TRANSITIONS FROM RICH-TO-LEAN SCHEDULES INCREASE ATTACK IN A LABORATORY MODEL OF SOCIAL AGGRESSION IN PIGEONS: II. FIXED-INTERVAL SCHEDULES.

Pigeons key pecked under two-component multiple fixed-interval (FI) schedules. Each component provided a different reinforcer magnitude (small or large), signaled by the color of the key light. Attacks toward a live, protected target pigeon were measured. Large- (rich) and small- (lean) reinforcer components alternated irregularly such that four different interval types (transitions) between the size of the immediately preceding reinforcer and the size of the upcoming reinforcer occurred within each session: lean-to-lean, lean-to-rich, rich-to-lean, and rich-torich transitions. The FI for each component was the same within each phase, but was manipulated across phases. For all pigeons, more attack occurred following the presentations of the larger reinforcer (i.e., during rich-to-lean and rich-to-rich transitions). For 2 of the 3 pigeons, this effect was modulated by the size of the upcoming reinforcer; attack following larger reinforcers was elevated when the upcoming reinforcer was small (i.e., during rich-to-lean transitions). This rich-to-lean effect on attack diminished or disappeared as the length of the FI schedule was increased (i.e., control over attack by the upcoming reinforcer size diminished with increases in the inter-reinforcement interval). For all pigeons and at all FIs, however, postreinforcement pauses were longest during the rich-to-lean transitions. These data (1) are consistent with the notion that postreinforcement periods during intermittent schedules function aversively and, thus, can precipitate aggressive behavior, and (2) suggest that rich-to-lean conditions may be especially aversive. They also indicate, however, that aversive effects of rich-to-lean transitions may differ across fixed-ratio (FR) and FI schedules, and that variables controlling attacking and pausing may not be isomorphic between these different schedule types.