2.2 | Study area and individual identification survey
We observed Indo-Pacific bottlenose dolphins around Mikura Island, Japan (33º90´N, 139º60´E) which is a small (approximately 20.54 km2) oceanic island. Ongoing individual identification surveys have been conducted since 1994 using underwater video recordings taken within 300 m of the coastline, at depths ranging from 2-45 m. The sex of each individual was determined by observing the genital and mammary slits in both males and females and/or an erection in males (see Kogi et al ., 2004). The nursing females were determined if the mother and calf pair was frequently observed swimming in echelon, infant, or lactating positions (see Kogi et al ., 2004). Kakudaet al. (2002) identified the Tursiops sp. around Mikura Island as Tursiops aduncus . From 1994–2022, a total of 382 individuals have been identified. The birth year is known for 243 individuals. In this study, 45 age-known individuals were used. The actual age of each individual was calculated by subtracting their birth year from the year of sample collection.