The Effects of Crowding on the Survival of Meadow Voles (Microtus Pennsylvanicus) Deprived of Cover and Water

This study investigated the effect of crowding on the survival time and endocrine response of laboratory—reared meadow voles deprived of cover and water. Crowding was experimentally varied in terms of amount of social contact (number of individuals/group) rather than density (number of individuals/unit area). The crowded situation consisted of ten animals (five of each sex) in a pen of 15 ft², the ᵤncrowded situation of two animals (one of each sex) in a pen of 3 ft²–one mouse per 1.5 ftʷ in both situations. Survival times were recorded for 504 experimental mice. Supplementary data on adrenal weight, spleen weight, thymus weight, body weight, and the amount of wounding were obtained for 888 mice. In the animals provided with an adequate supply of cover, crowding induced hypertrophy of the spleen and of the female adrenal, but it had no adverse effect on wounding or survival. In the absence of cover, however, crowding precipitated much fighting and mortality. Twenty—seven per cent of the crowded animals deprived of cover succumbed whereas none of those with cover died. This mortality was greatest among the males. Crowding lowered the survival time of meadow voles deprived of water with males having a lower survival time than females. Prolongation of the crowding period decreased the survival time in males but not in females. The ill effects of crowding on survival time disappeared within a few days after animals were released from the crowded situation. Crowded animals autopsied after succumbing has slightly enlarged adrenals and greatly enlarged spleens compared with uncrowded animals. The thymus was not affected by crowding. The incidence of wounding in males was greater in the crowded animals. In all cases animal deprived of water showed adrenal hypertrophy, splenic and thymic atrophy, and a loss in body weight. Dominant males in the social hierarchies of stabilized groups showed longer survival times without water than subordinate males. They also showed greater gains in body weight. Adrenal weight, thymus weight, spleen weight, and the amount of wounding in males also varied in relation to social rank but appeared to be more closely related to the stability of a social hierarchy. The females showed very little agonistic interaction and did not organize into recognizable social hierarchies. The low frequency of social conflict and high survival time in females, and the high frequency of social conflict and low survival time in males, suggest that crowding does affect the capacity of an animal to survive in stressful situations. It can be concluded that under the conditions applying in this study crowding operates to regulate populations of voles by increasing the vulnerability of the animals to various deleterious factors. Conclusive evidence remains to be presented that crowding does, in fact, operate to regulate populations in nature.