Application of DNA Methylation–Based Age Estimation to Construct an Age Structure of Humpback Whales in a Newly Emerged Wintering Ground Around Hachijojima Island, Tokyo Metropolis, Japan

Using a noninvasive DNA methylation (DNAm)-based age estimation method, we investigate the age structure of humpback whales that newly emerged around Hachijojima Island, Tokyo Metropolis, Japan to uncover the role of this area for this species. We measured DNAm frequencies at three age-related genes (<i>GRIA2</i>, <i>CDKN2A,</i> and <i>TET2</i>) from 26 biopsy skin samples of 21 unique humpback whales (15 males and 6 females) randomly collected in the winters of 2018-2021 and estimated their ages using the known age estimation model for humpback whales. The estimated ages of the 21 individuals were 2.95-30.40 years old with a mean of 12.02, and the resulting age structure in 5-year increments was roughly normally distributed with a peak at 10.00-14.99 class, suggesting the dominance of young adult males in this water. The observations (young males, male aggressive behavior for mating, whale song, and mother-calf pair) indicated that newly emerged humpback whales appeared to utilize Hachijojima Island as their new wintering ground, expanding the northern limit of the wintering area in the western North Pacific from previously known.