Estivation
Estivation, also known as aestivation, is a state of dormancy that some animals enter during hot and dry conditions, characterized by significantly reduced metabolic activity. This physiological response leads to a substantial decrease in processes such as respiration and heart rate, enabling diverse species to survive harsh environmental conditions. Estivation is commonly observed in poikilothermic animals, including mollusks, arthropods, fish, amphibians, reptiles, and some small desert mammals. For instance, lungfish utilize this strategy by burrowing into mud and sealing themselves off until rains replenish their habitats. Similarly, certain frogs and toads create protective cocoons or dig backward into the soil to minimize moisture loss. While the phenomenon is often compared to hibernation, it is triggered by heat and drought rather than cold. Estivating animals may remain dormant for extended periods, sometimes years, waiting for favorable conditions to resume their life processes. Understanding estivation highlights the remarkable adaptations of various species to cope with climate extremes.
Estivation
Estivation is a dormancy state of low activity and metabolism that some animals experience in response to hot, dry weather. During true estivation, sometimes spelled aestivation, metabolic processes, including oxygen consumption, respiration, heart rate, and neurological activity, decrease substantially. However, the term is often used loosely to describe the activity of any animal that spends part of the warmer, drier season in a torpid state. Except for the environmental stimuli that trigger them, estivation and hibernation are similar, although they differ physiologically and occur most often in the poikilotherms. Among the animals that experience estivation are mollusks, arthropods, fish, amphibians, and reptiles, although some small desert mammals also experience estivation.
Invertebrate Estivation
A seemingly lifeless desert may be teeming with estivating life underground, waiting for seasonal rains that will awaken them to resume their life cycles. Snails, slugs, earthworms, insects, spiders, and nematodes, along with cocoons, eggs, grubs, larvae, and pupae, may all lie dormant in the soil, in building foundations and rock crevices, or under rotting logs or other vegetation. The animals are not just dormant; some of them will also be in an arrested state of sexual development called diapause. Some tropical snails can estivate for years at a time. To prepare, a snail digs a deep burrow in moist ground or under rocks. Next, it forms an epiphragm (a sealing membrane for the shell) to prevent evaporation and desiccation. Finally, its metabolism slows dramatically until it detects favorable environmental cues such as increased moisture.
Lungfish Estivation
Among the fishes, the process of estivation is best known in the air-breathing lungfishes of Africa (genus Protopterus) and South America (genre Lepidosiren and Neoceratodus). Adult fish have paired lungs and vestigial gills. In fact, they will drown if held underwater. Both abilities—to estivate and breathe air—have contributed to their survival in areas that experience severe seasonal droughts. When the rivers, lakes, and marshes they inhabit dry up, lungfish dig burrows deep in the mud, leaving air passages to the surface. They curl up inside and wait out the arid conditions until rain fills up their burrows with water. Protopterus secretes a mucous coating that hardens, forming a tough, cocoonlike hygroscopic chamber with only one opening connected to its mouth. The lungfish can survive like this for months, but some have been documented in this state for up to four years. When the rains come and flood the passage, the dormant fish awakens with a cough. Lepidosiren burrows more deeply than Protopterus and plugs the entrance to its air tube with perforated clay. Its burrow is somewhat larger and usually contains some water. It coats the walls with a jellylike substance to maintain moisture. During estivation, lungfish are so torpid that they make easy prey for local fisherman, who spear them in their burrows. African clawed frogs (Xenopuslaevis) undergo a similar estivation strategy.
During estivation, energy required for reduced metabolism is provided by the breakdown of tissue protein, such as the muscles in their tail. The waste product is urea, which is excreted in large amounts once the fish is again submerged in water. Lack of oxygen can become a problem for lungfish sealed in their burrows, but the aerobic metabolism of an estivating Protopterus is only 20 percent of its normal resting metabolism.
Estivation in Reptiles and Amphibians
During the summer, many reptiles and amphibians estivate. Some frogs and toads insulate themselves in cocoons composed of many layers of unshed skin. The eastern spadefoot toad digs backward into sandy soil with its spade-shaped hind feet to estivate for weeks; experiments indicate that the sound of rain falling rather than moisture itself may trigger the toad’s arousal. The mud turtle (genus Kinosternon) abandons its drying pond for a burrow, where it estivates for up to three months. Desert tortoises of the North American Southwest spend much of their summers estivating in burrows but emerge to drink and browse after infrequent thunderstorms. In the same region, the lizard called the chuckwalla stops eating and estivates in rock crevices, emerging every third day for about an hour at sunset. Other reptiles and amphibians that enter a state of estivation include alligators, crocodiles, frogs, toads, and salamanders.
Estivation in Mammals and Birds
Among the few estivating mammals are small desert rodents, lemurs, and hedgehogs. Some ground squirrels remain dormant in their burrows from late summer, merging estivation with hibernation. They reappear in early spring to take advantage of new growth after winter rains. The round-tailed ground squirrel of southern Arizona (Xerospermophilus tereticaudus) avoids the hot, dry autumn by disappearing in August and September. Generally, the period of estivation coincides with the period most prone to scarcity of vegetation. The painted wooly bat (Kerivoula picta) enters a state of estivation during the day.
While a few birds may hibernate, it is unlikely that they estivate. They can migrate to more attractive regions, or, like many reptiles and most mammals, they cope with high temperatures by confining their activities to cooler parts of the day.
Among other estivating animals are particular species of moths, mosquitoes, ants, soldier beetles, ladybugs, earthworms, and weevils, as well as Australia's tropical freshwater crab (Austrothelphusa transversa) and yellowmargined leaf beetles (Microthecaochroloma).
Principal Terms
Epiphragm: Covering or sealing membrane
Hygroscopic: Able to retain moisture
Nematode: A long, cylindrical worm; some are parasitic
Poikilotherm: Cold-blooded or ectothermic; any organism having a body temperature that varies with its surroundings; in general, reptiles, amphibians, fish, and invertebrates
Torpid: Dormant, numb, sluggish in action
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