Materials and Methods
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Competitive Fighting Tests
Following weaning each female was placed in a 32xl0 inch pen with a female of approximately the same age. but from a different strain. This was done because previous research (Zook and Adams. 1977) has shown that rates of competitive fighting are greater when the opponent is not from the same litter as the subject. The pair was allowed to live together and habituated to each other for at least one week before being subjected to food deprivation and competitive fighting tests. All subjects were at least 100 days old before competitive fighting tests were conducted.
Testing was done under conditions of 24-hour food deprivation. A test was begun by placing a single pellet of lab chow into the food hopper located on the front of the cage. Due to the limited space in front of the hopper, only one animal at a time could gain access to the food pellet. In most cases, each animal jostled with its cagemate for position at the hopper, and, in some cases, attack then occurred. Test duration was 20 minutes and each test was carried out during the mid-afternoon under red-light illumination, which is the early portion of the noon to midnight cycle in our laboratory. Tests on the same pair of rats were repeated from one to five times, spaced at least one week apart.
Behaviors were recorded as described by Grant and Mackintosh (1963) and Adams (1976). In particular, the presence of competitive fighting was measured in terms of the occurrence of a bite-and-kick attack.
Isolation-Induced Fighting Tests
Male rats were used in tests of isolation-induced fighting. They were housed alone in a 32xl0 inch wire mesh cage with a plexiglas front for at least five weeks prior to testing. To facilitate the measurement of scent-marking behavior, a glass Petri dish was put in the cage with the rat. Once each week, the dish was removed from the cage, weighed, cleaned and replaced, allowing assessment of the amount of scent-marking material deposited on the dish by the rat. This substance, which is largely a product of the preputial gland, is deposited on the dish when the animal urinates. On the day of a test for isolation-induced fighting, the dish was weighed just prior to the test, replaced without washing, and weighed again 24 hours later.
At the end of the five-week isolation period, with the animal at least 100 days old, an intruder rat belonging to a different strain was placed into the home cage of the subject, and for twenty minutes all behaviors were recorded on a moving paper tape (see Lehman and Adams, 1977 for details). All tests were conducted in mid-afternoon under red light illumination. All animals were tested only once.
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