) 0.7 (0.2-1.8)
5.6 (4.0-10.8) 6.7 (2.3-8.0) 1.0 (0.0-1.7)
6.9 (4.3-12.4) 4.8 (3.7-8.7) 1.5 (0.0-2.0)
4.0 (2.0-6.0) 6.0 (4.0-7.0)
6.0 (3.0-7.0) 4.0 (3.0-7.0)
7.0 (6.0-9.0) 6.0 (4.0-7.5)
G6 G3
Number of availablepelletsper rat (a), number of food pelletsbrought back by each carrier rat (b), mean number of attacks per rat (c) and number of food thefts made by the noncarrier rats (d) (medians and quartiles), in the different types of groups on days 4, 10, and 16 (G6: six-rat groups; G3: differentiated three-rat groups; ND G3: nondifferentiated three-rat groups).
20
I H t i I . I , I E R EI A [
could be studied in the model described here. However, in the field of psychopharmacological research this situation needed some simplifications. The main aim of this study was to determine whether reducing group size could modify the expressions of behavioural differentiation when groups of rats were placed in a restricted food access situation. The behavioural differentiation between carrier and noncarrier rats was observed in each group of six rats. In groups of three rats, this phenomenon occurred less frequently, since only 50% were formed of both types of rats at the end of the experiment. It seems that reducing group size increases occurrence of the carrier type, leading to a smaller number of differentiated groups. The greater incidence of carriers does not seem to be due to greater difficulty in adapting to the situation when we placed the constraint. If we consider weight change as a global health indicator, rats in the groups of three showed the same characteristics as those in G6. The groups of three rats thus adapted without major problems to the experimental situation. Adopting a social role might be partly due to the interindividual variability to which a rat was confronted, i.e., social group complexity. Reduction of variability in groups of three rats could possibly modify the incidence of the noncarrier category. The second aim was to describe the behavioural characteristics of the groups of three rats and to compare their organization to that in the groups of six. If one only considers the differentiated groups of three rats (G3), they seemed to adapt in a similar way as G6 rats. The mean number of available pellets per rat was equivalent in G6 and G3. As far as carrying activity is concerned, rats brought back equivalent numbers of food pellets in G6 and
G3 when the experiment stopped. Food supplying was thus similar in both types of groups. The mean number of attacks was similar in G3 and G6 on day 16 and, lastly, noncarrier rats in both types of groups stole equivalent numbers of food pellets. Groups of three hence seemed to adapt without any problem to the situation and generally showed better stability in the experiment than G6. The organization established in G3 appeared to be more contrasted than compared with G6. Food m o v e m e n t between rats mainly occurred from carriers to noncarriers, making up almost all the food exchanges in small groups. Interactions between the two types of rats appear to be more significant in small groups and dependence of noncarriers on carriers is better emphasized. Decreasing group size restricts the range of possible types of interaction per individual as it diminishes possibilities for different behavioural profiles to appear. Time spent in scoring behaviour is considerably reduced in the groups of three rats, and behavioural profiles are more clear cut, typologically speaking. In this respect, these groups seem to be more interesting for etho-pharmacological research. However, behavioural differentiation is less frequent if group size is reduced, which may pose practical problems for future pharmacological use, these studies tending to need a large number of groups. A solution we propose testing would be to make up these groups from previously differentiated groups of six rats while keeping particular relationships between different types of rats. -~,CKNOWLEDGEMENT This research was supported by a grant provided by Duphar B.V. company, Weesp. The Netherlands.
REFERENCES
1. Barnett, S. A. An analysis of social behaviour in wild rats. Proc. Zool. Soc. Lond. 130:107-152; 1958. 2. Colin, C. Apports d'une situation de ditficult6 d'accrs ~tla nouriture dans l'&ude des relations sociales chez le rat. Universit6 de Nancy 1; 1989. Thesis. 3. Colin, C.; Desor, D. Diffrrenciations comportementales dans des groupes de rats soumis ft une difficult6 d'accrs fi la nourriture. Behav. Proc. 13:85-100; 1986. 4. Desor, D.; Colin, C.; Krafft, B. Assessment of drug effects in a complex social situation in the rat: The "swimming-pool" model. Neuroseience 22:360; 1987. 5. Desor, D.; Colin, C.: Thullier, F.; Krafft, B.; Mos, J. Experimental social differentiation in diving for food rats. Proceedings of the 21 st International Ethological Conference, Utrecht; 1989. 6. Erskine, M. S.; Denenberg, V. H.; Goldman, B. D. Aggression in the lactating rat: Effects of intruder age and test arena. Behav. Biol. 23:52-66: 1978. 7. File, S. E. Colony aggression: Effects ofbenzodiazepines on intruder behavior. Physiol. Psychol. 10:413-416; 1982.
