Competitive and Territorial Fighting

Offense testing. Tests of competitive fighting were conducted following 24- and 48-h deprivation each week. Food was withdrawn 24 h before the first test, and the animals remained deprived for an additional 24 h following the first test, so that the second test occurred under conditions of 48-h deprivation. All tests were conducted between 2 and 4 h into the dark portion of the artificial diurnal cycle.

At the start of each fighting test, a single food pellet was dropped into the food hopper. The fact that the food hopper was separated by wire screening from the animals made it difficult for both animals to chew on the same piece of food simultaneously, so they usually alternated eating and pushing each other out of the way until fighting occurred.

Warm up tests were conducted the first week, because the latency to feeding was often rather slow (a mean of 9 and 6 min for tests 1 and 2, respectively). By the second week, however, when scoring began, the latency had decreased to a mean of less than 2 min. Scoring began as soon as both animals had fed.

Scoring was done for 20 min. All offense behaviors were noted for each animal along with the minute in which they occurred: bite-and-kick attack, bite, offensive sideways posture, piloerection, on-top posture, upright posture, and chase.

Scoring was done by undergraduate observers. In the first week of warm up tests they kept independent records which were then compared to improve interobserver agreement, and their observations were monitored by experienced observers.

Surgical procedures. Gonadectomies were done a minimum of 3 months prior to testing to ensure that circulating levels of gonadal hormones were exhausted. Vasectomies and tube typing of other rats were done at the same time. Rats were prepared for surgery by weighing and injection with 3.5 cc/kg of the anesthetic Chloropent. Following standard surgical procedures, the rats were placed in an incubator and their temperatures were monitored until recovery from anesthesia.

Estrus testing. For the first half of the animals tested, all normal DA females were tested daily for estrus. A Fischer male was introduced into the female's cage; if mounting and lordosis were observed within a 10-min time, the female was considered in estrus. Vaginal smears were not used, because previous work had shown that the procedure would have induced pseudopregnancy. Because so few animals cycled (one Fischer and no DA), presumably because of the stress of deprivation and fighting, the procedure was discontinued for the remaining half of the animals.

Weighing. All rats were weighed weekly on the same day and on the same balance. The initial mean weights of DA males were 290 and 205 g. respectively, while the initial mean weights of the Fischers were 281 and 186 g. respectively.

Statistical analysis. A repeated measures analysis of variance was carried out on the attack frequency data (sum of bite and bite-and-kick attacks) with the following main effects: test order, level of food deprivation, sex opponent, sex of test animal, and intact vs. gonadectomized test animal. The frequency data were not transformed for analysis.

(End of section)