The
hypothalamus is considered to be the central control for reproduction due to its role in hormone regulation. 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.
Day length Seasonal breeding readiness 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 fertility period. "Long day" breeders cycle when days get longer (spring) and are in anestrus in fall and winter. Some animals that are long day breeders include
ring-tailed lemurs,
horses,
hamsters,
groundhogs, and
mink. "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. == See also ==