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

Abstract

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.

Exploratory personality is independent of telomere dynamics in a wild bird population

“…Alternatively, the association with age may reflect changes in other measures of intrinsic state with age, such as an increase in sex hormone titres (Boulton et al, 2015). For instance, testosterone levels often increase in early-life in passerines (Hau & Goymann, 2015;Těšický et al, 2022). However, it is not clear why exploration of a novel object shows a decline towards the end of an individual's life.…”

Guess who? Evaluating individual acoustic monitoring for males and females of the Tawny Pipit, a migratory passerine bird with a simple song

“…For example, increased testosterone levels promote singing in female European Robins (Kriner and Schwabl 1991;Schwabl 1992). The level of this hormone in females often fluctuates temporally (Ketterson et al 2005), and may differ substantially among individual females in various monogamous songbird species (e.g., Moreno et al 2014;Těšický et al 2022). Unfortunately, we lack any specific information about the status, age or physiological state of the females recorded in our study.…”

Nearly (?) sterile avian egg in a passerine bird

“…The sampling was conducted in a free-living great tit population breeding in artificial nest boxes in a deciduous forest at the edge of Prague, Czech Republic, EU (50˚08′12.4″N, 14˚27′57.2″E; see Těšický et al 2021 , 2022 for more details on the study site) during their breeding period in April and May 2018. In total, we collected 52 eggs to sample microbiome of the egg content (E0-egg), 118 eggs to sample E13 embryonic gut (unmanipulated, E13-nat, as well as Enterococcus -manipulated eggs, E13-Ent, see below), and 34 maternal female faecal samples (see Table S2 in Supporting Information 1 ( SI1 ) for number of samples in different categories).…”