Epigenetic aging and transposon dysregulation reflect size-related lifespan compression in dogs.

The extraordinary lifespan variation in domestic dogs provides a natural experiment for testing how intrinsic rates of biological aging shape lifespan. Using 1640 methylomes from 894 dogs, we developed an epigenetic clock that predicted mortality and demonstrated that epigenetic aging is fastest early in life. At orthologs of human age-associated genes, dogs exhibited concordant age effects on promoter methylation, highlighting conserved remodeling of immune pathways. We found that larger and male dogs, which are shorter lived, exhibit accelerated molecular aging. Distinct epigenetic architectures mediated these effects: Sex-dependent methylation changes were concentrated on the X chromosome, whereas size-associated methylation was especially pronounced at transposable elements (TEs). These findings show that epigenetics reflects lifespan differences in dogs and identifies TEs as potential mediators of size-associated lifespan.