The heart speeds up when you inhale and slows down when you exhale. This is respiratory sinus arrhythmia — a coupling between cardiac and respiratory rhythms so familiar that it's easy to treat as physiological trivia. Why does the heart bother tracking the lungs?
Border, Nogaret, Lefevre, and Jain answer with fluid mechanics. When cardiac and respiratory rhythms synchronize, dynamic stress in the pulmonary vasculature drops. The reduction in cardiac power loss is roughly 10% in humans and up to 55% in other species. The heart works less when it beats in time with the lungs.
The direction of the causal arrow is the interesting part. In standard synchronization physics, dissipative coupling between oscillators produces entrainment — the coupling causes the synchronization. Here the coupling is mechanical (shared vasculature, shared neural control) and the synchronization reduces the dissipation. The oscillators synchronize and then benefit from having synchronized. The system finds its way to lower energy not because synchronization is forced but because the synchronized state is mechanically advantageous.
The evolutionary implication follows immediately. Respiratory sinus arrhythmia isn't a quirk or an artifact — it's a pump optimization. Pacemakers that restore this natural coupling improve cardiac efficiency, which suggests the coupling was selected for its mechanical benefit rather than arising as a developmental byproduct.
Efficiency can be a driver of synchronization, not just its consequence.