Seasonality in animals

In temperate climates, many species are seasonal breeders. Sheep, goats and horses are examples of them. This means that periods of sexual activity (the oestrus season) are alternated with periods of sexual inactivity (the anoestrus season). In sheep for instance, sexual activity starts when the day length increases (long-day breeders). The difference in breeding season between species is connected with the differences in gestation length. In temperate and cold climates, the result is that both horses and sheep give birth to their young in spring, a period with sufficient food, giving them the best chance of survival.

The pineal gland is the main regulatory organ in seasonality. Via the eyes and complex neural connections day length is registered in the pineal gland. The pineal gland produces indoleamins of which melatonin is the most important. Melatonin is produced and secreted during the night. When days become shorter, the exposure of the animal to melatonin increases. This has, through not yet fully elucidated connections, a stimulating effect on the GnRH secretion by the hypothalamus in short-day breeders like sheep. In long-day breeders (horse) the increased exposure to melatonin during long nights (short days) inhibits the GnRH release by the hypothalamus. In this way the day length differences are recognized and translated into signals that turn on and off sexual activity. Of the domesticated animals, the cow lost most of its seasonality due to the process of domestication over the centuries.

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