Age determination is essential in ecological and conservation research, particularly for long-lived species, including cetaceans. Age data provide insights into life-history traits, including maturation, longevity, and population dynamics. In addition to determining age in years, estimating birth periods at a finer temporal resolution allows for the analyses of birth seasonality, early development of behavior, and mother-infant interactions. However, comprehensive age datasets for wild cetacean populations are scarce. This study presents a dataset of the age (in years) and estimated birth period of Indo-Pacific bottlenose dolphins (Tursiops aduncus) around Mikura Island, Japan. Age determination was based primarily on long-term individual identification surveys initiated in 1994. Two methods were used to estimate the age of individuals with unknown birth years: (1) speckle pattern analysis; and (2) epigenetic clock analysis using DNA methylation from fecal samples. Age data were available for 84.4% of all individuals identified in the Mikura population (n = 313), including 172 individuals with known birth years, 118 individuals with estimated ages based on speckle pattern analysis, and one individual whose age was estimated using epigenetic analysis. Birth periods were inferred from field observations recording the interval between the mother’s last solitary sighting and her first sighting with a newborn. Birth periods were determined for 120 neonates, with at least 75.7% of births occurring before July and 94.7% occurring before August. This dataset provides a valuable resource for studying life history, population dynamics, and conservation strategies of T. aduncus and offers a methodological framework for age estimation in cetacean populations.

Age is necessary information for the study of life history of wild animals. A general method to estimate the age of odontocetes is counting dental growth layer groups (GLGs). However, this method is highly invasive as it requires the capture and handling of individuals to collect their teeth. Recently, the development of DNA-based age estimation methods has been actively studied as an alternative to such invasive methods, of which many have used biopsy samples. However, if DNA-based age estimation can be developed from fecal samples, age estimation can be performed without touching or disrupting individuals, thus establishing an entirely non-invasive method. We developed an age estimation model using the methylation rate of two gene regions, GRIA2 and CDKN2A, measured through methylation-sensitive high-resolution melting (MS-HRM) from fecal samples of wild Indo-Pacific bottlenose dolphins (Tursiops aduncus). The age of individuals was known through conducting longitudinal individual identification surveys underwater. Methylation rates were quantified from 36 samples. Both gene regions showed a significant correlation between age and methylation rate. The age estimation model was constructed based on the methylation rates of both genes which achieved sufficient accuracy (after LOOCV: MAE = 5.08, R2 = 0.34) for the ecological studies of the Indo-Pacific bottlenose dolphins, with a lifespan of 40-50 years. This is the first study to report the use of non-invasive fecal samples to estimate the age of marine mammals.