The evolutionary and ecological drivers of senescence in two wild passerine systems

<p dir="ltr">Senescence, defined as a decline in the body’s function with age leading to death, is a widespread phenomenon across virtually all living organisms. Despite this, key questions around senescence remain: Why is there variation in senescence across individuals and how did it evolve? What environmental factors mediate senescence? More specifically, how do increased anthropogenic stressors link with both senescence and general performance in wild animals?</p><p dir="ltr">In my PhD project, I examined the ultimate (evolutionary) and proximate (ecological) drivers of senescence in two wild house sparrow (<i>Passer domesticus</i>) systems, using telomere dynamics as a biomarker of senescence. Telomeres are protective DNA sequences capping the ends of chromosomes, and they progressively shorten with age and oxidative stress. Shortened telomeres have been linked with cellular and organismal senescence, and therefore serve as an important measure bridging physiology and body performance.</p><p dir="ltr">To study the evolution of telomere dynamics, I first confirmed that telomere length has a genetic component across vertebrates, via a meta-analysis on the heritability of telomere length. Next, using a long-term sparrow monitoring project on Lundy Island, UK, I found that telomere length had evolutionary potential, and that patterns of genetic variation in telomere length increased with age, congruent with evolutionary theories of senescence. Using the Lundy sparrow population, I also found links between telomere dynamics and fitness components, namely that longer telomeres were linked with higher survival and lifetime reproductive success, signifying selection. Moving to proximate drivers, I then investigated the links between telomere dynamics and heavy metal pollution using the Broken Hill house sparrow population, where individuals are under chronic exposure to lead (Pb) due to mining activities. I found that shorter telomeres were associated with higher average blood Pb content, but the effect is weak, potentially due to local adaptation. I also further looked at the effects of Pb pollution on antipredator behaviours, which is key to mortality, and found Pb was associated with heightened wariness but diminished escape performance. Overall, my findings have provided support for evolutionary theories of senescence in the wild, thereby aided in our understanding on the evolutionary mechanism of senescence, and how senescence in wild populations might be impacted under increasing human influences in the Anthropocene. I encourage future research to investigate specific selection patterns, as well as addressing behaviours as an overlooked potential mediator of senescence.</p>