Phyniology and Behavior, Vol. 14, pp. 171-175. Brain Research Publications Inc., 1975. Printed in the U.S.A.
Effects of Age and Food Deprivation on the Development of Muricidal Behavior in Rats JEFFREY B. MALICK 1
Department o f Pharmacology, Biological Research Division Schering Corporation, Bloomfield, New Jersey 0 7003
(Received 1 June 1973) MALICK, J. B. Effects of age and food deprivation on the development of muricidal behavior in rats. PHYSIOL. BEHAV. 14(2) 171-175, 1975. - The induction of muricidal behavior in the rat, under various experimental conditions, was studied in order to determine whether mouse killing is a form of predatory aggression that is closely related to feeding behavior. Rats of various ages were socially isolated and subjected to various degrees of food deprivation to determine whether the same laws that govern feeding also govern muricidal behavior. The age of the rat at the beginning of the study (8, 12 or 18 weeks of age) did not appear to significantly affect the induction of muricide by food deprivation. Chronic food deprivation caused non-killer rats to kill mice. In addition, the severity of the food deprivation influenced the induction of mouse killing such that the greater the degree of deprivation, the greater the number of rats that became ldllers. The effects of food deprivation on the induction of mouse killing were reversed by giving the rats access to food ad h'b. Since many of the laws that govern feeding also appear to be the laws which govern mouse killing, the two behaviors may be highly inter-related. Killer rats
Induction of muricide
Food deprivation
Influence of age
KARLI'S [7] original observations on wild and domesticated rats under laboratory conditions clearly showed that there were distinct differences in their killing responses to the mouse. Whereas 70% of wild rats were killers, only 12% of the domesticated rats would kill mice. Since most killers eat part or all of the mice they kill, Karli attempted experimentally to elucidate the role hunger played in the killer response. He found that food deprivation failed to induce the majority of both wild and domesticated rats to kill mice, in fact, most rats starved to death without ever exhibiting any hostility toward the mice. Even after consuming dead mice presented to them, most rats failed to kill subsequent animals and starved to death. Several studies [4, 5, 14] have indicated that the incidence of killing behavior can be increased if rats are exposed repeatedly to a combination of cyclic food deprivation and food competition; in these studies, the rats were food deprived for 22 hr each day and were then allowed to compete for a common food supply with another rat for about 2 hr each day. From the results of these studies, it was imposs~le to conclude whether the increase in killing was due to the competition for food or the food deprivation. Whalen and Fehr [ 15 ], utilizing experimental designs similar to the aforementioned studies, concluded that cyclic food deprivation appeared to be of primary importance in increasing the incidence of the killing response and that food competition, as such, was of relatively little importance. Paul and co-workers [ 11] have since demonstrated that most rats maintained on a food deprivation schedule (23 hr deprivation and 1 hr ad lib access to food/day) for 2 weeks killed mice.
Predatory aggression
In a recent review on the physiological basis of aggression, Moyer [9] has analyzed mouse-killing by rats in terms of predatory aggression and has implied that the killing response is a food-getting behavior. If Moyer [9] is correct in assuming that killing is a food-seeking response, then mouse-killing behavior should obey the same laws that govern feeding. Feeding responses vary in direct relation to the degree of food deprivation; therefore, if mouse-killing is a food-seeking behavior, its probability of occurrence should vary according to the degree of food deprivation. Paul [ 12] has failed to demonstrate a significant difference between the killing behavior of rats reduced to either 75% or 90% of normal body weights. One of the purposes of these studies was to further investigate this very important question: i.e., does varying the severity of food deprivation influence the probability of mouse killing? Myer [ 10] has demonstrated that the percentage of rats that killed on first exposure to mice did not differ significantly with age. In this study, rats of varying ages were studied since, presumably, food deprivation would differentially affect rats of different ages. In this study, domesticated rats of different ages were systematically, chronically exposed to varying levels of food deprivation in order to ascertain whether hunger i n c r e a s e d the development of the killer (muricidal) response. METHOD
Animals and Procedure Male hooded Long-Evans rats (Blue Spruce Farms,
Reprint requests to: Dr. Jeffery B. Malick, Department of Pharmacology, IC! United States, Inc., Wilmington, Delaware 19899 171
172
MALICK
