Estimating Narwhal ( <scp> <i>Monodon monoceros</i> </scp> ) Age Using Epigenetic Analysis of Skin

ABSTRACT Age is an important parameter to understand species life‐history characteristics. Recently, DNA methylation methods have emerged as innovative tools to estimate the age of marine mammals. Narwhal ( Monodon monoceros ) epigenetic age estimates were computed using beluga ( Delphinapterus leucas ), killer whale ( Orcinus orca ), bowhead ( Balaena mysticetus ), odontocete, cetacean, and mammalian epigenetic clocks and were compared with age estimates from counting growth layer groups in tusks ( n = 7) and/or aspartic acid racemization of eye lenses ( n = 25). Pearson's correlation coefficients ( R ) ranged 0.68–0.87 for the clocks, but the absolute accuracy of age estimates was quite low, with median absolute error (MAE) between epigenetic and reference age ranging 7–37.5 years. To address this lack of accuracy, we more broadly investigated DNA methylation levels at cytosine‐guanine sites (CpGs) for this same set of aged narwhals to develop a narwhal‐specific epigenetic clock. Using a penalized regression model on 1009 CpG sites, ten CpGs (plus intercept) were selected, resulting in a model with a R = 0.70 and MAE of 8.6 years. This is the first study to age narwhals using DNA methylation patterns and to develop a narwhal‐specific epigenetic aging clock, which will assist with understanding important life‐history characteristics for sustainable conservation management.