| Considerable interest exists in the hypothesized trade-off between current reproduction and future survival; however, the mechanisms that mediate this trade-off have been elusive. One mechanism that can affect both reproduction and survival is the immune system. Activation of the immune system during reproduction will likely increase survival, but may use energetic resources that could have been directed towards reproduction. Similar arguments can be made for a trade-off between survival and development; mounting an immune response may increase survival of developing young, but may use resources necessary for growth. I present the results of four studies that address life-history trade-offs. For the first, I used captive Japanese Quail to test whether energy (as measured by CO2 production) directed towards an immune response varied between intact and castrated males. The second describes a pathogenic parasite of Florida Scrub-Jays that demonstrates that there are indeed possible risks to immune suppression. In the third, I investigated variation in immune response and condition of nestlings from both a resource-limited suburban environment and a natural wildland environment. The findings demonstrate that suburban nestlings reduce immune function to maintain body condition. This suggests that when resources are limited, it may be more important to maintain growth, which reduces predation risk, than the development of the immune system. The last study compared immune and stress parameters of breeder and non-breeder Florida Scrub-Jays, before breeding, and when birds are feeding dependent young which has been hypothesized to be an energetic bottleneck. I found that immune function, as measured by a bacterial killing assay, was suppressed while feeding young and that it also was lowest during a resource-poor year. Breeders that invested more in reproductive effort were in poorer body condition and were less likely to survive, a finding that supports the hypothesized trade-off between reproduction and survival. However, no evidence existed to support a role for the immune system in this trade-off because immune suppression during reproduction did not appear to increase the risk of death. |
