The reproductive patterns of an animal are a consequence of the interaction of the endogenous regulatory mechanisms – mainly endocrine (thyroid gland) and environmental signals. These signals may affect reproductive mechanisms deeply, i.e., females can stop cycling in some periods of the year, or can ovulate and come into estrus during the seasonal non-cycling period. Domestic species are broadly classified as seasonal and non-seasonal breeders depending upon the number of times they breed during a year;

  • Continuous breeders: These are the organisms which mate or breed throughout the year irrespective of the season. Example: poultry, rabbit and cattle etc.
  • Seasonal Breeders: Seasonal breeders are those animal species that reproduce only during a specific season of the year. Example: sheep, mare, bitch etc.

Seasonal breeders are further sub-divided into two categories based on a day-length.

Long day breeders: Those animals that breed during spring and summer season are called long day breeders. Example:  quail, horse and hamster etc.

Short-day breeders: Animals that breed during fall/winter season are known as short day breeders. Example: sheep and goat.

Long-day breeders and short-day breeders

Factors affecting seasonal breeding:

  • Photoperiod (Day Length)
  • Change in estrogen concentration
  • Availability of food and water
  • Housing
  • Climate

When female seasonal breeder is in season, or fertile and receptive to mating; they experience one or more estrus cycles. Male seasonal breeders, on the other hand, have fluctuating testosterone levels, testes weight, and fertility depending on the time of year.

Seasonal breeding is affected mainly by melatonin hormone by the following ways:

  • Melatonin produced during dark hours of a day
  • Melatonin is more potent in female
  • Long duration of melatonin has stimulatory effect
  • Short duration has inhibitory effect
  • Melatonin give signals to the neural structures controlling the secretion of GnRH (Reproductive Hormone)

Example: Seasonal Reproduction in Livestock species (sheep).

Most sheep breeds show reproductive seasonality, normally breeding in late summer and autumn. Both ram and ewes have seasonal variations in breeding activity, but females are more seasonal than males. Temperature, nutrition, social influences, lambing date and lactation period can modulate photo-periodical influences. It is possible to manipulate breeding season timing by altering the photoperiod with artificial lightning or with melatonin implants that simulate melatonin blood concentrations similar to the breeding season.