Most environments on our planet are highly seasonal, reflecting the tilt in the Earth’s axis relative to the sun. As a consequence, the majority of life forms have evolved profound seasonal variations in their behaviour and physiology that allow them to anticipate these patterns in food supply and to optimize their reproductive strategy. Reproduction is an energetically costly process in mammals, for example, in supporting pregnancy and then lactation in females. There has been strong selective pressure to ensure births occur in the optimal seasonal for survival. Terrestrial mammals indigenous to temperate and polar regions tend to give birth in spring, when the climatic conditions and food availability are conducive to survival. Seasonal cycles of reproduction also occur in equatorial regions, where they may be linked to wet and dry seasons. Only the species that have been domesticated by man or intensively bred, such as the laboratory strains of mice and rats, fail to display this seasonality. Given the intimate link between energy availability and reproductive success, it is no surprise that body systems regulating energy intake, storage and expenditure are themselves highly seasonal.

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