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Biology

The Neurobiology of Circadian Rhythms and Seasonal Breeding in Mammals

Quick fact

Mammals detect the changing season by the length of the night, not the day. The pineal gland releases the hormone melatonin only during darkness, and the duration of that nightly melatonin pulse is the chemical message that tells the brain whether to turn reproductive systems on or off—even though the seasons are caused by changes in day length, it is the night that carries the critical information.

Why this is interesting

Why do most mammals give birth in spring, even though they mated months earlier? The answer lies not in a calendar, but in a brain region that reads the changing length of daylight.