Physiological responses to social status and housing conditions in male mice subject to food competition tests

Abstract

Seventy eight male mice were individually housed at weaning. After 14 days of isolation, the males were allocated to 13 permanent groups of three mice (Triads), 13 permanent groups of two mice (Couples) and 13 mice remaining in isolation for 21 days. Four 10‐min observation periods, each preceded by 24 hours of food deprivation, were carried out in a neutral cage. In one of the shorter walls of this cage a plastic tube was connected to the cage. A pellet of food was firmly attached to the inner surface of the tube and the behaviour of the mice was recorded on a videotape. Twenty‐four hours after finishing the last observation, the mice were killed and selected organs cleaned and weighed. Relative organ weight differences between dominant and subordinate males showed similar tendencies in both Triads and Couples. Dominants had heavier testes, thymuses, intact and squeezed seminal vesicles, squeezed preputial and prostates and lighter adrenals than subordinates. However, Triads and Couples showed differences of degree. While significant differences between dominant and subordinated mice were evident in Triads for testes, thymuses, intact and squeezed seminal vesicles, adrenals and prostates, in Couples, significance was achieved only for the thymus, adrenal and prostate. The number of attacks directed towards other mice (higher in Triads than Couples) thus seems to have a more important influence on relative organ weights than being attacked. Isolated males appeared partly stressed by isolation and/or food deprivation (showing a relative increase in adrenal and lower thymus, squeezed preputial and prostate weights when compared to dominants) but they were closer to the dominants of social groups in to having heavier testes than subordinated mice.