8. Gandolfi, G.; Parisi, V. Ethological aspects of predation by rats, Rattus norvegicus (Berkenhout), on bivalves, Unio pictorurn L. and Cerastoderma lamarcki (Reeve). Boll. Zool. 40:69-74; 1973. 9. Malatynska, E.; Kostowski, W. The effect of antidepressant drugs on dominance behavior in rats competing for food. Pol. J. Pharmacol. Pharm. 36:531-540; 1984. 10. Nieder, L.; Cagnin, M.; Parisi, V. Burrowing and feeding behaviour in the rat. Anim. Behav. 30:837-844; 1982. 11. Olivier, B.; van Aken, H.; Jaarsma, I.; van Oorschot, R.; Zethof, T.: Bradford, L. D. Behavioural effects of psychoactive drugs on agonistic behaviour of male territorial rats (resident-intruder model). In: Miczek, K. A.; Kruk, M. R.; Olivier, B., eds. Ethopharmacological aggression research. New York: Alan R. Liss, Inc.; 1984:137-156. 12. Olivier, B.; Mos, J.; van Oorschot, R. Maternal aggression in rats: Effects of chlordiazepoxide and fluprazine. Psychopharmacology (Berlin) 86:68-76; 1985. 13. Whishaw, I. K.; Tomie, J. Food wresting and dodging: Strategies used by rats (Ranus norvegicus) for obtaining and protecting food from conspecifics. J. Comp. Psychol. 101:202-209; 1987.
0031-9384/92 $5.00 + .00 Copyright © 1992PergamonPress Ltd.
Printed in the USA.
Effect of Group Size on Social Organization in Rats with Restricted Access to Food F. T H U L L I E R , .1 D. D E S O R , * J. M O S ¢ A N D B. K R A F F T *
*Laboratoire de Biologie du Comportement et Physiologie, URA C.N.R.S., 1293, Universit6 de Nancy I, B.P. 239, 54506 Vandoeuvre les Nancy Cedex, France, and i'Solvay Duphar B. V., Department of Pharmacology, P.O. Box 900, 1380 DA Weesp, Holland R e c e i v e d 5 S e p t e m b e r 1991 THULLIER, F., D. DESOR, J. MOS AND B. KRAFFT. Effect of group size on socialorganization in rats with restrictedaccess to food. PHYSIOL BEHAV 52(1) 17-20, 1992.--In groups of rats subjected to a experimental design in which access to the feeder was made difficult, a behavioural differentiation developed. Some rats brought back food pellets to the cage (carrier rats) while the others stayed in the home cage (noncarrier rats). We compared the social organization underlying the adoption of these roles in groups of three and six rats. Reducing group size increased the incidence of carrier rats, leading to a smaller number of differentiated groups. However, differentiated groups seemed to adapt to the situation in the same way as the groups of six rats. In both cases, carriers could be distinguished from noncarriers by their way of obtaining food and by how food possession stopped. Moreover, behavioural variables generally exhibited better stability in small groups and noncarrier/carrier rat relations were more extensive, resulting in more typical organization. The potential use of such groups for pharmacological studies is discussed. Rat
Social organization
Social behaviour
Group size
METHOD
COLIN and Desor (3) and Desor et al. (4) recently described the development of behavioural differentiation in groups of rats where food accessibility was made difficult by progressively immersing the only way of access with water. During the course of the test two main categories of rats appeared: carrier rats which dive and bring food back to the home cage and noncarrier rats which never dive, but steal food from other members of the group. This experimental situation discloses several behaviours also described in the wild rat such as diving and carrying and stealing food (8,10). This differentiation regularly happens, and observations clearly indicate the stability for up to at least several months (5). The rats have similar weight curves as laboratoryhoused animals and remain healthy, demonstrating a proper adaptation to the conditions. This experimental situation, in which rats exhibit differentiated behavioural profiles, seems interesting in the field of psychoactive drug research. Previous studies have always been done with groups of six rats. However, the behavioural analysis is time-consuming and the emerging social structure is complex. We, thus, studied the possibility of using groups of three rats, which is far more convenient for behaviour scoring. We determined whether reducing group size could modify expressions of behavioural profiles or global health of the rats. Since group size can influence social organization (1), groups of three rats were directly compared to ones of six, especially with respect to similarities in social organization.
