Group housing accelerates growth and induces obesity in adult hamsters

To determine whether variable body heat loss may influence control of growth and regulation of body fat content in hamsters, mature female hamsters were housed for 7-14 wk in one of three conditions: individually in metal cages (n = 6), individually in plastic boxes with bedding (n = 6), or communally (6 per box) in plastic boxes with bedding. Their resting metabolic rate (RMR) was measured individually or in pairs of two between days 65 and 75. When thermal properties of shelter varied alone, singly housed hamsters regulated energy balance fairly accurately by compensatory changes in food intake, and their rate of growth was unaffected. In contrast, group housing induced acceleration of growth and obesity without hyperphagia, was associated with an acute inhibition of RMR in hamsters tested in pairs, and was associated with a chronic inhibition of RMR in hamsters tested individually. We conclude that conspecific body contact in mature group-housed hamsters accelerates somatic growth and increases fat deposition. Energy for this anabolism is derived, in part, from reduced RMR and, in part, from a slight increase in food consumption.