LETARGO. — This is a characteristic phenomenon of certain mammals (rodents, insectivores, chiropterans, carnivores, etc.), represented by the reduction, to a minimum, of physiological organic activities (respiratory, circulatory, etc.), so that the animal assumes a state of apparent deep sleep; l., although associated with sleep, is a phenomenon distinct from II. Other animals as well, such as, among the heterothermic vertebrates, amphibians, reptiles, and certain fish (e.g., dipnoans), display similar phenomena, which are more appropriately designated by the terms estivation or hibernation. Estivation is also displayed by invertebrates (e.g., earthworms, snails, and certain crustaceans), and hibernation by many insects.
Through l., estivation, and hibernation, animals overcome unfavorable environmental conditions in summer and winter, particularly those relating to temperature and, in summer, also to the lack of water; they are therefore annual cyclical phenomena. There is no example of l. among birds, because they overcome these conditions through migrations.
With l., metabolic and assimilative activity is suspended; the respiratory quotient, between emitted CO_{2} and absorbed O_{2}, is reduced to one third; body temperature (in poikilotherms) decreases considerably. At the end of l., body weight is found to have diminished substantially.
L., hibernation, and estivation are phenomena that may be associated with that of latent life, but in this field there are far more notable phenomena, involving encystment and the life of spores and plant seeds.
Encystment, characteristic of many invertebrates (rotifers, tardigrades, worms, etc.), is marked by extremely pronounced dehydration and by the formation of envelopes that may derive from the animal’s own integument; the internal anatomical condition is also profoundly modified. Under favorable conditions the animal takes up water again and, with the restoration of its organs, returns to active life. Latent life was studied by Spallanzani, who called it « minimum life ». Spores and seeds likewise exist in a state of dehydration and, if conditions are unfavorable, may preserve minimum life for a very long time.
The phenomena of hypnosis and catalepsy, which represent pathological conditions or conditions brought about by hypnotic procedures in neuropathic or hysterical individuals, should not be confused with l. (v. LETARGIA).