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Introduction
Page 1
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Experiment 1:
Intro/Method
Pages 2-3
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Experiment 1:
Results
Page 4
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Experiment 1:
Discussion
Page 5
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Experiment 2:
Intro/Method
Page 6
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Experiment 2:
Results
Page 7
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Experiment 2:
Discussion
Page 8
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Experiment 3:
Intro/Methods
Page 9
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Experiment 3:
Results
Page 10
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Experiment 3:
Discussion
Page 11
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General Discussion
Page 12
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Figures 1-2-3
Pages 13-14-15
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Tables 1-2-3
Pages 16-17-18
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This experiment tested the effects of the following variables on competitive fighting: strain, sex of test animal, sex of opponent, gonadectomy, and level of food deprivation.
Whereas in territorial fighting the resident rat is almost always dominant over an intruder, in competitive fighting the two opponents may attack each other and require many sessions to establish a dominance relationship. To simplify the experiment in this regard, we used pairs of rats from two highly inbred strains that differ dramatically in competitive fighting (1). The Dark Agouti rats, which show more competitive fighting, were considered to be the test animals. They were paired with Fischer albinos, which show much less competitive fighting. The different coat colors also made behavioral scoring easier.
Method
Subjects. Two highly inbred strains of rats were used as subjects. The Dark Agouti (DA) strain was developed at Yale University and has been bred by brother-sister pairings at Wesleyan University since 1970 (now in their 35th generation at Wesleyan). The Fischers were obtained from Charles River Breeding Laboratories in 1986 and have been bred since then by brother-sister matings.
Housing. The rats were housed as pairs throughout the experiment in 81 X 25 X 25 cm cages with wire mesh walls and Plexiglas fronts. There were 64 cages in all, set up in eight racks of eight per rack. Attached at the end of each cage was a water bottle and a food hopper that allowed two animals to eat side by side. The water bottles were always fined. The food, Purina Lab Chow, was freely available except for 48 h of food deprivation which occurred once a week. An artificial diurnal cycle was created by lighting the rooms from midnight until noon with a white bulb, and with a right light for the remainder of the cycle.
Experimental design. Competitive fighting was tested in 64 pairs of rats, each pair consisting of one Dark Agouti (DA) test animal and one Fischer opponent who were cagemates. Half of the DA test animals were male and the other half were female.
The experiments were conducted over a period of 5 weeks. In the first week the rats were tested by the normal procedure, but because the animals were becoming habituated to the test situation and often took several minutes to begin feeding, the tests were considered as warmup, and the data were not included in the final data.
Formal testing began in the second week. After the tests in the second week, the Fischer cagemates of the DAs were switched so that one sex opponent was substituted for the other. In weeks 3 and 4, the DAs were tested against this new cagemate. After the tests in week 4, the cagemates were again switched and the original Fischer cagemate was returned to the DA. In this way, data were counterbalanced for order effects because DA data from weeks 2 and 5 against Fischer cagemates of one sex were compared to data from weeks 3 and 4 against Fischer cagemates of the opposite sex.
Half of the DA test animals were gonadectomized (16 castrated males and 16 ovariectomized females). Fischer cagemates of normal test animals were vasectomized or tube tied to prevent pregnancies; of course, this was not necessary for the Fischer cagemates of the ovariectomized and castrated DA test animals.
Two tests were conducted each week, one after 24-h deprivation and the other after 48-h deprivation, as described in the following section on offense testing.
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