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.