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Seasonal breeders are animal species that successfully mate only during certain times of the year. These times of year allow for the births at a time optimal for the survival of the young in terms of factors such as temperature, food and water. Related sexual interest and behaviors are expressed and accepted only during this period. Female seasonal breeders will have one or more estrus cycles only when she is "in season" or fertile and receptive to mating. At other times of the year, they will be anestrus. Similarly, male seasonal breeders may exhibit changes in testosterone levels, testes weight and fertility depending on the time of year.
The hypothalamus is considered to be the central control for reproduction. Hence, factors that determine when a seasonal breeder will be ready for mating affect this tissue. This is achieved specifically through changes in the production of the hormone GnRH. GnRH in turn transits to the pituitary where it promotes the secretion into the bloodstream of the gonadotropin LH, a pituitary hormone critical for reproductive function and behavior. Changes in gonadotropin secretion initiate the end of anestrus in females.
Factors that determine time of fertility
When a seasonal breeder is ready for mating is strongly regulated by length of day (photoperiod) and thus season. Photoperiod likely affects the seasonal breeder through changes in melatonin secretion by the pineal gland that ultimately alter GnRH release by the hypothalamus. 
Hence, seasonal breeders can be divided into groups based on when they are fertile. "Long day" breeders cycle when days get longer (spring) and are anestrus in fall and winter. "Short day" breeders cycle when the length of daylight shortens (fall) and are anestrus in spring and summer. Domestication has allowed cattle and swine to be liberated from breeding seasonality. Day length variations with latitude can also impact breeding. For instance, sheep and goats in tropical climes may breed throughout the year while those in more polar arctic areas may have a shortened season.
Females are generally more sensitive to changes in day length. For instance, unlike mares, stallions remain fertile year-round, suffering only some declines in sexual behavior and sperm production out of season.
Other factors that affect breeding time include the presence of a ready and available mate. For instance, the presence of a fertile male will induce an estrus cycle in a doe shortly after introduction.
Partial list of seasonal breeders
Long day breeders
Short day breeders
- Prendergast BJ (2005). Internalization of seasonal time. Horm. Behav. 48 (5): 503–11.
- M. N. Lehman, R. L. Goodman, F. J. Karsch, G. L. Jackson, S. J. Berriman, H. T. Jansen (1997). The GnRH System of Seasonal Breeders: Anatomy and Plasticity. Brain Res. Bull. 44 (4): 445–57.
- Garel M, Solberg EJ, Saether BE, Grøtan V, Tufto J, Heim M (2009). Age, size, and spatiotemporal variation in ovulation patterns of a seasonal breeder, the Norwegian moose. Am. Nat. 173 (1): 89–104.
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