Seasons give life on Earth a calendar: a yearly rhythm of plenty and scarcity that plants and animals plan around. Nearly every big event in their year, from flowering to breeding to sleeping through winter, is timed to it. That pressure has driven some of life’s most extreme adaptations and helped generate the variety of species we see today.1PubMed Central. Understanding Evolutionary Impacts of Seasonality: An Introduction to the Symposium Because so much rides on timing, a warming climate that nudges the seasons can leave species out of step with what they depend on.
What Causes the Seasons
Earth’s axis is tilted rather than standing straight up, by about 23.4 degrees.2NASA Space Place. Why Are Seasons Important to Life on Earth? As Earth circles the Sun, that tilted axis keeps pointing the same direction.3NASA Space Place. Why Are Seasons Important to Life on Earth? So different parts of the planet receive the Sun’s direct rays at different times of year.4NASA Space Place. Why Are Seasons Important to Life on Earth? In June, the Northern Hemisphere is tilted toward the Sun and sunlight hits it most directly, which makes it summer there.5NASA Space Place. Why Are Seasons Important to Life on Earth? By December it is the Southern Hemisphere’s turn to tilt toward the Sun, and the north has winter.6NASA Space Place. Why Are Seasons Important to Life on Earth?
Many people think summer happens when Earth is closest to the Sun. In fact Earth is closest in early January, in the middle of northern winter.
Direct rays heat more because they concentrate energy on a small patch of ground, like a flashlight pointed straight down. Slanted winter light spreads the same energy over a wider area, so each bit of ground gets less. Summer days are also longer, so more hours of heating stack up.
Near the equator, sunlight stays strong all year and temperature changes little. There, the seasons show up mainly as shifts in rainfall, with wet and dry seasons. So “season” means different things in different places.
How Plants and Animals Know What Season It Is
Day length is the main signal. Many plants read it to flower in the best season to reproduce.7PubMed Central. Photoperiodic Flowering: Time Measurement Mechanisms in Leaves Their leaves contain machinery that measures day length by combining the plant’s daily internal clock with light signals.8PubMed Central. Photoperiodic Flowering: Time Measurement Mechanisms in Leaves When days reach the right length, the leaves switch on a flowering signal that travels through the plant.9PubMed Central. Photoperiodic Flowering: Time Measurement Mechanisms in Leaves
Gardeners know the result: some plants bloom only during long summer days and others only as days shorten. Many plants are better described as measuring the length of the uninterrupted night, which is why “long-day” and “short-day” are only loose labels. A short burst of artificial light in the middle of a long night can reset the count for sensitive short-day plants and stop them from flowering.
Animals use day length too. It, rather than cold weather, triggers seasonal shedding and the growth of a winter coat.10Alaska Department of Fish and Game. White for Winter Shorter days prompt hormones such as melatonin and prolactin that start the winter coat.11Alaska Department of Fish and Game. White for Winter So animals kept indoors can be pushed into winter or spring coats just by changing the lighting.12Alaska Department of Fish and Game. White for Winter
Day length works better than temperature as a calendar because temperature swings wildly, and a warm week in February could fool an animal. Day length on a given date is the same every year.
Living things also carry an internal yearly clock. Seasonal rhythms keep cycling even when outside cues are removed.13Wu Tsai Neurosciences Institute. How animals keep time annually: molecular mechanisms of the seasonal rhythm Hamsters kept under unchanging light still cycled through summer and winter states in weight, body temperature and reproductive organs.14PubMed. A circannual clock drives expression of genes central for seasonal reproduction A hormone signal in the brain rose and fell on this yearly cycle, switching breeding on and off.15PubMed. A circannual clock drives expression of genes central for seasonal reproduction
In nature, day length acts as the reset that keeps this clock matched to the real calendar. Without it, the clock in animals kept in constant conditions drifts, running a bit shorter or longer than exactly twelve months. Seeds, meanwhile, can use cold instead, needing a certain number of hours of winter chill before they sprout in spring.16Seed Science Research. Cold stratification in winter is more than enough for seed dormancy-break of summer annuals in eastern North America: implications for climate change
Surviving Winter
Hibernation is a state of deep inactivity that many mammals use to save energy through winter.17PubMed Central. Why bears hibernate? Redefining the scaling energetics of hibernation Hibernators can go months without eating or moving while keeping their muscles, bones and organs healthy.18Cahiers de Nutrition et de Dietetique. Metabolic flexibility in mammalian hibernation That works because the body actively controls the slowdown; it isn’t just a result of getting cold.19Cahiers de Nutrition et de Dietetique. Metabolic flexibility in mammalian hibernation Gram for gram, a hibernating bat burns energy at about the same rate as a hibernating bear 20,000 times its size.20PubMed Central. Why bears hibernate? Redefining the scaling energetics of hibernation Hibernation seems to push cells close to the lowest energy level they can survive on.21PubMed Central. Why bears hibernate? Redefining the scaling energetics of hibernation
It is not just a long sleep. Ordinary sleep keeps body temperature and metabolism close to normal, while hibernation drops body temperature far lower and slows the heart and breathing dramatically. Hibernators also aren’t perfectly still: many wake periodically during winter, warming up before sinking again. Those wake-ups are costly in energy, so an animal disturbed repeatedly can burn through the fat it needs to reach spring.