Animals" One hundred and eight Wistar males (IFFA Credo, France), weighing 250-300 g at the start of the experiment, were used. Twelve groups of six rats and twelve groups of three rats were randomly constituted and housed in wire mesh cages (50 X 40 X 30 cm).
Experimental Setup Each cage was linked to an aquarium (120 X 20 X 30 cm) by a tunnel (Fig. 1). A sliding door controled access to the tunnel at the end of which a feeder was placed, distributing one food pellet at a time. The m a x i m u m water level was fixed by a perspex cover over the aquarium, forcing the rats to swim underwater.
Procedures From day l of the experiment, access to the feeder was limited to a daily 3-hour period. The aquarium was filled according to the following time schedule: on days l and 2 there was no water; on days 3 to 7 the way of access was progressively immersed until the m a x i m u m water level was attained. The first day of m a x i m u m level was called M W L day l, and rats had to dive to reach the feeder. The whole experiment lasted 2 months, and was run in two successive series, each comprising six groups of six rats and six
Requests for reprints should be addressed to F. Thullier.
17
18
IHI I III-:R VI
C,,
D......... , ~
:
F
FIG. 1. Experimental setup: A, aquarium: C, cage; D, door: F, feeder: T, tunnel).
three-rat groups. Data from both series were pooled for the analysis. Behavioural Scoring
Because most food carrying occurs at the beginning of the session (2), behavioural analyse was conducted by video-recording the first hour of the session on M W L days 4, 10, and 16. For technical reasons, the sessions were recorded in daylight (0700-1900). Behavioural data were directly entered into a computer. We only took analysed behavioural items related to feeding in this context. In this situation, a food possession period (Fig. 2), i.e., when a rat touches food, may start in one of the three following ways: by carrying food, by stealing food, or by picking up food from the ground. It may end in one of the four following ways: the rat eats up the food pellet, the pellet is stolen by an other rat, the pellet is lost (during an antagonistic encounter), or the pellet is left on the ground. The owner of the food may be attacked by other rats. Stealing attempts start by an approach to which the owner may respond by fleeing or kicking (13). Some approaches end by full attack, some of which evolve to simultaneous biting of the pellet for at least 1 second. The attacks may or may not lead to appropriation of the food. RESULTS
\I
16. no signiticant diflierence was observed between the categories of rats ••(4) .... 5.358, NS. 1-o compare feeding behaviour in the rats, we divided the number of pellets brought back during the first hour of the session by the n u m b e r of rats living in the cage (Table la). -Ihe number of pellets available per rat increased in the groups of six (Friedman's test: X~(2) -- 15.16, p < 0.001: Wilcoxon's test: T !1. p < 0.05 in MWI, day 10/16 comparison). Fhe n u m b e r of tood pellets available was stable in the groups of three, [x~(2) - 0.273, NS in G3: X~(2) : 1.444, NS in ND G3]. Compared with diiL ferentialed groups, nondifferentiated ones obtained more food on d a y s 4 a n d 1011/(2): 7.221, p < 0 . 0 5 , L' : 6.5, p < 0 . 0 5 ; 11(2) 5.355, 0.05 < p < 0.10, ~: = 5.5, p < (t.03]. However, the different types of groups had equivalent feeding possibilities [t1(2) - 3.978, NS] at the end of the experiment. The n u m b e r of pellets brought back by each carrier rat represents its workload (Table lb). It slightly increased in the groups ofsix [X~(2) 7.875. p < 0.02: - - 3.1 t, p < 0.01 in MWL day 10/16 comparison], and remained stable in the groups of three [X~(2) '-: 0.059, NS; x~;(2) :,: 0.368, NS, in G3 and ND G3 respectively]. However, at days 4 and 16 the carriers in the differentiated groups of six or three rats showed no significant difference in carrying activity [MWL day 4: H(2) - 7.361, p < 0.