Figure 1. The sequence of experimental events for establishing individual levels of motivated responding for aggression and fighting performance during a daily fixed interval (FI) trial for aggression reward.

Prior to their confirmation of territorial aggression, resident male mice are initially housed with females for at least 1 month. In subsequent daily resident–intruder confrontations, each resident male encounters a novel male intruder for less than 5 minutes in the resident’s home cage. After establishing an aggressive phenotype, each resident is trained during a daily FI schedule that is reinforced by the presentation of an intruder (delivered into the resident’s home cage). The duration of the FI is progressively increased from 1 second to 10 minutes over the course of about a month. Resident mice then are allowed to establish a consistent pattern of FI responding. The upper left panel displays typical FI10 scalloped patterns of responding by mice for an aggression reward. Individual responses (tick marks) that are typically generated during a 10-min FI are indicated at the bottom left for varying levels of anticipatory arousal. The first response initiated after the completion of the FI allows the introduction of an intruder into the resident’s home cage. The upper right panel displays one element of fighting behavior (attack bites) that can be examined and potentially altered by experimental manipulations. Other aggressive elements that can be quantitatively and qualitatively examined include the latency to a first attack, number of attack bouts (bottom right), and biting topography (not shown). Changes in appetitive motivational responding can occur with or without alterations in fighting performance.