Overwintering is the biological process by which non-migratory organisms survive periods of environmental hardship, most commonly the cold, dark, and resource-scarce conditions of winter. This survival strategy involves a state of reduced activity or dormancy that allows the organism to persist until favorable conditions return. For many species, enduring the winter is an alternate strategy to migrating to warmer climates. This process is key to understanding how temperate and polar ecosystems maintain their biodiversity.
The Definition and Scope of Overwintering
Overwintering describes the strategies an organism uses to pass through a period when normal activity or survival is challenging due to factors like low temperatures, limited food, or reduced light. This strategy is different from migration, which involves leaving a habitat to seek more hospitable conditions elsewhere. Overwintering applies to organisms that remain in their home ranges and persist through seasonal adversity, which may involve surviving intense dryness instead of extreme cold in some biomes.
A wide array of life forms engages in seasonal persistence, including insects, amphibians, reptiles, plants, and certain mammals. Many insects, such as the mourning cloak butterfly, overwinter as adults, while others survive in the egg stage. Among vertebrates, certain bats and small mammals like groundhogs are overwinterers, as are many reptiles and amphibians. Overwintering also occurs in plants, involving dormancy where above-ground growth ceases and energy reserves are stored to fuel regrowth in the spring.
Physiological Mechanisms for Winter Survival
The internal processes organisms use to survive winter dormancy are diverse, focusing on metabolic suppression and protection against cellular damage. Mammals and birds often employ torpor, a state characterized by a significant drop in metabolic rate, lowered heart rate, and reduced body temperature. True hibernators, such as groundhogs, can drop their core temperature from around 37 degrees Celsius to near freezing, sometimes as low as 0 degrees Celsius, which drastically conserves energy.
Cold-blooded animals, or ectotherms, like reptiles and amphibians, enter a similar state called brumation, where their metabolism slows substantially in response to ambient temperatures. During brumation, species must drastically slow their heart and respiratory rates, often while submerged underwater. This physiological adjustment allows them to maintain a prolonged period of inactivity without needing to eat or move.
Insects and some crustaceans rely on a hormonally regulated state of suspended development known as diapause. Diapause is triggered by environmental cues like shortening day length, which initiates a cascade of internal changes regardless of the immediate temperature. During this developmental arrest, insects significantly lower their metabolic rate and alter their internal chemistry to prepare for cold exposure.
A powerful defense against freezing is the production of cryoprotectants, which are molecules that prevent or control ice formation within the body. Freeze-avoiding species, such as the codling moth, accumulate polyols to depress the temperature at which their body fluids freeze. This process, called supercooling, is often a direct result of metabolic pathway alterations linked to diapause.
Other species, like the wood frog, are freeze-tolerant, meaning they can survive controlled ice formation in their extracellular spaces. These organisms flood their cells with glucose from the liver, which acts as a cryoprotectant. This protects internal cell structures from the damage caused by dehydration and freezing. This mechanism prevents destructive ice crystals from forming inside the cells, allowing the organism to survive with much of its body frozen solid.
Preparation and Habitat Selection
The success of overwintering depends on the preparation undertaken by the organism in the autumn months before the cold sets in. A primary preparatory step is the accumulation of substantial energy reserves, often through fat loading, which sustains the animal through its period of inactivity. For many insects, this phase also involves controlled dehydration of body tissues to enhance cold tolerance and lower the supercooling point of their body fluids.
Behavioral cues, such as shortening photoperiod and decreasing average daily temperatures, signal to organisms that it is time to seek appropriate shelter. The selection of a suitable microclimate is important, as the chosen location must provide a stable temperature that remains above the lethal freezing point or above the frost line. These specialized winter retreats are often referred to as hibernacula.
Many species burrow deep into the soil, utilizing the insulating properties of the ground where temperatures remain relatively constant and warmer than the surface air. Amphibians and reptiles, such as box turtles, dig into loose soil or leaf litter, while aquatic turtles may spend the winter submerged at the bottom of ponds. Insects frequently seek refuge under loose tree bark, beneath leaf litter, or deep within plant stems. The choice of shelter is important, and some species, like garter snakes, return to the same communal den year after year to survive the cold in large groups.