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Fuji-Hakone-Izu NP, Tokyo
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In 2012, 8-year-old “Hoppe-chan” cared for a calf after its mother died, staying with it for 102 days; researchers saw her nurse it—the first documented wild-cetacean case of foster nursing.
A dolphin’s belly speckles are a natural age clock: as they spread and grow denser, researchers can estimate age without touching it. At Mikurajima, the method estimated ages for 89 previously age-unknown dolphins.
That little calf beside an adult may have a babysitter, not its mother: Mikurajima researchers record young females accompanying calves and confirm mother–calf pairs only after repeated sightings.
A Mikurajima-linked study found fat-receptor genes and fat-breaking enzymes on the tongue of a young Indo-Pacific bottlenose dolphin. Researchers suggest this may help calves detect fatty acids in milk.
In 2019–20, researchers recognized 6 dolphins previously identified around Mikurajima near neighboring Toshima. At least 1 was later recorded back near Mikurajima: these island residents can travel between islands.
Mikurajima dolphins change companions over time rather than staying in fixed pods. Researchers compared 234 underwater observations with surface sightings to find a way to track groups as their members shift.
The swim has a “let the dolphin decide” rule: no touching, feeding or chasing, and no re-entry with a group that doesn’t approach. Swimmers keep clear of calves; scuba and noisy gear are also prohibited.
Since 1994, teams have identified Mikurajima dolphins from underwater video—not tags—using natural fin nicks and scars. A village-backed volunteer project began the work; the tourist association now leads monitoring.
Underwater footage captured pre-adult males, ages 1–10, in social sexual behavior, sometimes switching roles and even in mother–son pairs. Researchers suggest it may help with practice or bonding, but its function isn’t settled.
