Environmental enrichment enhances delayed-type hypersensitivity in both short- and long-day Siberian hamsters.

Darwinian fitness reflects trade-offs between reproduction and survival. Mechanisms have evolved in small nontropical mammals and birds to maximize reproductive output during the summer when thermoregulatory demands are relatively low and food is abundant and to shunt energy to processes that presumably increase the odds of survival during the winter when thermoregulatory demands are high and food is scarce. In order to predict the onset of winter, many seasonally-breeding mammals use day length (photoperiod) information. Seasonal adjustments of immune responses may be one mechanism to enhance survival; short days enhance cell-mediated immune function in seasonally-breeding rodents. The goal of the present study was to determine whether delayed-type hypersensitivity in hamsters is constrained or if photoperiod merely establishes a baseline level of immune response that can then be fined tuned by other environmental conditions. To test this, we used environmental enrichment, a manipulation that enhances many aspects of immune function. Hamsters were assigned to either long or short photoperiods and further assigned into either singly-housed or environmentally-enriched cages. After 10 weeks of concurrent photoperiod and housing treatment, delayed-type hypersensitivity (DTH) was induced. Although short days enhanced DTH responses compared with long days, environmental enrichment enhanced swelling responses in both short days and long days, suggesting that even during potential energetic bottlenecks or during maximal reproductive investment, hamsters can modulate their investment in immune function.