Melatonin and Seasonality in Sheep

Lynda Williams, R J A HELLIWELL

Research output: Contribution to journalLiterature reviewpeer-review

18 Citations (Scopus)


Seasonality in sheep and other mammals involves profound changes in metabolism, appetite and coat growth as well as reproductive status. The pineal hormone melatonin influences all of these changes and acts as an accurate reflection of the ambient photoperiod. In sheep, exposure to short daylengths or administration of melatonin by daily dosing or by continuous release implants significantly advances the breeding season and appears to enhance ovulation rate and embryo survival but only after a delay of approximately 60 days. During this delay, in the ewe, daily melatonin administration must be sustained and there appears to be no requirement for a gradual increase in luteinising hormone (LH) secretion. Sheep exposed to a constant 'stimulatory' melatonin signal, however, eventually become refractory to that signal and enter anoestrus. If sheep are deprived of the changing melatonin signal either by pinealectomy or by exposure to constant light regimes they will stm exhibit annual cycles of seasonal breeding. This indicates that there is a circannual rhythm of reproduction which is modulated by the changing melatonin signal, ensuring that reproduction coincides with the appropriate season. Ambient photoperiod and hence the melatonin signal is even more critical for the timing of the first reproductive cycle in pubertal animals. Evidence has accumulated that the photoperiodic history of an animal commences in utero with the foetus being receptive to the maternal melatonin signal from early in gestation. It appears that maternally experienced photoperiod is transmitted to the foetus and alters the subsequent response of the neonate to photoperiod. The phenomenon of seasonality and the influence of melatonin is discussed in the light of recent studies on the location and characteristics of the melatonin receptor. These findings are also discussed in relation to the applications of melatonin in manipulating the breeding season.

Original languageEnglish
Pages (from-to)159-182
Number of pages24
JournalAnimal Reproduction Science
Issue number1-4
Publication statusPublished - Oct 1993


  • hamster phodopus-sungorus
  • ovine pars tuberalis
  • male syrian hamster
  • rat suprachiasmatic nucleus
  • mice peromyscus-leucopus
  • luteinizing-hormone LH
  • white-footed mouse
  • binding-sites
  • ovarian activity
  • Djungarian hamster


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