What does individual housing mean to a mouse?

Abstract

Some recent studies relating the effects of individual housing (isolation) and group housing to behavior, physiology and neurochemistry in laboratory rats and mice are reviewed and these accounts related to comparable information derived from experiments employing “social stresses” e.g. subjecting the animal to defeat. The data is discussed in relation to the problem of whether individual housing constitutes a “stress” (in terms of adrenocortical and adrenal medullary functioning) in these species, as it appears to do in primates. In spite of the large number of papers which ascribe the behavioral and endocrine changes obtained in isolation versus grouping comparisons to the effects of “the isolation-induced stress syndrome”, it is concluded that, in terms of adrenal function, there is little evidence that isolation per se constitutes a stress in rats and mice, although there is some evidence that adrenocortical reactivity is increased by housing animals in this manner. It should be noted that the wild progenitor of the laboratory strains of house mouse often appears to evidence territoriality. The view is advanced that the isolated condition in male mice may result in changes characteristic of territorial dominance. This may represent a mechanism for inducing social or territorial stability in this species. It appears that experiments involving physical isolation in laboratory rodents are unlikely to provide good models for the effects of “social deprivation” in man. It is thought that more studies employing measurement of hormone titers in biological samples, obtained as a result of non-stressful procedures, will lead to a clearer understanding of the effects seen in isolation versus grouping studies. Organ weight studies often appear to be very misleading, particularly in female rodents. It is also hoped that other studies will concern themselves with effects of isolation with respect to other social cues in rodents including odors and ultrasound.