High-speed photography was used to compare the pigeon's response to unsignalled shock with the pigeon's key-peck response. During shock, pigeons flex their neck (i.e., the distance between their eyes and shoulders decreases). Following shock, the neck is extended. During key pecking, the neck remains extended and the head moves toward the key in a slight arc as though attached to a fixed fulcrum. Response topography during pecking and shock appear to be incompatible, and it is concluded that the difficulty in key-peck training pigeons to escape electric shock is due to interference from the unconditioned flexion response. This conclusion supports the species-specific defense theory of escape and avoidance behavior.Several investigators (Hoffman and Fleshler, 1959;Rachlin and Hineline, 1967;Hineline and Rachlin, 1969;Macphail, 1968) have reported failure, or great difficulty, in training pigeons to escape electric shock when escape is made contingent on the key-peck response. However, successful avoidance and escape behaviors in pigeons have been conditioned when responses other than key pecking have been selected. Hoffman and Fleshler (1959) obtained conditioning when a head-lifting response was used; Macphail (1968) and Bedford and Anger (1968) reported successful conditioning using a shuttle response; Smith and Keller (1970)

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Two groups of 12 Long-Evans rats were alternately isolated and crowded. Order of housing and familiarity of the subjects with the test apparatus were also varied systematically. Subjects were deprived of food, trained to bar press, and allowed to compete in pairs for sugar water in a Skinner box. Isolated pairs produced more agonistic and contact behaviors and fewer bar presses than did crowded pairs. Weight was found to increase more rapidly during free feeding when animals were isolated. Familiarity of only 1 isolate with the testing environment led to the display of less aggression than was observed in other isolate conditions.Animal experimental findings seem equivocal regarding the hypothesis that crowding leads to aggression. This uncertainty exists because investigators have employed 2 different manipulations of the independent variable. Studies have been designed to examine the effects of (a) differences in population size and (b) differences between grouped and isolated housing conditions.Two conclusions may be drawn from such studies: (a) Larger groups of animals exhibit more aggression than do smaller groups (Gregor, Smith, Simons, & Parker, 1972); and (b) isolated animals exhibit more aggression than do grouped animals

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