Pathological togetherness: the animal evidence.
Animals studies relating to high population density are reviewed. While these experiments cannot be extrapolated to human populations, it is impossible to ignore the similarities to some human findings such as poverty, poor physical and mental health, and high infant mortality. Fertility, mortality, morbidity, maturation rates, and endocrine activity have all been shown to be sensitive to crowding. Population flux is normal, but too great a crowding foolowed by an extremely small population leads to local extinction. Overexploitation of habitation leads to wastage of young and decreases the efficiency of the utilization of the resources. Crowding takes the greatest toll among the embryos and infants due to failure in lactation, neglect of infants, and increased maternal mortality and difficult labor. Only under severe conditions, such as the Sika deer on Jamestown Island in Maryland, is the mortality of adults significantly affected. These animals showed signfiicant decrease in adrenal weight, seemingly the result of crowding stress. There was no evidence of starvation. Dispersal of subordinate animals is a normal reaction to crowding. Under severe crowding, fighting, catatonic withdrawal, aberrations in sexual and maternal behavior, and syndromes of stress and psyiological breakdown occur. More recent experiments indicate that stress induced by crowding affects production of adrenocorticotropin which can interfere with sexual development, pregnancy, lactation, and the disease-resistant mechanism. Thes complex feedback mechanisms also affect courtship behavior and the desire of the mother to care for her young. In severly crowded rat colonies infant mortality reached as high as 96%.