White hair. Pink nose. Tail. The mouse is three months old. He’s been in a cage for four weeks in a period of enforced solitude. But today he will get a visitor.
A new mouse enters his cage, “our” mouse sizes him up — there’s “an initial pattern of exploratory activity”, as the researchers running this trial will put it. Then suddenly our mouse makes a startling move. He stands on his back legs, rattles his tail and aggressively bites the intruder, wrestling him to the ground.
The ensuing fight — brutal, violent and prompted simply by the introduction of another mouse — is videotaped by the researchers. They have seen this play out before. In almost all cases, the longer a mouse is isolated, the more aggressive it is to the newcomer.
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