Independent avian epigenetic clocks for aging and development

Information on individual age is a fundamental aspect in many ecological
and evolutionary studies. However, accurate and non-lethal methods that
can be applied to estimate the age of wild animals are often absent.
Furthermore, since the process of aging is accompanied by a physical
decline and the deterioration of biological functions, the biological age
often deviates from the chronological age. Epigenetic marks are widely
suggested to be associated with this age-related physical decline, and
especially changes in DNA methylation are suggested to be reliable
age-predictive biomarkers. Here, we developed separate epigenetic clocks
for aging and development in a small passerine bird, the great tit (Parus
major). The aging clock was constructed and evaluated using erythrocyte
DNA methylation data of 122 post-fledging individuals, and the
developmental clock using 67 pre-fledging individuals from a wild
population. Using a leave-one-out cross validation approach, we were able
to accurately predict the ages of individuals with median absolute
deviations of 0.40 years for the aging and 1.06 days for the development
clock. Moreover, using existing data from a brood-size manipulation we
show that nestlings from reduced broods are estimated to be biologically
older compared to control nestlings, while they are expected to have
higher fitness. These epigenetic clocks provide further evidence that, as
observed in mammals, changes in DNA methylation of certain CpG sites are
highly correlated with chronological age in birds and this opens up new
avenues for broad applications in behavioural and evolutionary ecology.