(15, = - 1.692, NS; MWL day 10: t1(2) 3.218, NS: MWL day 16: H(2) - 10.444, p < 0.01, _- - 0.256, NS, respectivelyl. The total n u m b e r of attacks observed in the groups (data in Table lc) was very, low and stable throughout the experiment in the nondifferentiated groups [x](2) - 1.077, NS]. The mean n u m b e r of attacks tended to decrease in G3 [×~(2) = 5.333. p < 0.10], and was stable in G6 [X~(2) = 1.911, NSI. However, on a day-to-day basis no differences in aggression were found in both types of groups (U .... 32, NS: U 36. NS: ~' ~ 26, NS on days 4, 10, and 16). CARRYING FOOD
STEALING PICKINGUP FOOD FOOD
FLIGHT OF THE OWNER \
/
,mr., ~
APPROACH
KICKING
/
/
/ /
In all six-rat groups a behavioural differentiation occurred at all days tested (called G6). By contrast, the differentiation was lower in the three-rat groups: at the end of the experiment, only half the groups showed behavioural differentiation (called G3) and the other six ones (called N D G3) were composed only of three carriers (number of differentiated three-rat groups: 7/12 on day 4, 7/12 on day 10, 6/12 on day 16). In the groups of six rats, the most frequent pattern of differentiation was three carrier rats on day 4, and four carriers and two noncarriers from day 10 onwards (Fig. 3). In the groups of three rats, the n u m b e r of all-carder rat groups slightly increased and the differentiated groups were equally divided into one-carder groups and two-carder groups. Weight change is an indicator of rat adaptation in the experimental situation. During the first week of m a x i m u m water level, all the rats experienced a weight loss, irrespective of the group size or their role (carrier or noncarrier) [Kruskal-Wallis' test: H(4) = 2.938, NS]. This initial phase was followed by compensation, and from day 10 the rats' weight increased. On day
THEFT OF FOOD 13, ct~ 1.,
FAILURE IN
/ATTACK
STEALING
t
z 0
B
I I
,s a._
THEFT OF FOOD
8 8
f FAILUREIN STEALINGFOOD
BOTHRATSBITE /...~ THEPELL~...-'~/ ~ " ~ END OF FEEDING
FOOD STOLEN
LEAVING
LOOSING THE PELLET THE PELLET
FIG. 2. The behavioural events during a food possession period.
GROUP SIZE AND BEHAVIOURAL PROFILES
19
o) ' o- - 3
,o- 3
displayed by noncarrier rats are reported in Table ld. G6 noncartier rats significantly increased in number of stolen pellets [X2(2) = 19.22, p < 0.001; z = 2.98, p < 0.01 in the comparison MWL day 10/16], but G3 ones did not [x2(2) = 1.47, NS]. However, statistical analysis did not show any significant difference between rats from G6 or G3 (z = 1.807; z = 0.667; z = 1.620 on days 4, 10, and 16) at any session. At the end of the experiment, food possession period for a carrier rat in G6 generally started with food carrying. The other two ways of obtaining food, especially stealing, were not extensively used (carrying: 82.63%; stealing: 5.53%; picking up: 11.83% of food possession periods). The rats were able to keep part of the food (eaten up: 40.11%, stolen: 42.54%) and few pellets were lost or left (9.89% and 7.46%). Noncarrier rats in G6 often started a food possession period by stealing food (82.48%). It was kept until the end more frequently than with carriers (57.99%). We noted that their food was rarely stolen (10.04% of stolen food; 8.18% of lost food; left food: 23.79%). There do not seem to be major changes in strategy between groups of six and groups of three rats. In G3, carrier rats mainly obtained food by diving to the feeder (carrying: 84.41%; stealing: 1.83%; picking up: 13.76%). On the other hand, noncarriers generally started a feeding period by stealing food (73.42%). The food of carrier rats was often stolen (eaten up: 30.58%; stolen: 48.76%; lost: 11.57%; left: 9.09%), while the noncarriers ate up their food and few pellets were stolen (eat up: 65.82%; stolen: 1.27%; lost: 7.59%; left: 25.32%).
