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Energetic Costs and Consequences of Environmental Stressors in BirdsEPA Grant Number: U916011
Title: Energetic Costs and Consequences of Environmental Stressors in Birds
Investigators: Martin, Lynn B.
Institution: Princeton University
EPA Project Officer: Graham, Karen
Project Period: January 1, 2001 through January 1, 2004
Project Amount: $130,000
RFA: STAR Graduate Fellowships (2001) RFA Text | Recipients Lists
Research Category: Fellowship - Zoology , Academic Fellowships , Biology/Life Sciences
All organisms schedule their lives to maximize their lifetime reproductive success. Interestingly, however, no species is a "jack-of-all-trades." By focusing on certain reproductive strategies, such as having large litters or an early age of first reproduction, organisms must relinquish other life history options, such as long life span or large offspring size—this concept is known as a life history tradeoff. Although examples of these tradeoffs are well known in nature, the mechanisms underlying these tradeoffs are poorly understood. One potentially fruitful perspective for understanding these life history tradeoffs involves examinations of their physiological underpinnings. Because physiological systems serve as the interface between the genetic and environmental signals that eventually form the organism's phenotype, the life histories strategies of organisms may be understood as manifestations of the incompatibilities of different physiological states. Certain physiological strategies may optimize some life-history strategies, but this optimization is detrimental to other strategies. The objective of this research project was to test for the existence of such a physiology-life history nexus by examining variation in one important physiological system, the immune system, in two populations of a passerine bird (the House Sparrows, Passer domesticus) that exhibit distinct life-history strategies.
The energetic cost of one type of immune response was comparable to the cost of producing an egg in this species. Thus, birds, and probably all vertebrates, cannot use their immune systems without paying substantial energetic penalties. Strategies of allocation to immune function versus reproduction varied between populations of House Sparrows with different life histories. One population of House Sparrows, which lays small clutches over a long breeding season (slow life history), showed no seasonal variability in immune activity; another population, which lays large clutches in a short breeding season (fast life history), exhibited very weak immune activity when reproductive effort was at a maximum followed by an increase after cessation of breeding. To determine if these differences were ecologically plastic or fixed (genetic) traits, I performed a common garden experiment in which I held birds from both populations under standardized environmental conditions. After 5 months in captivity, all birds had lower responses than they showed in the wild at the same time of year. After 18 months in captivity, the relative differences detected in the wild returned.
I investigated the assumptions of an immunological assay that I used to characterize immune activity in the two populations. Surprisingly, I discovered that the reasons for the differences in immune activity between sparrow populations were because of differences in the immune cell populations between the groups—"slow" sparrows showed more adaptive immune activity than "fast" sparrows. This suggested that variation in the seasonality of immune activity in the two sparrow populations was because of the unique immunological architecture underlying the responses. To follow up on this finding, I investigated the specific differences in the architecture of the immune system between the sparrow populations using multiple immunological assays. I found that "slow" sparrows showed a much stronger reliance on antibody defenses and nonspecific innate defenses than the "fast" population.
Lastly, I performed a single study that integrated all of my previous findings. I investigated whether: (1) tradeoffs between one life history trait, feather regeneration, and immune activity occurred at all in House Sparrows; and (2) if so, was immunosuppression during molt more dramatic in "fast" birds because of a higher molt rate in that population? I found no difference in molt phenology between the two populations; this did not support my hypothesis that a more demanding life-history strategy would be more detrimental to immune function. However, I found evidence for tradeoffs between molt and immune function in both populations.