Annual cycles of changing daylength are important cues that influence many animals. Photoperiodism is the scientific term for the responses of organisms to the relative length of day and night. Such responses allow animals to synchronize their behavior with favorable environmental conditions, which leads to greater survival and reproductive success. Many animals respond both behaviorally and physiologically to changes in daylength, which regulates seasonal activities including migration, reproduction, molting, and hibernation.
Animals must begin migration before conditions have deteriorated where they are, or they will miss the optimal conditions at their destination. Photoperiodic changes not only cue animals to depart but also influence physiological changes that prepare them for extended travel, such as the accumulation of fat and changes in muscle mass.
If animals wait to breed until the food supply is already high, they will be too late to take advantage of optimal food resources. The peak of food availability will have passed by the time their young are mature enough to need it. They must breed in anticipation of high food availability so their young will be born when weather conditions are favorable and food is abundant, ideally peaking during the period of development when they need to consume the largest quantities. The increasing light of longer days stimulates the hypothalamus, which triggers the pituitary to release hormones that lead to reproductive behavior.
Seasonal molting of feathers and shedding of fur are similarly regulated by photoperiod. That鈥檚 important because proper timing allows animals to maintain appropriate amounts of insulation as well as to have the right color of camouflage to match the environment. Arctic foxes and ptarmigans change from brown to white fur and brown to white feathers respectively as daylength shortens which helps them avoid detection by predators during the snowy winter.
Photoperiodism also stimulates hibernation in some mammals. As daylight hours shorten in late summer and fall, animals experience physiological changes such as lower metabolic rates and increased fat storage that prepare them for winter. The onset of these changes is largely triggered by diminishing light, ensuring that animals enter hibernation before extreme cold and food scarcity become dangerous.
Animals behave differently during different seasons in adaptive ways so that they are feeding, breeding, hibernating, molting, and migrating at the most advantageous times. Because daylength is consistent year after year, it鈥檚 a dependable indicator of the season. Animals who respond to it benefit by reacting appropriately in anticipation of the changes to come. Other big seasonal changes such as temperature and other weather conditions are less reliable. There are many blips of unseasonably low and unseasonably high temperatures and other weather variations.
Daylength has historically been the most trustworthy way to detect the season and so behave in a seasonally appropriate and beneficial way. However, rapid climate change has thrown off this predictor of seasonal changes and resource fluctuations. Daylength isn鈥檛 as reliable an indicator of upcoming changes as it once was because it鈥檚 out of sync with new patterns of temperature, weather, snow cover, and food availability.