Dolphins from a prehistoric midden imply long-term philopatry of delphinids around Tokyo Bay.

This article investigates the long-term maternal site fidelity (philopatry) of delphinids around Tokyo Bay by analyzing mitochondrial DNA (mtDNA) from prehistoric dolphin remains excavated at the Shomyoji Midden, a Jomon-era shell mound dated 5000–3000 years before present. The study successfully sequenced the mtDNA D-loop region from skeletal remains of bottlenose dolphins (Tursiops aduncus and T. truncatus), Pacific white-sided dolphins (Lagenorhynchus obliquidens), and a false killer whale (Pseudorca crassidens), finding that several prehistoric haplotypes persist in modern populations nearby. While genetic diversity in Pacific white-sided dolphins appears stable over time, the modern bottlenose dolphin population near Tokyo Bay shows reduced diversity compared to prehistoric samples. These findings suggest that maternal groups of these delphinid species have inhabited the Tokyo Bay region continuously for at least 4000 years, representing one of the longest documented cases of cetacean philopatry.