Mice

Mice sit at the opposite end of the square-cube law from elephants. Small animals have high surface area relative to volume, losing heat rapidly through their proportionally large surface. This is why mice eat constantly - they're fighting continuous thermodynamic loss. A mouse that stops eating for a day dies; an elephant can fast for weeks. The contrast demonstrates that different scales face fundamentally different physical constraints, requiring different operating parameters, not just scaled versions of the same approach.

Across mammals from mice to whales, total heartbeats per lifetime remains roughly constant at ~1.5 billion. Mice have faster heart rates (scaling as body mass to the -0.25 power) but shorter lifespans (scaling as mass to the 0.25 power), producing the same total heartbeats as whales. This power law reveals that biological time runs differently at different scales - a mouse lives its entire life in the same objective time a whale experiences as a brief phase.

Jacques Miller's 1961 thymus experiments used mice to reveal that what appeared redundant was actually essential. Thymectomized newborn mice developed severe immunodeficiency - they couldn't reject skin grafts, succumbed rapidly to infections, showed dramatic lymphocyte depletion. The thymus wasn't backup; it was the organ producing T cells. The mouse lesson: small size creates constant energy crises, biological time scales with body mass, and apparent redundancy often masks essential functions.