From climate warming to accelerated cellular ageing: an experimental study in wild birds

Climate change is increasing both the average ambient temperature and the frequency and severity of heat waves. While direct mortality induced by heat waves is increasingly reported, sub-lethal effects are also likely to impact wild populations. We hypothesized that accelerated ageing could be a cost of being exposed to higher ambient temperature, especially in early-life when thermoregulatory capacities are not fully developed. We tested this hypothesis in wild great tits (Parus major) by experimentally increasing nest box temperature by ca. 2{degrees}C during postnatal growth and measuring telomere length, a biomarker of cellular ageing predictive of survival prospects in many bird species. While increasing early-life temperature had no detectable effect on growth or survival to fledging, it accelerated telomere shortening, and although non-significantly, reduced medium-term survival from 34% to 19%. Heat-induced telomere shortening was not explained by oxidative stress, but more likely by an increase in energy demand (i.e. higher thyroid hormones levels, increased expression of glucocorticoid receptor, increased mitochondrial density) leading to a reduction in telomere maintenance mechanisms (i.e. non-significant decrease in the gene expression of telomerase and protective shelterin). Our results thus suggest that climate warming can affect rate of ageing in wild birds, with potential impact on population dynamics and persistence.