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MWL day 4
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rats
MWL day 10
MWL day 10
7
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6
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8.5
8.5
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MWL day 16
MWL day 16
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Effective behavioural models for screening psychoactive drugs increasingly call for analysis of social interactions in small groups. The rat is used in different study models for dominance behaviour (9), maternal aggression (6,12) and territorial behaviour in a resident-intruder paradigm (11) and in a colony-intruder model (7). Those models assess alterations in social interactions between individuals with previously assessed behavioural characteristics. Social relations of behaviourally differentiated rats
6
09
5 TABLE 1
£ 4
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FIG. 3. Distribution of the six-rat groups (A) and three-rat groups (B) according to the number of carrier rats per group. We compared the relative contribution of the four types of food exchange on day 16 that resulted from attacks. In both group sizes, the main flow was from carrier to noncarrier rats (79.2% versus 95%). Contrary to this, food exchange from noncarriers to carriers was low in G6 and did not occur in G3. Only a small percentage of food exchange was intracategorial in both types of groups, particularly in the small groups where it was practically nonexistent. Survival of noncarrier rats in this experimental design depends on their efficiency in stealing food. The number of food thefts
MWL,Day 10
MWL,Day 16
G6 G3 ND G3
4.7 (3.5-5.8) 5.3 (4.7-5.9) 6.0 (5.9-7.0)
4.9 (3.9-5.9) 4.7 (4.4-6.1) 6.7 (5.7-7.7)
6.3 (5.2-6.8) 5.2 (3.3-6.3) 6.5 (6.3-8.3)
G6 G3 ND G3
7.0 (6.0-10.0) 10.0 (7.0-13.2) 6.0 (6.0-7.0)
8.5 (6.0-10.0) 8.0 (7.0-12.5) 6.0 (5.2-8.2)
9.0 (8.0-12.0) 10.0 (7.7-12.2) 6.0 (5.0-8.0)
G6 G3 ND G3
7.3 (5.4-13.2) 13.0 (6.3-14.9) 0.7 (0.2-1.8)
5.6 (4.0-10.8) 6.7 (2.3-8.0) 1.0 (0.0-1.7)
6.9 (4.3-12.4) 4.8 (3.7-8.7) 1.5 (0.0-2.0)
4.0 (2.0-6.0) 6.0 (4.0-7.0)
6.0 (3.0-7.0) 4.0 (3.0-7.0)
7.0 (6.0-9.0) 6.0 (4.0-7.5)
G6 G3
Number of availablepelletsper rat (a), number of food pelletsbrought back by each carrier rat (b), mean number of attacks per rat (c) and number of food thefts made by the noncarrier rats (d) (medians and quartiles), in the different types of groups on days 4, 10, and 16 (G6: six-rat groups; G3: differentiated three-rat groups; ND G3: nondifferentiated three-rat groups).