That fat is built up during the plentiful months. Some other animals instead hide stores of seeds or nuts and eat from the stash in the cold. Either strategy depends on the timing cues described earlier, since preparation has to begin while food is still easy to find. Bears, bats and ground squirrels wait it out, while many birds, which can travel long distances, leave.
Insects fight freezing with chemistry. Many overwintering insects fill their body fluids with sugars and antifreeze proteins.22PubMed Central. Antifreeze proteins govern the precipitation of trehalose in a freezing-avoiding insect at low temperature The proteins lower the freezing point and slow ice growth.23PubMed Central. Antifreeze proteins govern the precipitation of trehalose in a freezing-avoiding insect at low temperature Some insects instead survive being frozen solid, down to -40 to -70°C.24PubMed. Antifreeze proteins in Alaskan insects and spiders
Color change and snow shelter help too. Animals like snowshoe hares turn white mainly to blend in against snow, with day length setting the timing.25PubMed. Function and underlying mechanisms of seasonal colour moulting in mammals and birds: what keeps them changing in a warming world? Under the snow lies a sheltered layer where many plants and animals survive winter.26Frontiers in Ecology and the Environment. The subnivium: a deteriorating seasonal refugium Small rodents spend most of the winter in the space between snow and ground.27PubMed. Small rodent winter survival: snow conditions limit access to food resources
Fluffy snow traps air, which slows heat loss from the ground, so soil under deep snow stays near freezing even in bitter cold above. Thin or patchy snow exposes plants and small animals to harsher cold. Icy snow can also seal plants in and cut rodents off from food, lowering their survival.28PubMed. Small rodent winter survival: snow conditions limit access to food resources
Seasons Set the Timing of Food, Breeding and Migration
Seasons create bursts of food. In temperate deciduous forests, the capacity for photosynthesis changes through the season, falling in mid and late summer even before leaves age.29Plant, Cell & Environment. Leaf age affects the seasonal pattern of photosynthetic capacityand net ecosystem exchange of carbon in a deciduous forest In oceans, the spring plankton bloom starts when winter mixing of the water gives way to calmer conditions near the surface.30ICES Journal of Marine Science. Characterizing upper-ocean mixing and its effect on the spring phytoplankton bloom with in situ data That bloom feeds the ocean food chain each spring. In the tropics, forests grow more in the wet season, and dry spells strongly limit productivity.31Forest Ecology and Management. Precipitation influences on the net primary productivity of a tropical seasonal rainforest in Southwest China: A 9-year case study
Breeding has to line up with these peaks so young can grow.32Canadian Journal of Zoology. Timing of breeding, peak food availability, and effects of mismatch on chick growth in birds nesting in the High Arctic Arctic shorebird chicks that hatched outside the peak in insect food grew more slowly.33Canadian Journal of Zoology. Timing of breeding, peak food availability, and effects of mismatch on chick growth in birds nesting in the High Arctic In temperate places, spring is baby season for this reason. Insects, leaves and seeds surge in late spring, parents need the most food when young are growing fast, and hatching or birth is timed so peak demand meets peak supply.
The surge doesn’t last, so the window for raising young may be only a few weeks. Animals that breed too late find the food already declining while their young still need the most energy.
Many animals can’t simply breed when food appears. Pregnancy or egg development takes time before the young arrive. Animals therefore rely on cues such as day length to begin breeding earlier, so the young arrive when the food does. Gestation length is one reason timing differs between species: one with a long pregnancy must mate in a different season than one with a short pregnancy, even if both give birth in spring.
Long-distance migrants track green, food-rich seasons across continents.34PubMed Central. Resource tracking within and across continents in long-distance bird migrants Most long-distance migratory birds follow their preferred climate and habitat as seasons change.35Journal of Biogeography. Do long‐distance migratory birds track their niche through seasons? A migrating bird enjoys summer abundance in the north and escapes winter scarcity by flying to milder regions. The cost is a dangerous, energy-draining journey.
