Aging involves physical changes over time, but time is not the primary driver of these changes. Large-breed dogs age faster than small breed dogs, and there is great individual variation in the manifestations of aging. A key lesson we have learned is that chronological age and biological age are not identical. While we can measure chronological age easily, knowing the biological age of an individual is more useful in predicting and mitigating the health effects of aging.
The extent of DNA methylation can serve as a measure of both chronological and biological age. Epigenetic clocks are measurements of DNA methylation at multiple sites which correlate with chronological age. This may not seem very useful since we often know chronological age directly. However, epigenetic clocks also accurately predict future mortality even when other risk factors for death and disease are accounted for. In this way, they can measure biological age as well. Such epigenetic clocks may help us measure aging and predict health outcomes as well as assess the impact of anti-aging treatments.
Epigenetic clocks have been developed for dogs, and they have given us further insight into patterns of aging within the species. One study** developed a clock for dogs and grey wolves that correlates strongly with chronological age. This clock also demonstrates that age acceleration (the difference between chronological and biological age) is greater for larger breed dogs, again showing that these dogs age faster (Figure 1).
**Thompson, M. J., von Holdt, B., Horvath, S., & Pellegrini, M. (2017). An epigenetic aging clock for dogs and wolves. Aging, 9(3), 1055–1068. https://doi.org/10.18632/aging.101211