Age estimation based on blood DNA methylation levels in brown bears

Age is an essential trait for understanding the ecology and management of
wildlife. A conventional method of estimating age in wild animals is
counting annuli formed in the cementum of teeth. This method has been used
in bears despite some disadvantages, such as high invasiveness and the
requirement for experienced observers. In this study, we established a
novel age estimation method based on DNA methylation levels using blood
collected from 49 brown bears of known ages living in both
captivity and the wild. We performed bisulfite pyrosequencing and obtained
methylation levels at 39 cytosine-phosphate-guanine (CpG) sites adjacent
to 12 genes. The methylation levels of CpGs adjacent to four genes showed
a significant correlation with age. The best model was based on DNA
methylation levels at just four CpG sites adjacent to a single
gene, SLC12A5, and it had high accuracy with a mean absolute
error of 1.3 years and median absolute error of 1.0 year after
leave-one-out cross-validation. This model represents the first epigenetic
method of age estimation in brown bears, which provides benefits over
tooth-based methods, including high accuracy, less invasiveness, and a
simple procedure. Our model has the potential for application to other
bear species, which will greatly improve ecological research,
conservation, and management.