Mating, Sociosexual, and Nursing Behavior of Wild East Asian Finless Porpoises in the Seto Inland Sea, Japan

ABSTRACT The mating and sociosexual (nonconceptive) behaviors of narrow‐ridged finless porpoises ( Neophocaena asiaeorientalis ) have been extensively studied in captivity. However, such behaviors may differ significantly from those that have developed in wild populations. Drone videos captured the behaviors of wild finless porpoises including a total of 60 copulatory attempts and 76 sociosexual and 103 nursing behavioral events from April 2022 to December 2023 during boat and land‐based surveys in the northern Aki Nada region of the Seto Inland Sea, Japan. The majority of copulatory attempts were initiated by adults in groups (61.7%). Receivers evaded 65% of copulatory attempts, while 35% were accepted. Sociosexual behaviors observed included body‐to‐body rubbing, copulation attempts on nonadults, and homosexual interactions. Most sociosexual events involved the receivers accepting the actions rather than evading them. Despite occasional observations of independently swimming and solitary calves, mother–calf pairs were seen predominantly in the infant position (51.4% of the time), suggesting extensive maternal care in wild conditions. These observations illustrate the variability of sexual behaviors in wild finless porpoises, with most being solitary except for reproduction and individuals living in groups.