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As horses lose their heavy winter hair in the spring, their owners may note that some horses seem to have a much thicker coat than others. Why is this the case?

Hair growth in equines is related to photoperiod, meaning that horses’ bodies respond to seasonal variations in the hours of daylight as they grow and shed their hair. Because the earth’s equator is tilted in its relation to the sun, Northern Hemisphere regions get more light for half the year, while those in the Southern Hemisphere get more light during the other half. Day length varies in each hemisphere, with the least variation occurring near the equator.

As the days begin to get shorter in autumn, horses start to grow heavier, thicker coats. For horses in the Northern Hemisphere, those at higher latitudes experience an earlier and more extreme drop in daylight than horses in more southern areas. They begin to develop a winter coat sooner, and thus have a thicker covering that starts to shed later in the spring when the daylight hours lengthen. Horses in more southerly climates produce a lighter coat that comes in later in the season and begins to shed earlier in the spring.

The length and density of a winter coat also depend on the horse’s breed, age, health status, and nutrition. In general, horses with draft breeding grow heavier coats than those of lighter breeds, but there is considerable variation between individual horses of the same breed.

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