Rollings 3 & Mats olsson 1* telomeres, the protective, terminal parts of the chromosomes erode during cell division and as a result of oxidative damage by reactive oxygen species (RoS). ectotherms rely on the ambient temperature for maintaining temperature-dependent metabolic rate, regulated through behavioural thermoregulation. their temperature-dependant metabolism, hence also the RoS production, is indirectly regulated through thermoregulation. Consequently, a potential causal chain affecting telomere length and attrition is: temperature (in particular, its deviation from a species-specific optimum)-metabolism-ROS production-anti-oxidation-telomere erosion. We measured telomere length in sand lizards (Lacerta agilis) using qPCR on blood samples from 1998-2006. Effects of climatological parameters (mean temperature and average sunshine hours) in the summer and winter preceding telomere sampling were used as predictors of telomere length in mixed model analysis. During the lizards' active period (summer), there was a largely negative effect of mean temperature and sun on telomere length, whereas a combined measure of age and size (head length) was positively related to telomere length. During the inactive period of lizards (winter), the results were largely the opposite with a positive relationship between temperature and sunshine hours and telomere length. in all four cases, thermal and age effects on telomere length appeared to be non-linear in the two sexes and seasons, with complex response surface effects on telomere length from combined age and thermal effects. Telomeres are the terminal parts of the chromosomes with simple tandem repeats ending as a single-stranded t-loop 1. They keep the chromosomes intact, protecting them from degradation and end-to-end fusion 2. Telomeres shorten during replication and may reach an unstable, critical point, which induces cell arrest and apoptosis 3. Aerobic metabolism generates highly reactive and cell-damaging reactive oxygen species (ROS) 4,5 , to which G-rich telomeres are particularly sensitive 6,7. Thermal shifts restrain the oxygen delivery system and ROS production increases during heat stress, hence telomere erosion is elevated by a rise in temperature, at least in some taxa 8. The loss of telomeric bases is counterbalanced by telomerase, which elongates telomeres with its RNA template 2 , in species-specific-, chromosomal location-specific-, and chromosomal-and telomere length-specific manners (e.g. 9,10 ,). Recent years has seen an increased focus on telomere dynamics in natural populations with a focus on links to life history, reproduction, ageing trajectories and survival (e.g. 9-13 ,). Specifically, there is much interest in the relationship between environmental conditions and telomere dynamics and their long-and short-term effects in a range of taxa, but thus far ectotherms have been poorly represented in the literature 9. As ectotherms, reptiles rely on environmental conditions for maintaining their body temperature and regulate this via behavioural th...