There was a man-made mouse utopia called Universe 25. It started with 4 males and 4 females. The colony peaked at 2200 and from there declined to extinction. Once a tipping point was reached, the mice lost instinctual behaviors. Scientists extrapolate this model to humans on earth.
Lone females retreated to isolated nesting boxes on penthouse levels. Other males, a group Calhoun termed “the beautiful ones,” never sought sex and never fought—they just ate, slept, and groomed, wrapped in narcissistic introspection. Elsewhere, cannibalism, pansexualism, and violence became endemic. Mouse society had collapsed.