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Seasons of Life – Russell G Foster and Leon Kreitzman

Small mammals such as the Syrian (golden) hamster, which has a paired testicular weight of about 5 grams when reproductively active but only about 0.2 grams when in the non breeding state, also regress their gonads.  There is relatively less marked seasonal change in the size of the reproductive organs in larger animals such as sheep and other ungulates.  The scrotal circumference – a measure of paired testis size – of a ram increases from about 30cms in the non breeding state to about 36 cms when breeding.  Even though the change in gonad size in large mammals such as ungulates is fairly small, sperm production in the male and ovulation in the female is often shut down for many months of the year.

Depending on the species, in most birds and mammals it takes between one and two months to reactivate a fully regressed set of reproductive organs.  These physical changes are accompanied by behavioural changes, which frequently involve establishing and maintaining a breeding territory to attract a mate.  After courtship and copulation, yet more time is needed for the young to develop before they can emerge into the world.  This period of development between fertilisation and birth or hatching, is largely fixed for each species.  The whole business of pro-creation can take a couple of months for a bird or small mammal, to over a year in a large mammal such as a horse and nearly two years in an elephant.  If a peak in food availability were the trigger for reproduction, it would be long gone before the young arrived.  So, to return to the key question, what triggers seasonal reproduction?