Species that display seasonal variation in sickness intensity show the most intense response in the season during which they have the highest body mass, suggesting that sickness intensity may be limited by an animal's energy stores. Siberian hamsters (Phodopus sungorus) display lower body masses and less intense sickness when housed in short, winter-like days as opposed to long, summer-like days. To determine if reduced sickness intensity displayed by short-day hamsters is a product of seasonal changes in body mass, we food-restricted long-day hamsters so that they exhibited body mass loss that mimicked the natural photoperiod-induced loss of body mass in short-day hamsters. We then experimentally induced sickness with lipopolysaccharide (LPS) and compared sickness responses among long-day food restricted and long- and short-day ad libitum fed groups, predicting that long-day restricted hamsters would show sickness responses comparable to short-day ad libitum hamsters and attenuated in comparison to long-day ad libitum hamsters. We found that long-day restricted hamsters showed attenuated LPS-induced anorexia, loss of body mass, and hypothermia compared to long-day ad libitum animals; however, anorexia remained elevated in long-day restricted animals as compared to short-day ad libitum animals. Additionally, LPS-induced anhedonia and decreases in nest building were not influenced by body mass. Results of hormone assays suggest that cortisol levels could play a role in the attenuation of sickness in long-day restricted hamsters, indicating that future research should target the roles of glucocorticoids and natural variation in energy stores in seasonal sickness variation.

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