Epigenetic aging of humpback whales in three oceans

Age is a key parameter in population ecology, but rarely known in whale populations. We used an epigenetic assay based on DNA methylation levels at three CpG sites to estimate the age of 1,013 humpback whales skin samples obtained from 1993 through 2014. The assay was calibrated with 76 individuals of known age (<1 to 30 years) and had an R2=0.718 (p=2.88e-22) and a standard deviation of 3.562 years. Samples of unknown exact age were obtained from one feeding ground (the Gulf of Maine, n=609), one extralimital wintering area off the southeast U.S. (n=33) and breeding grounds in the North Atlantic (West Indies, n=122), the North Pacific (Hawaii, n=41) and the South Pacific (American Samoa, n=115). The maximum estimated age at the time of sampling was 50.9 years. Two females, both from the Gulf of Maine, reached the maximum observed age of 64 years when later re-sighted (one with a dependent calf in 2016). Whales of both sexes in the Gulf of Maine were younger on average than in those in the West Indies, and whales sampled off the southeast U.S. coast in winter were younger than both North Atlantic feeding and breeding populations. Together, these results suggest a lower tendency of juveniles to undertake or complete migration to the breeding grounds. Age frequency differences among oceanic breeding populations were generally consistent with historical exploitation histories, but not clearly indicative of recovery. This is the first study of age in live humpback whales based on large sample sizes and multiple populations. Epigenetic aging has broad potential to advance understanding of whale biology and ecology and to facilitate conservation. Despite its average precision, we recommend caution when applying this assay to small sample sizes and individual case studies