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Amygdalotomized, sham-operated and intact wild-derived house mice of both sexes were group-caged (8–10) or singly caged for 3 weeks. The weights of ovaries (p < 0.05) and uteri (p < 0.01) were less in intact and sham-operated grouped females compared to singly caged intact and amygdalotomized females. The weight of the reproductive organs in amygdalotomized grouped female mice was not different from singly caged controls. Body weights among amygdalotomized grouped females were heavier (p < 0.05) than singly caged controls. Seminal vesicle weights of intact and sham-operated grouped males were less (p < 0.05) than that of singly caged controls and grouped amygdalotomized males. Plasma LH values from the various groups coincided with the morphological data. These results support the hypothesis that increased population density impairs reproductive function in mammals. Perhaps the effects of decreased pituitary-gonadal function associated with overcrowding are mediated via amygdalar nuclei.

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