This research is designed to identify the mechanisms that comprise the process of aging and to demonstrate its fitness consequences within a natural population of free-living vertebrates. Birds are noteworthy for having longer life spans than similar sized mammals. Tree swallows are the model system used because a large number of known-aged individuals can be repeatedly captured and sampled from year to year, and their reproductive success is easily measured across their life span. This research examines natural variation in physiological factors associated with aging in free-living birds. Longitudinal samples from individual tree swallows will be used to track oxidative stress, dysregulation of telomeres (chromosomal caps that protect coding DNA but shorten with cell division), and immune function decline. The extent to which individual variation in physiological mechanisms accounts for observed differences in survival and reproductive success will allow the researchers to identify the relative importance of these alternative pathways through which aging may occur. Samples from a cross-section of birds of different ages will identify how population averages for each physiological trait change with age, and comparison of the longitudinal and cross sectional samples will separate effects of individual aging from effects of selection between individuals with different trait values. The research will include an experiment designed to increase metabolic rates of individual swallows to test the hypothesis that oxidative metabolism and the attendant production of free radicals is causally important to the progression of aging and telomere shortening. It will provide laboratory and field training of high school, undergraduate, and graduate students as well as involvement by high school science teachers. This research will enable researchers interested in aging to link phenotypic trait values and risks of mortality, and identify evolutionarily important mechanisms of aging. In addition, this research will provide new tools to substantially strengthen the study of trade-offs in life history strategies.
This research was designed to determine the physiological mechanisms that underlie aging, the progressive decline in function, as individuals grow older, in free-living vertebrate animals. To do that, we recorded the survival and reproductive success of a large sample of individual tree swallows over a five-year period. From each individual, we collected annual blood samples and assessed immune function, total antioxidant capacity, oxidative stress, oxidative damage to DNA, and telomere length to determine how those traits are related to life span and reproductive success. We also tested the hypothesis that oxidative stress accelerates aging and decreases lifespan by equipping some birds with backpacks to increase their flight load, and hence metabolic expenditure by about 5% for an entire year. Tree swallows (Tachycineta bicolor) were chosen because they readily adopt nest boxes where they can be easily captured. They breed over a large area of the US and Canada and winter in the southern US, Mexico, and islands in the Gulf of Mexico, but individuals usually return to the same nesting sites. We sampled birds from populations at Ithaca, NY and Ames, IA, and followed 297 birds for up to 5 years after their first nesting attempt, plus 25 birds that were already 5 years old when the study began. Five years is almost twice the expected average life span for these populations, and our oldest birds (10 years) were close to the maximum recorded life span for tree swallows. Telomeres are repeated sequences of DNA at the ends of chromosomes. They protect gene-coding sequences within the chromosome against damage that would otherwise result from the normally incomplete copying of chromosomal DNA during each cell replication. Telomeres generally grow shorter with age because of that incomplete copying. They are also relatively vulnerable to oxidative damage, and oxidative stress increases shortening rate. Loading did not affect innate immune competence, total antioxidants or DNA damage other than telomere shortening. The telomeres of individuals loaded with backpacks, however, shortened about 5 times as much during that year as did telomeres of control birds of matching chronological age. The odds ratio of return during subsequent years was affected by both telomere length (longer telomeres were associated with higher survival) and rate of telomere shortening (greater shortening decreased survival). These data support the oxidative stress model for aging, and suggest that telomere length and its rate of shortening reflect past stresses and predict future survival. Measurement of telomere length and rates of shortening can therefore be used as powerful markers of biological age, even in the absence of information on calendar age. Telomere measurements provide an integrative index of individual quality and stress history, and predict future ramifications of experimental procedures or environmental changes. Our longitudinal analyses of the relationships between antioxidant status, free radical levels, unrepaired DNA damage, immune function, reproductive success and survival are still in progress, but data analyzed to date suggest that in surviving individuals, there is little change in these traits across years. The ability of birds to maintain those systems may help account for the observation that birds in general live longer than similar-sized mammals, even though birds have higher rates of metabolism and presumably of free radical production. Telomere length can be measured in several ways, some of which denature the DNA (break the bonds that link two strands of DNA in a double helix) before measurement. As part of our telomere work, we examined the effect of denaturing DNA on estimates of telomere length. Chromosomes include telomere-like sequences that are internal, and not part of telomeres. We found that such interstitial sequences can be more than 35% of the telomere-like DNA in some species, and vary between individuals. Using denatured DNA therefore results in overestimates of telomere length and reduces power to detect differences in length between groups or ages. The backpacks we used to load birds contained data loggers that colleagues in Canada used to determine migration patterns of tree swallows. In the autumn, tree swallows cross the Gulf of Mexico in a single flight of more than 850 km, but only on nights with favorable tailwinds. However, in the spring they must return to the US by flying over land to the west of the Gulf (three times further) because of persistent unfavorable winds. This grant provided resources to train 4 high school students, 12 undergraduates, 3 graduate students and 3 postdocs. The majority of these were women and nearly all have remained in science-related fields. A high school teacher was supported, and she used knowledge acquired under the grant to guide about 6 science fair projects, using resources in our lab. Numerous presentations at scientific meetings and to the general public have presented ideas summarized here. Eleven publications resulted from this research and several more are in preparation.