Altamont, New York) were used throughout this study. A total of 400 rats were used and they were divided by age and weight into three groups as follows: 8 weeks old (200g); 12 weeks old (300g); and 18 weeks old (400g). Rats from each of the aforementioned age groups (the number of rats/group are indicated on the appropriate figures) were subjected to varying levels of food deprivation for 6 weeks as follows: ad lib food; 15 g of food per day; and 7.5 g of food per day. A fourth group was placed on a complete deprivation (starvation) regimen for a limited time (l 1 days). All rats received water ad lib throughout the study. Prior to use in this study, all rats were housed 5 per cage. At the beginning of this study, rats were individually housed (social isolation; i.e., rats could not see neighboring rats but could perceive auditory and visual cues) in cages having an inside measurement of 14 × 7 x 7 in. The rats were isolated for 24 hr and then preselected so that they would be non-killers at the beginning of the study. Rats were tested for killer responses by gently placing a mouse in the front of the rat's home cage and noting whether the intruder was killed in a 5 min period, except during pre-selection when the mouse remained in the cage for 1 hr. All rats were tested for muricidal behavior between 9 and 11 a.m. and were fed appropriately (whenever applicable) at 2 p.m.; thus, there was an interval of 3 - 5 hr between testing and food presentation. All rats, except those on complete starvation regimens, were retested for their muricidal behavior at each of the following intervals after isolation and initiation of applicable food deprivation schedules: 3, 7, 14, 21, 28, 35 and 42 days. All rats on complete food deprivation schedules (16 rats from each of the 3 age groups) were tested for killing responses daily for 11 days at which time the study was terminated since the rats were believed to be too debilitated to kill mice. In another study, a group of 36 (12 weeks old) rats were given 7.5 g of food per day for 21 days and then given food ad lib for the remaining 21 days of the study in order to determine whether food deprivation-induced muricidal behavior was permanent or whether returning the rats to ad lib food schedules would abolish the response. Statistical analysis. In the analysis of the percentage of killers, Fisher's Exact Probability test was used to compare the three deprivation groups for each of the three age groups [13]. The McNemar test (one-tailed) was used to analyze the results of the experiment in which a group of rats was subjected to food deprivation and then given food ad lib to test for reversal of deprivation-induced changes in behavior [ 13 ]. RESULTS
Table 1 compares the induction of muricidal behavior in rats of different age groups under varying levels of food deprivation. This table indicates the percentage of rats showing the muricidal response after 6 weeks of isolation on the appropriate feeding regimen and the percentage that became killers after 11 days on starvation schedules. At the end of 6 weeks of a regimen of 15 g of food per day (slight deprivation) all 3 age groups exhibited significantly greater percentages of muricidal behavior (p
Effects of Age and Food Deprivation on the Development of Muricidal Behavior in Rats JEFFREY B. MALICK 1
Department o f Pharmacology, Biological Research Division Schering Corporation, Bloomfield, New Jersey 0 7003
(Received 1 June 1973) MALICK, J. B. Effects of age and food deprivation on the development of muricidal behavior in rats. PHYSIOL. BEHAV. 14(2) 171-175, 1975. - The induction of muricidal behavior in the rat, under various experimental conditions, was studied in order to determine whether mouse killing is a form of predatory aggression that is closely related to feeding behavior. Rats of various ages were socially isolated and subjected to various degrees of food deprivation to determine whether the same laws that govern feeding also govern muricidal behavior. The age of the rat at the beginning of the study (8, 12 or 18 weeks of age) did not appear to significantly affect the induction of muricide by food deprivation. Chronic food deprivation caused non-killer rats to kill mice. In addition, the severity of the food deprivation influenced the induction of mouse killing such that the greater the degree of deprivation, the greater the number of rats that became ldllers. The effects of food deprivation on the induction of mouse killing were reversed by giving the rats access to food ad h'b. Since many of the laws that govern feeding also appear to be the laws which govern mouse killing, the two behaviors may be highly inter-related. Killer rats
Induction of muricide
Food deprivation
Influence of age
KARLI'S [7] original observations on wild and domesticated rats under laboratory conditions clearly showed that there were distinct differences in their killing responses to the mouse. Whereas 70% of wild rats were killers, only 12% of the domesticated rats would kill mice. Since most killers eat part or all of the mice they kill, Karli attempted experimentally to elucidate the role hunger played in the killer response. He found that food deprivation failed to induce the majority of both wild and domesticated rats to kill mice, in fact, most rats starved to death without ever exhibiting any hostility toward the mice. Even after consuming dead mice presented to them, most rats failed to kill subsequent animals and starved to death. Several studies [4, 5, 14] have indicated that the incidence of killing behavior can be increased if rats are exposed repeatedly to a combination of cyclic food deprivation and food competition; in these studies, the rats were food deprived for 22 hr each day and were then allowed to compete for a common food supply with another rat for about 2 hr each day. From the results of these studies, it was imposs~le to conclude whether the increase in killing was due to the competition for food or the food deprivation. Whalen and Fehr [ 15 ], utilizing experimental designs similar to the aforementioned studies, concluded that cyclic food deprivation appeared to be of primary importance in increasing the incidence of the killing response and that food competition, as such, was of relatively little importance. Paul and co-workers [ 11] have since demonstrated that most rats maintained on a food deprivation schedule (23 hr deprivation and 1 hr ad lib access to food/day) for 2 weeks killed mice.
