Understanding some of the evolutionary history of horses is helpful when pondering their reproductive complexities. After evolving into a grazing animal with great agility and speed (about a million years ago), the modern horse migrated to various parts of the Earth in response to changing physical and weather conditions. After near-extinction 10,000 years ago, the species came back in Asia and gradually repopulated parts of the world as man domesticated and transported the animals. Their return to North America was only 500 years ago, when the Spanish explorers re-introduced horses.
In each diverse area of the world, reproductive function is molded by the environment. Since the gestation period of equids is nearly one year in most cases, adaptation to a breeding season that corresponded to the best weather and grass seasons was important to survival. This seasonality is controlled by several factors, with light being the primary agent. The interaction between lengthening days and melatonin, secreted by the pineal gland, is the control mechanism. Melatonin, which has an inhibitory effect on reproductive function, is reduced as day length increases, releasing the hormonal control of the mare’s cycle.
Thus, the horses of the desert, or other equatorial regions with earlier increases in day length, produce foals throughout a broad season, and the ponies which originate in far northern areas have the shortest breeding and foaling seasons. Breeds that were developed in Britain, northern Europe, and North America are at peak reproductive efficiency between May and August in the northern hemisphere. Arabians, Paso Finos, and breeds of more southerly origin might reproduce all year.