Physiology: efficiency of equine express postal systems

Express postal systems relied on horses for a period that spanned two millennia (540 BC to AD 1861), and these systems showed remarkable consistency in the average mail-courier speed and in the distance between horse-changing stations. Here I show how this adopted speed and distance combination was ideal for optimal horse performance and can be explained with a modern understanding of equine physiology and with reference to recent endurance records. The parameters of the historical systems were chosen to avoid heat stress deriving from an otherwise over-boosted metabolic machine and to reduce the risk of the animal falling lame.