Box 1 Generalizations about photoperiodism in animals (Bradshaw & Holzapfel 2007); Box 2 provides a brief glossary of terms
Being in the right physiological, developmental or reproductive condition at the right time and place is an essential component of fitness in seasonal environments. A wide variety of vertebrates and invertebrates in marine, fresh water, and terrestrial habitats use the length of day (photoperiodism) to anticipate and prepare for seasonal transitions of major events in their life histories,
1 Unlike temperature or rainfall, the annual change in day length at any location is invariant from year to year. Day length therefore provides a highly reliable anticipatory cue for future seasonal conditions.
2 A specific photoperiodic response is based on selection through evolutionary time for the optimal seasonal time to develop, migrate, reproduce or go dormant.
3 Photoperiodism regulates a go/no-go response that initiates a cascade of physiological, developmental or reproductive processes that are generally irrevocable within the lifetime of the individual or are not reversed before completion of the seasonal event under selection.
4 In ectotherms, photoperiodism may act in concert with temperature to regulate subsequent continuous rate processes and thereby fine-tune the actual timing of the seasonal event in a thermal environment that varies from year to year.
5 Photoperiod tends to provide the most important cue for events that are distant in time or space; temperature, food and other ecological conditions become more important closer to the actual event itself.
6 Animals may respond to either absolute or changing day lengths; reliance on absolute day length is more prevalent in short- than long-lived animals.
7 Critical photoperiod, threshold day length or the incidence of photoperiodism within and among species tends to increase with latitude or altitude in the temperate zone.
8 Animals use day length in conjunction with circannual rhythmicity and refractory periods to keep track of seasonal time not only at temperate latitudes but also at tropical overwintering localities with constant day length, during migration through zones of rapidly changing day length, during polar summers with constant light, and in winter hibernacula or during polar winters with constant darkness.
Box 2 Glossary
Adaptation: genetic change in a population due to natural selection, leading to improvement of some function or increased suitability to some aspect of its environment.
Circannual rhythms: internal (endogenous) physiological rhythms with a period of about a year; circannual rhythms are most often ‘set’ by photoperiod.
Critical photoperiod: the length of day that induces a 50% long-day and 50% short-day response in a population or cohort; the length of day that causes an individual to switch from a long- to a short-day response, and vice versa.
Diapause: arthropod dormancy, may be hibernal or aestival.
Heritability: strictly, the ratio of additive to phenotypic variance; colloquially, the amount of genetic variation that is exposed to selection; a measure of the efficiency of response to selection.
Parturition: giving birth.
Phenology: the annual timing of life-history events in a population.
Phenotypic plasticity: the ability of an individual to develop any of several phenotypes, depending on the environment.
Photoperiodism: the ability to assess the length of day or night to regulate behaviour, physiology, development or reproduction.
Refractory: inability to respond to day length; herein, the inability to respond to long days. Refractoriness may be induced spontaneously or by long days themselves and may be terminated spontaneously or in response to short days or low temperature.