If an animal can boost its immune system in response to shortening days, why would it not keep its immune system operating at that higher level all the time? Is there some physiological cost to maintaining a higher level of immune response?

Michael Lehner
Greenfield, N.H.

According to Staci D. Bilbo of Johns Hopkins University in Baltimore, it’s too “energetically expensive” for hamsters to maintain a high level of immune function. She adds, “During reproduction in the spring and summer, immune function may be compromised because (1) infection is less likely to be harmful due to less stressful conditions and (2) reproduction takes precedence during this time.” –S. Milius