Dormancy

Dormancy is a temporary halt in an organism’s growth, development, and, for animals, physical activity. By slowing metabolism, it helps conserve energy. This state is closely tied to environmental conditions, and organisms can time their entry into dormancy using either predictive or consequential methods.

Predictive dormancy happens before harsh conditions arrive. For instance, plants use changes in day length and falling temperatures to anticipate winter. Consequential dormancy occurs after adverse conditions have already begun, which is common in unpredictable climates. While sudden environmental shifts can cause high mortality for animals relying on this strategy, it allows them to stay active longer and make better use of available resources.

**Animals**

**Hibernation** Many mammals use hibernation to cut energy use and survive winter food shortages. It can be either predictive or consequential. Animals prepare by building a thick layer of body fat in late summer and autumn to fuel the dormant period. During hibernation, heart rate can drop by as much as 95%, and body temperature falls. Some hibernators also generate heat through non-shivering thermogenesis—a process in brown adipose tissue where the mitochondrial proton gradient produces heat instead of ATP—to avoid freezing. Hibernating animals include bats, ground squirrels and other rodents, mouse lemurs, European hedgehogs and other insectivores, monotremes, and marsupials. Though almost exclusive to mammals, a few birds, like the common poorwill, also hibernate.

**Diapause** Diapause is a predictive strategy set by an animal’s genetics. It is common in insects, allowing them to pause development between autumn and spring, and in mammals like the roe deer—the only ungulate with embryonic diapause—where a delay in embryo attachment ensures offspring are born in favorable spring conditions.

**Aestivation** Aestivation (also spelled estivation) is a consequential dormancy during a dry season, typically summer, triggered by food or water shortages. This often, but not always, means hot weather. The behavior is ancient: fossilized aestivation burrows of lungfish appear in rocks from the Devonian to Cretaceous periods, and amphibian burrows from the Permian. Aestivation occurs across a wide range of animals. Among arthropods are beetles like ladybirds, flies like mosquitoes, lepidopterans like bogong moths, and crustaceans such as the inland freshwater crab *Austrothelphusa transversa*. Among vertebrates, it is most common in reptiles and amphibians, including the desert tortoise, spotted turtle, California tiger salamander, and California red-legged frog. Modern lungfish, like the African lungfish, aestivate just as their ancestors did.

**Brumation** Endotherms and other heterotherms are described as hibernating, but the dormancy of ectotherms like lizards in cold conditions is different. The term brumation was coined in the 1920s for this process. It differs from hibernation in that energy is stored in glycogen instead of or alongside fats, and periodic water intake is required.

**Plants** In plant physiology, dormancy is a period of arrested growth. It is a survival strategy that helps many plant species endure harsh conditions or climates where part of the year—such as winter or a dry season—is unsuitable for growth. Many dormant plants have a biological clock that signals when to slow activity and prepare soft tissues for freezing or drought. Dormancy can also be triggered after a normal growing season by falling temperatures, shorter days, or reduced rainfall. Chemical treatments can effectively break dormancy, especially in woody plants like grapes, berries, apples, peaches, and kiwis. Hydrogen cyanamide stimulates cell division and growth in dormant plants, causing buds to break when the plant is near the end of dormancy. Slight cell injury may play a role, likely by increasing membrane permeability. This injury is linked to the inhibition of catalase, which stimulates the pentose phosphate cycle. Hydrogen cyanamide also interacts with the cytokinin metabolic cycle, triggering a new growth cycle.

**Seeds** When a mature, viable seed fails to germinate under favorable conditions, it is dormant. This is called embryo dormancy or internal dormancy, caused by the embryo’s own characteristics that prevent germination. This should not be confused with seed coat dormancy, external dormancy, or hardheadedness, which results from a hard seed coat that blocks water and oxygen from reaching the embryo—a physical barrier.