A Comparison Between Agonistic Behavior of Rats of Wistar and Wezob Strain (<em>Rattus Norvegicus</em>)

Agonistic behavior of rats of the Wistar and WEzob strain was compared. Two male rats of the same strain or two male-female pairs of the same strain were confronted with each other for a period of one hour. Individuals and pairs were unknown to each other. In the week before the pairs or the single males were placed in the two compartments of a box. By removing a separating panel, encounters could take place. Behavior was videotaped and analyzed afterwards. During the confrontation session, males from pairs displayed far more consummatory acts (clinch fights and attacks, Le., biting or attempting to bite) than the individual housed males. The individually housed males, however, showed overall longer agonistic behavior, but this consisted of mainly appetitive agonistic behavior such as lateral attack, keeping down, keep off lying, them males from pairs. There were striking differences between strains: Wistar males from pairs showed more frequent and longer clinch fights and attacks than those of the WEzob strain. The former got far more wounds. It seems that belonging to the losing strain in interstrain encounters is not a good predictor of the amount of intrastrain aggression. The WEzob females displayed more frequent and longer appetitive behavior than the Wistar females. In male-female interactions the WEzob rats displayed more fi-equently some appetitive behaviors than the Wistar rats, but the Wistar rats displayed more and longer clinch fights than the WEzob rats. The enhancement of male aggression by the presence of a female seems to be a strain-dependent phenomenon. Usually male rats display agonistic behavior against intruders in their residence, especially if this intruder is a male. This agonistic behavior of the resident male is labelled as territorial behavior, Le., behavior to retain territory. Rats trying to extend their territory at the cost of neighbouring rats also display sigonistic behavior (Calhoun, 1962; Moyer, 1971). The resident rat is usually the winner. This phenomenon is known as the prior residence or ownership effect (Waser and Wiley, 1979). If the loser cannot flee, as is common in experimental situations, a dominant-submissive relation is established. Social isolation enhances agonistic behavior of male rats (Blanchard & Blanchard, 1979) and so does the presence of a female or her odor in the cage (Flannely & Lore, 1977). In interstrain encounters WEzob rats (a Dutch strain) are defeated by S3 rats (Tryon Maze dull rats) but win over Wistar rats (Van de Address correspondence to Dr. A Nyssen, University of Amsterdam, Subfaculty of Psychology, Weesperplein 8, 1018XA Amsterdam, The Netherlands. The International Journal of Comparative Psychology, Spring 1988 1 79 © 1988 Human Sciences Press 180 THE INTERNATIONAL JOURNAL OF COMPARATIVE PSYCHOLOGY Poll, Smeets, Van Oyen & VanderZwan, 1982). This suggests that S3 rats are more aggressive than WEzob rats and WEzob rats more than Wistar rats. This is in agreement with the statements of many investigators that Wistar rats are tame or dull animals. In the catalogue of the Dutch governmental institute for breeding laboratory animals the WEzob is characterized as aggressive. In the experiments of Van de Poll, et al. (1982), interstrain encounters between WEzob and Wistar rats were observed. Pilot studies of intrastrain encounters in Wistar rats as well as in WEzob rats showed there might be differences between intraand interstrain agonistic behavior. Environmental conditions are very important for the appearance of fighting and fleeing. Environmental factors can be the origin on strain differences. Rats must be housed in the experimental closure at least a couple of days to establish a territory. A simulation of a natural territory is impossible because in nature home ranges amount to several hundreds of meters in length with a width of 15 meters (Taylor, 1978). As it is known that social isolation, especially in youth, can increase aggressive behavior, the males that would be housed individually during the experiment, had to grow up in same sex male groups to make sure that the agonistic behavior of the single males could not be attributed to social isolation. A short period of isolation, one week, either does not or minimally influences that behavior (Adams, 1976; Timmermans, 1978). The Wistar and WEzob rats were investigated under two conditions. In one condition, two individually housed male rats from the same strain were confronted with each other. In the other condition, two male-female pairs from the same strain were confronted with each other. In both conditions the rats stayed in the experimental box the week before testing. This set up presents a problem in the case of the male-female pairs. When the two male-female pairs confront each other, the males will try to copulate with the unknown female. But in the week before testing when the females were in the company of a male, they tended to be in oestrus at least once. Therefore, they were pregnant at testing if the male and female were typical. The pregnant female would then reject the males and a fight between them would arise. Agonistic behavior has two aspects: attack and flight. Attack behaviour consists of a number of acts and postures which ultimately lead to biting the opponent. Biting and attempts to bite are the end or the goal of the whole offensive behavioral pattern and were therefore considered the consummatory acts of the offensive or attack behavior. The other acts of this pattern leading to the goal behavior were considered appetitive attacking behavior. Flight behavior, too, is divided into these two categories. Flight behavior either leads the rat to a place in which it does not meet danger, or to behavior which minimizes attacking behavior. Fleeing therefore was seen as consummatory, other nonA. NIJSSEN and MJ. VAN RIJSWIJK 1 81 combative withdrawing acts were considered appetitive flight behavior (Archer, 1976). Rats which chased away the others or were avoided were classified as winners and dominsint; fleeing rats or rats which avoided other rats were classified as losers and submissive. Most probably the winners would be in control of the whole enclosure: its own chamber and that of the loser.