Estivation is summer dormancy, that is, long-term torpor during summer for survival of hot and dry periods. Many desert plants survive extended periods of high temperature and low rainfall. Some survive as desiccated seeds (5-10% water content), particularly annual species, but some survive desiccation as adults. These 'resurrection' plants, such as the Rose of Jericho (Selaginella), can desiccate to about 5% water content during dry periods, but survive and 'come back to life' after rain. Pincushion lilies similarly re-activate by regenerating from buds after rain.
Among invertebrates (e.g., earthworms and insects) estivation usually involves an inactive stage with a
water-resistant covering. For example, estivating earthworms form a mucus cocoon to resist desiccation, and many insect pupae are remarkably resistant to water loss. Amongst vertebrates, fishes, amphibians, and reptiles enter a similar estivation state. Fishes and amphibians often form a cocoon of dried mucus (e.g., African lung-fishes) or shed epidermal layers (e.g., some desert frogs; Figure 3) to resist epidermal water loss; the cocoon covers the entire body surface except for the nostrils. Reptiles have a relatively water-impermeable epidermis and do not need to form a cocoon to reduce evaporative water loss. Estivating ectotherms typically have an intrinsic metabolic depression for energy conservation.
Some mammals also estivate (Table 1). For example, desert ground squirrels enter a long-term estivation state that is physiologically similar to hibernation except for the higher ambient temperature (Ta) and Tb. Other mammals such as cactus mice and kangaroo mice use single-day torpor cycles during summer.
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