Predatory aggression
In a recent review on the physiological basis of aggression, Moyer [9] has analyzed mouse-killing by rats in terms of predatory aggression and has implied that the killing response is a food-getting behavior. If Moyer [9] is correct in assuming that killing is a food-seeking response, then mouse-killing behavior should obey the same laws that govern feeding. Feeding responses vary in direct relation to the degree of food deprivation; therefore, if mouse-killing is a food-seeking behavior, its probability of occurrence should vary according to the degree of food deprivation. Paul [ 12] has failed to demonstrate a significant difference between the killing behavior of rats reduced to either 75% or 90% of normal body weights. One of the purposes of these studies was to further investigate this very important question: i.e., does varying the severity of food deprivation influence the probability of mouse killing? Myer [ 10] has demonstrated that the percentage of rats that killed on first exposure to mice did not differ significantly with age. In this study, rats of varying ages were studied since, presumably, food deprivation would differentially affect rats of different ages. In this study, domesticated rats of different ages were systematically, chronically exposed to varying levels of food deprivation in order to ascertain whether hunger i n c r e a s e d the development of the killer (muricidal) response. METHOD
Animals and Procedure Male hooded Long-Evans rats (Blue Spruce Farms,
Reprint requests to: Dr. Jeffery B. Malick, Department of Pharmacology, IC! United States, Inc., Wilmington, Delaware 19899 171
172
MALICK
Altamont, New York) were used throughout this study. A total of 400 rats were used and they were divided by age and weight into three groups as follows: 8 weeks old (200g); 12 weeks old (300g); and 18 weeks old (400g). Rats from each of the aforementioned age groups (the number of rats/group are indicated on the appropriate figures) were subjected to varying levels of food deprivation for 6 weeks as follows: ad lib food; 15 g of food per day; and 7.5 g of food per day. A fourth group was placed on a complete deprivation (starvation) regimen for a limited time (l 1 days). All rats received water ad lib throughout the study. Prior to use in this study, all rats were housed 5 per cage. At the beginning of this study, rats were individually housed (social isolation; i.e., rats could not see neighboring rats but could perceive auditory and visual cues) in cages having an inside measurement of 14 × 7 x 7 in. The rats were isolated for 24 hr and then preselected so that they would be non-killers at the beginning of the study. Rats were tested for killer responses by gently placing a mouse in the front of the rat's home cage and noting whether the intruder was killed in a 5 min period, except during pre-selection when the mouse remained in the cage for 1 hr. All rats were tested for muricidal behavior between 9 and 11 a.m. and were fed appropriately (whenever applicable) at 2 p.m.; thus, there was an interval of 3 - 5 hr between testing and food presentation. All rats, except those on complete starvation regimens, were retested for their muricidal behavior at each of the following intervals after isolation and initiation of applicable food deprivation schedules: 3, 7, 14, 21, 28, 35 and 42 days. All rats on complete food deprivation schedules (16 rats from each of the 3 age groups) were tested for killing responses daily for 11 days at which time the study was terminated since the rats were believed to be too debilitated to kill mice. In another study, a group of 36 (12 weeks old) rats were given 7.5 g of food per day for 21 days and then given food ad lib for the remaining 21 days of the study in order to determine whether food deprivation-induced muricidal behavior was permanent or whether returning the rats to ad lib food schedules would abolish the response. Statistical analysis. In the analysis of the percentage of killers, Fisher's Exact Probability test was used to compare the three deprivation groups for each of the three age groups [13]. The McNemar test (one-tailed) was used to analyze the results of the experiment in which a group of rats was subjected to food deprivation and then given food ad lib to test for reversal of deprivation-induced changes in behavior [ 13 ]. RESULTS
Table 1 compares the induction of muricidal behavior in rats of different age groups under varying levels of food deprivation. This table indicates the percentage of rats showing the muricidal response after 6 weeks of isolation on the appropriate feeding regimen and the percentage that became killers after 11 days on starvation schedules. At the end of 6 weeks of a regimen of 15 g of food per day (slight deprivation) all 3 age groups exhibited significantly greater percentages of muricidal behavior (p