20
I H t i I . I , I E R EI A [
could be studied in the model described here. However, in the field of psychopharmacological research this situation needed some simplifications. The main aim of this study was to determine whether reducing group size could modify the expressions of behavioural differentiation when groups of rats were placed in a restricted food access situation. The behavioural differentiation between carrier and noncarrier rats was observed in each group of six rats. In groups of three rats, this phenomenon occurred less frequently, since only 50% were formed of both types of rats at the end of the experiment. It seems that reducing group size increases occurrence of the carrier type, leading to a smaller number of differentiated groups. The greater incidence of carriers does not seem to be due to greater difficulty in adapting to the situation when we placed the constraint. If we consider weight change as a global health indicator, rats in the groups of three showed the same characteristics as those in G6. The groups of three rats thus adapted without major problems to the experimental situation. Adopting a social role might be partly due to the interindividual variability to which a rat was confronted, i.e., social group complexity. Reduction of variability in groups of three rats could possibly modify the incidence of the noncarrier category. The second aim was to describe the behavioural characteristics of the groups of three rats and to compare their organization to that in the groups of six. If one only considers the differentiated groups of three rats (G3), they seemed to adapt in a similar way as G6 rats. The mean number of available pellets per rat was equivalent in G6 and G3. As far as carrying activity is concerned, rats brought back equivalent numbers of food pellets in G6 and
G3 when the experiment stopped. Food supplying was thus similar in both types of groups. The mean number of attacks was similar in G3 and G6 on day 16 and, lastly, noncarrier rats in both types of groups stole equivalent numbers of food pellets. Groups of three hence seemed to adapt without any problem to the situation and generally showed better stability in the experiment than G6. The organization established in G3 appeared to be more contrasted than compared with G6. Food m o v e m e n t between rats mainly occurred from carriers to noncarriers, making up almost all the food exchanges in small groups. Interactions between the two types of rats appear to be more significant in small groups and dependence of noncarriers on carriers is better emphasized. Decreasing group size restricts the range of possible types of interaction per individual as it diminishes possibilities for different behavioural profiles to appear. Time spent in scoring behaviour is considerably reduced in the groups of three rats, and behavioural profiles are more clear cut, typologically speaking. In this respect, these groups seem to be more interesting for etho-pharmacological research. However, behavioural differentiation is less frequent if group size is reduced, which may pose practical problems for future pharmacological use, these studies tending to need a large number of groups. A solution we propose testing would be to make up these groups from previously differentiated groups of six rats while keeping particular relationships between different types of rats. -~,CKNOWLEDGEMENT This research was supported by a grant provided by Duphar B.V. company, Weesp. The Netherlands.
REFERENCES
1. Barnett, S. A. An analysis of social behaviour in wild rats. Proc. Zool. Soc. Lond. 130:107-152; 1958. 2. Colin, C. Apports d'une situation de ditficult6 d'accrs ~tla nouriture dans l'&ude des relations sociales chez le rat. Universit6 de Nancy 1; 1989. Thesis. 3. Colin, C.; Desor, D. Diffrrenciations comportementales dans des groupes de rats soumis ft une difficult6 d'accrs fi la nourriture. Behav. Proc. 13:85-100; 1986. 4. Desor, D.; Colin, C.; Krafft, B. Assessment of drug effects in a complex social situation in the rat: The "swimming-pool" model. Neuroseience 22:360; 1987. 5. Desor, D.; Colin, C.: Thullier, F.; Krafft, B.; Mos, J. Experimental social differentiation in diving for food rats. Proceedings of the 21 st International Ethological Conference, Utrecht; 1989. 6. Erskine, M. S.; Denenberg, V. H.; Goldman, B. D. Aggression in the lactating rat: Effects of intruder age and test arena. Behav. Biol. 23:52-66: 1978. 7. File, S. E. Colony aggression: Effects ofbenzodiazepines on intruder behavior. Physiol. Psychol. 10:413-416; 1982.
8. Gandolfi, G.; Parisi, V. Ethological aspects of predation by rats, Rattus norvegicus (Berkenhout), on bivalves, Unio pictorurn L. and Cerastoderma lamarcki (Reeve). Boll. Zool. 40:69-74; 1973. 9. Malatynska, E.; Kostowski, W. The effect of antidepressant drugs on dominance behavior in rats competing for food. Pol. J. Pharmacol. Pharm. 36:531-540; 1984. 10. Nieder, L.; Cagnin, M.; Parisi, V. Burrowing and feeding behaviour in the rat. Anim. Behav. 30:837-844; 1982. 11. Olivier, B.; van Aken, H.; Jaarsma, I.; van Oorschot, R.; Zethof, T.: Bradford, L. D. Behavioural effects of psychoactive drugs on agonistic behaviour of male territorial rats (resident-intruder model). In: Miczek, K. A.; Kruk, M. R.; Olivier, B., eds. Ethopharmacological aggression research. New York: Alan R. Liss, Inc.; 1984:137-156. 12. Olivier, B.; Mos, J.; van Oorschot, R. Maternal aggression in rats: Effects of chlordiazepoxide and fluprazine. Psychopharmacology (Berlin) 86:68-76; 1985. 13. Whishaw, I. K.; Tomie, J. Food wresting and dodging: Strategies used by rats (Ranus norvegicus) for obtaining and protecting food from conspecifics. J. Comp. Psychol. 101:202-209; 1987.
