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 ambient temperature, food and water availability, and even changes in the predation behaviors of other species. 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. Unlike reproductive cyclicity, seasonality is described in both males and females. Male seasonal breeders may exhibit changes in testosterone levels, testes weight, and fertility depending on the time of year.
The breeding season is the most suitable season, usually with favourable conditions and abundant food and water, for breeding. Abiotic factors such as the timing of seasonal rains and winds can also play an important role in breeding onset and success.
Many species breed in colonies or large communities which is known as communal breeding. It is common to see large congregations of these species in particular favourable locations in their breeding seasons. These breeding colonies and their location are generally protected by wildlife conservation laws to keep the species from going extinct. Some species have evolved for communal breeding in large breeding colonies and can not breed in smaller numbers or pairs alone. These species can be threatened by imminent extinction if they are hunted on their breeding grounds or if their breeding colonies are destroyed. The Passenger pigeon is a famous example of probably the most numerous land bird on the American continent which had evolved for communal breeding that went extinct due to large scale hunting in its communal breeding grounds during the breeding season and its inability to breed in smaller numbers.
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 of the gonadotropins LH and FSH, both pituitary hormones critical for reproductive function and behavior, into the bloodstream. 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 in anestrus in fall and winter. "Short day" breeders cycle when the length of daylight shortens (fall) and are in anestrus in spring and summer. The decreased light during the fall decreases the firing of the retinal nerves, in turn decreasing the excitation of the superior cervical ganglion, which then decreases the inhibition of the pineal gland, finally resulting in an increase in melatonin. This increase in melatonin results in an increase in GnRH and subsequently an increase in the hormones LH and FSH, which stimulate cyclicity.
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
- Rut (mammalian reproduction), the mating season of various ungulate species
- Prendergast BJ (2005). "Internalization of seasonal time". Horm. Behav. 48 (5): 503–11. doi:10.1016/j.yhbeh.2005.05.013. PMID 16026787.
- Jerram L. Brown, 1978
- 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. doi:10.1016/S0361-9230(97)00225-6. PMID 9370210.
- L. Senger, Phillip (2005). Pathways to Pregnancy and Parturition (2nd Revised ed.). p. 154.
- 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. doi:10.1086/593359. PMID 19072136.