Elevated epigenetic age in wild compared to lab-raised house mice

Age is a key parameter in population ecology, with myriad biological processes changing with age as organisms develop early in life and later senesce. As age is often hard to accurately measure with non-lethal methods, epigenetic methods of age estimation (epigenetic clocks) have become a popular tool in animal ecology, and are often developed or calibrated using captive animals of known age. However, few studies have directly compared epigenetic age estimates between wild and captive or lab-reared animals of the same species, even though age-related epigenetic changes can be influenced by environmental conditions. Here, we built an epigenetic clock from standard laboratory house mice (C57BL/6, Mus musculus) and then used it to estimate age in a population of wild mice ( Mus musculus domesticus ) of unknown age. We show that this clock accurately predicts adult wild mice to be older than juveniles and that wild mice typically increase in epigenetic age over time, but with wide variation in epigenetic ageing rate among wild individuals. We further show that, for a given body mass, wild mice are epigenetically older than lab mice, and that this difference is not explained by accelerated ageing post-capture but is observed even among the smallest juvenile mice. This suggests different epigenetic age profiles in mice with contrasting genetic and environmental backgrounds arise very early in life and may be driven by peri- and postnatal effects on offspring DNA methylation.