Inter-annual repeatability and age-dependent changes in plasma testosterone levels in a longitudinally monitored free-living passerine bird.

While seasonal trends in testosterone levels are known from cross-cohort studies, data on testosterone inter-annual individual repeatability in wild birds are rare. Also, our understanding of hormonal age-dependent changes in testosterone levels is limited. We assessed plasma testosterone levels in 105 samples originating from 49 repeatedly captured free-living great tits (Parus major) sampled during the nesting to investigate their relative long-term repeatability and within-individual changes. Furthermore, we examined the inter-annual repeatability of condition-related traits (carotenoid- and melanin-based plumage ornamentation, ptilochronological feather growth rate, body mass, and haematological heterophil/lymphocyte ratio) and their relationships to testosterone levels. We show that testosterone levels are inter-annually repeatable in females, with a non-significant pattern in males, both in absolute values and individual ranks (indicating the maintenance of relative status in a population). In males, we found a quadratic dependence of testosterone levels on age, with a peak in midlife. In contrast, female testosterone levels showed no age-dependent trends. The inter-annual repeatability of condition-related traits ranged from zero to moderate and was mostly unrelated to plasma testosterone concentrations. However, males with elevated testosterone had significantly higher carotenoid-pigmented yellow plumage brightness, a trait presumably involved in mating. Showing inter-annual repeatability in testosterone levels, this research opens the way to further understanding the causes of variation in condition-related traits. Based on a longitudinal dataset, this study demonstrates that male plasma testosterone undergoes age-related changes that may regulate resource allocation. Our results thus suggest that, unlike females, male birds undergo hormonal senescence similar to mammals.

Longitudinal evidence for immunosenescence and inflammaging in free-living great tits.

The first-line effector mechanisms of immune defence, including inflammation and oxidative burst, contribute significantly to host-pathogen resistance. Whether these immune responses undergo age-related changes in birds remains unknown. Here, we tracked selected inflammatory parameters in 54 free-living great tits (Parus major) of known age, captured repeatedly over three consecutive years, with the aims to investigate long-term repeatability and age-dependent changes in cellular oxidative burst responsiveness upon in vitro stimulation with bacterial lipopolysaccharide (LPS), and to identify its relationships with leukotriene B4 (LTB4) levels and haematological traits. In addition, we linked these immunological traits to selected physiological markers (antioxidants and oxidative stress markers). LTB4 levels increased with age and we have shown a similar non-significant tendency also for absolute granulocyte counts, indicating propagating chronic inflammation over the bird's lifetime, consistent with the inflammaging hypothesis. In contrast, cellular oxidative burst followed a quadratic trend of dependency on age with a peak in midlife individuals, in line with the immunosenescence hypothesis. Interestingly, LTB4 levels were positively associated with general oxidative damage, but negatively with antioxidant glutathione peroxidase activity, indicating links to redox balance. This longitudinal study demonstrates the contrasting patterns of age-related changes in background and acute markers of pro-inflammatory immunity contributing to immunosenescence in birds and thus provides basis for interpretation of the tested inflammatory markers in cross-cohort datasets.