Photoperiod
The relative length of light and dark periods in a 24-hour cycle. Many organisms use photoperiod as a reliable cue for seasonal timing of reproduction, migration, and dormancy.
Used in the Books
This term appears in 3 chapters:
"... without waiting for genetic evolution. Arctic foxes grow white fur in winter and brown fur in summer. Same fox, same DNA, different coloration. The photoperiod (day length) triggers hormonal cascades that switch which pigment genes are expressed. Desert locusts exist in two forms: a solitary phase (brown, a..."
"The margin matters. Before entering hibernation, bears engage in this feeding frenzy with singular focus. The trigger is photoperiod - decreasing daylight signals winter's approach (Paul et al., 2008). The bear's metabolism shifts. Leptin and insulin resistance increase, preventing..."
"...enetically silenced by prolonged cold exposure.) This is a two-signal system. Signal 1: Cold (vernalization) removes the block. Signal 2: Long days (photoperiod) activate flowering. Think of it as a two-key lock: both must turn for the door to open. Without vernalization, photoperiod can't trigger flowering...."
Biological Context
Unlike temperature, which fluctuates unpredictably, day length is a reliable predictor of season. Plants use photoperiod to time flowering; birds use it to trigger migration; mammals use it to time reproduction. The pineal gland detects photoperiod through melatonin secretion patterns.
Business Application
Business 'photoperiods': reliable leading indicators of market cycles. Quarterly earnings seasons, annual budget cycles, and industry trade shows create predictable patterns. Companies that align their rhythms to market photoperiods optimize timing of launches and investments.