LETHARGY. - It is a phenomenon characteristic of certain mammals (rodents, insectivores, bats, carnivores, etc.), consisting in the reduction, to a minimum, of physiological organic activities (respiratory activity, circulatory activity, etc.), such as to bring the animal into a state of apparent deep sleep; lethargus, although associated with sleep, is a phenomenon distinct from it. Other animals too, such as, among the heterothermic vertebrates, amphibians, reptiles and certain fish (e.g., lungfish), exhibit similar phenomena that are better designated by the terms estivation or hibernation. Estivation is also exhibited by invertebrates (e.g., earthworms, snails, certain crustaceans) and hibernation by many insects.
By means of lethargus, estivation and hibernation, animals overcome unfavorable environmental conditions of summer and winter, especially those relating to temperature and, in summer, also to the lack of water; they are therefore annual cyclic phenomena. Among birds there is no example of lethargus because they overcome these conditions by migration.
During lethargus, metabolic and assimilative activity is suspended; the respiratory ratio between CO_{2} emitted and O_{2} taken in is reduced to as little as one third; body temperature (in poikilotherms) decreases considerably. At the end of lethargus the body weight is found to be considerably diminished.
Lethargus, hibernation and estivation are phenomena that can be associated with that of latent life, but in this field far more remarkable phenomena occur with the conditions of encystment and of the life of spores and seeds of plants.
Encystment, characteristic of many invertebrates (rotifers, tardigrades, worms, etc.), is characterized by a very considerable dehydration and by the formation of envelopes that may be derived from the animal's own integument; even the internal anatomical condition is profoundly modified. Under favorable conditions the animal reabsorbs water and, with the reconstitution of the organs, returns to active life. Latent life was studied by Spallanzani, who called it "minimal life." Spores and seeds are likewise in a condition of dehydration and, if conditions are not favorable, can maintain minimal life for a very long time.
The phenomena of hypnosis and catalepsy are not to be confused with lethargus; these represent pathological conditions or conditions induced by hypnotic procedures on neuropathic or hysterical individuals (v. LETARGIA).