Seasons Shape Biodiversity and Human History
Seasonality shapes evolution through competing pressures between summer and winter.36PubMed Central. Understanding Evolutionary Impacts of Seasonality: An Introduction to the Symposium It also appears to be key to why tropical regions hold more species than high latitudes.37Paleobiology. Seasonality, the latitudinal gradient of diversity, and Eocene insects Where winters are harsh, species must tolerate or avoid the cold, which narrows the range of lifestyles that can succeed. Where conditions are steadier, species can specialize more finely. This is one explanation among several, and scientists see seasonality as a major factor rather than the only one.
There are three broad ways to cope with change. Some species keep varied genetic forms suited to different seasons. Some individuals flexibly change their bodies or behavior. Others spread risk, like seeds that sprout at different times. Animals sharing an area can also avoid competing by shifting their activity times with the seasons.38PubMed Central. Coexistence of predators in time: Effects of season and prey availability on species activity within a Mediterranean carnivore guild
Seasons helped launch agriculture as well. At the end of the last Ice Age, the Near East developed a strongly seasonal climate with very dry summers.39American Anthropologist. The Ecology of Seasonal Stress and the Origins of Agriculture in the Near East Wild cereals and legumes that complete their life in one season thrived.40American Anthropologist. The Ecology of Seasonal Stress and the Origins of Agriculture in the Near East People stored food for the long dry season and settled in one place.41American Anthropologist. The Ecology of Seasonal Stress and the Origins of Agriculture in the Near East Those conditions helped set the stage for farming.42American Anthropologist. The Ecology of Seasonal Stress and the Origins of Agriculture in the Near East
How Seasons Affect Human Health
Our bodies change with the seasons too. In Europe, human immune genes show a stronger inflammatory pattern in winter.43PubMed Central. Widespread seasonal gene expression reveals annual differences in human immunity and physiology Blood levels of inflammation markers such as C-reactive protein, which your blood test may list as CRP, are higher in winter.44PubMed Central. Seasonal and daytime variation in multiple immune parameters in humans: Evidence from 329,261 participants of the UK Biobank cohort This may help explain why heart disease peaks in winter.45PubMed Central. Seasonal and daytime variation in multiple immune parameters in humans: Evidence from 329,261 participants of the UK Biobank cohort Winter also brings peak rates of some psychiatric and autoimmune conditions.46PubMed Central. Widespread seasonal gene expression reveals annual differences in human immunity and physiology These are patterns across populations, so they can’t predict what will happen to any one person.
It also means a blood test taken in winter might show a somewhat higher marker like CRP than the same person’s result in summer. A doctor looks at the whole picture, including whether you have a current infection, rather than one number. If you compare results over time, remember when each sample was taken.
In winter, it helps to stay active even when days are short, get outdoor daylight when possible, and keep up vaccinations and routine checkups. If winter mood dips are severe, ask a doctor.
When Climate Change Shifts the Seasons
A change in one species’ seasonal timing affects the species it eats or is eaten by.47PubMed. Prey-predator phenological mismatch under climate change Species respond to warming at different rates, so ocean plankton communities are already falling out of step between food-chain levels.48PubMed. Impact of climate change on marine pelagic phenology and trophic mismatch Plentiful food can make up for a small timing gap, though.49PubMed. Timing and abundance as key mechanisms affecting trophic interactions in variable environments
A predator breeding on a fixed day-length schedule may have young to feed after its prey has peaked and declined. The effect can pass up and down a food chain, because a change at one level affects the species above and below it.
Day-length cues can become a trap. Day length doesn’t change with climate, but temperature and snow do, so animals following day length may act on a schedule that no longer fits. Species that use temperature cues may shift faster, widening gaps. Species that adjust flexibly, responding to temperature as well as day length, have a better chance of keeping pace. Those tied rigidly to a single cue can’t easily shift, even if they would benefit. Migrants face a particular problem, since they set off from places far away where they can’t sense conditions at the destination.
Camouflage and snow shelter are at risk. Snowshoe hares can’t change when their coats start to turn, so a white hare on bare ground is easy prey.50PubMed Central. Camouflage mismatch in seasonal coat color due to decreased snow duration They can only speed up the spring change to brown, not shift their fall timing.51PubMed Central. Camouflage mismatch in seasonal coat color due to decreased snow duration The protective snow layer is also shrinking as winters change.52Frontiers in Ecology and the Environment. The subnivium: a deteriorating seasonal refugium Colder snow layers and more freeze-thaw cycles threaten hibernating mammals, insects and amphibians.53PubMed Central. Winter Conditions and Land Cover Structure the Subnivium, A Seasonal Refuge beneath the Snow In the tropics, dry seasons are lengthening as forests demand more water.54PubMed Central. Rising ecosystem water demand exacerbates the lengthening of tropical dry seasons