Food supplements modulate changes in leucocyte numbers in breeding male ground squirrels

Immunosuppression may be an important cost of reproduction in breeding males. It can result from elevated levels of testosterone or stress hormones and may serve to lower the energetic cost of maintaining immune function at a time of high demand. This suggests that greater access to energy resources could reduce immunosuppression as a cost of reproduction, minimizing the trade-off between energetic investment in current reproductive effort and survival. I examined the impact of food availability on immune function by provisioning male Belding's ground squirrels in the field from the time they emerged from hibernation to the start of breeding. Temporal changes in immune status, measured by leucocyte counts, differed between provisioned males and un-provisioned controls. Provisioning advanced the increase in lymphocytes and neutrophils from after breeding to before. At the start of breeding, the leucocyte count was three times greater in provisioned males than in controls and was still nearly twice as great at the end of breeding. Control males increased all leucocyte numbers after breeding. This experiment demonstrates that variation in food intake can lead to individual variation in the extent of immunosuppression during breeding and therefore that reduced immune function may not be an obligatory cost of reproduction.