Portland State University. Department of Psychology
Date of Award
Master of Science (M.S.) in Psychology
1 online resource (46 leaves : ill. ; 28 cm.)
Crowding stress, Mice -- Behavior
Previous research has found that if a population were allowed to exceed a comfortable density level, then many catastrophic events occurred such as increased mortality among the young, cannibalism, homosexuality, and lack of maternal functions. The most influential researcher in this area is Calhoun (1962), after whose experimental design a pilot study was fashioned to replicate his results. The results of this pilot study inspired a more detailed research project of which this thesis is an account. Forty-eight albino mice of the Swiss Webster strain were divided into three groups of sixteen each. Each group consisted of ten females and six males chosen randomly; two groups were to serve as experimental groups and the other group as the control. The experimental groups were placed into apparatus 15 5/8” x 20 1/2"x 8" and the control group in an apparatus 47 7/8" x 61 1/2" x 8". The three groups were allowed to multiply freely with nesting material, food and water provided proportionately as their numbers grew. The experimental groups were allowed to overpopuate while the control group was not. There were six behavior variables noted as the experiment proceeded: (1) grooming, (2) homosexuality, (3) nest building, (4) retrieving of young, (5) fighting, and (6) mortality of the young. It was predicted that grooming, nest building, and retrieving of the young would decrease in frequency as the population increased, while fighting, homosexuality and mortality of the young would increase with the rising population density. The experiment was conducted for six months and fourteen days. The result of this experiment was a total lack of overpopulation. The two experimental groups never weaned any pups though they produced many, and the control group grew to the comfortable limits of its apparatus and then ceased weaning any further pups. In an effort to ascertain the reasons for these results, one of the experimental groups was artificially reduced in number; whereupon it promptly weaned forty-one percent of its first litter, thirty percent of its second, and none of its third. At the time of its first weaning, this group was technically overpopulated. In conclusion a hypothesis is proposed to explain the results. It is felt that each population has an innate knowledge of its comfortable limits with regard to density and will maintain this crucial density level if necessary. The group's ability to control its popu1ation is directly related to a time factor in that if a population were allowed to approach its crucial density level gradually it would not exceed it; however if there were little or no approach time, then this level would be exceeded.
Hammock, James Robert, "Behavioral changes due to overpopulation in mice" (1971). Dissertations and Theses. Paper 1429.