genetically determined, in which this process is initiated. This form of diapause is most well studied in rodents and marsupials but has been identified in many other species, including non-mammals. It is not clear how well the mechanisms studied for the onset, maintenance and release from facultative diapause in the rodent model apply to these other species. Obligate diapause Obligate (adj: by necessity) diapause (a.k.a. seasonal delayed implantation) is a mechanism ensuring the birth of offspring is timed during optimal environmental conditions, to ensure maximal survival. The proposed mechanism is to separate conception and parturition (birth) so that each can occur at the most favourable time of year. Obligate diapause is activated and deactivated by changes to the number of daylight hours within a day (photoperiod) and hence, occurs within specific seasons. While obligate diapause occurs in a variety of species in different groups, there are significant variations in diapause length. Skunks (Spilogale gracilis) have a diapause of around 200 days while minks (Neovision vison) only have a diapause of around 14 days. Similarly to facultative diapause, a series of hormonal changes arrest the blastocyst development, prior to implantation, preventing continued growth of the embryo. However, in obligate diapause, the blastocyst shall enter into the dormant state in every reproductive season. This means every blastocyst a mother produces shall enter a period of diapause. Close regulation of obligate diapause is essential for survival of the mother and offspring. Premature diapause can result in forgone growth and breeding opportunities and late diapause can result in death due to adverse conditions. Prior to the vernal equinox, the photoperiod is less than 12 hours. This increases the production of melatonin in the pineal gland. Due to the inhibitory relationship between melatonin and prolactin, this increase in melatonin decreases prolactin secretion from the pituitary gland. The decrease in prolactin consequently decreases progesterone production in the corpus luteum, preventing development of the blastocyst. This induces embryonic diapause. After the vernal equinox, the photoperiod is greater than 12 hours. This decreases the production of melatonin in the pineal gland and, therefore, increases the prolactin and progesterone production in the pituitary gland and corpus luteum respectively. The increase in prolactin induces expression of the gene Odc (ornithine decarboxylase). The Odc gene produces the ODC protein, a rate-limiting enzyme in the production of the polyamine, putrescine, within the uterine environment. The presence of putrescine may indicate a role