The Journey of Phyllis the Elephant Seal — The Deep-Diving Record-Setter of Año Nuevo

A journey measured in depth, not just distance

Nicole the shark, Adelita the turtle, the record humpback and E7 the godwit all crossed the surface of the ocean. Phyllis did something none of them did: she lived her whole journey vertically.

A northern elephant seal is one of the deepest-diving animals on Earth. Adult females dive almost continuously, day and night, mostly to 400–700 metres — and sometimes past 1,000 metres, more than a mile down — staying underwater 83–90% of the time, surfacing for only a few minutes between dives. They keep this up for the entire months-long migration. While the other animals here travel across the top of the sea, Phyllis spent her whole journey commuting up and down the dark water column beneath it.

Out from California, past the dateline

In spring 2016, after giving birth, nursing her pup and moulting on the beach at Año Nuevo Reserve in California, Phyllis — an eight-year-old, 1,200-pound female named after the late philanthropist Phyllis Sooy — slipped back into the sea for her long post-moult foraging migration.

Female elephant seals from Año Nuevo usually head out into the open North Pacific and forage along a rich boundary zone where two great rotating currents meet. Phyllis went farther than almost any tracked female of her colony: out across the open ocean, across the International Date Line, and almost to Japan before turning for home. By the time she landed, her loop measured about 7,400 miles (some 11,900 km) — the longest of any elephant seal tracked from Año Nuevo in over two decades, and perhaps closer to 10,000 miles counting all her diving.

The silence, and the homecoming

Then, on New Year's Eve 2016, about 683 miles from home, her satellite tag went quiet. Its battery had died. Because roughly one in seven tagged elephant seals is lost at sea, her researchers feared the worst.

For two weeks they heard nothing. Then, on 14 January 2017, an observer spotted Phyllis resting in a rocky cove at Año Nuevo State Park — alive after all. Five days later she gave birth to a healthy pup. The professor who runs the program simply called her "an amazing seal."

Because her tag had died 683 miles out, that last stretch home was never recorded. We know she made it only because she came back to the beach — which is exactly why the map draws it differently.

Why Phyllis is drawn honestly

Phyllis's track shows the layer's honesty rule at work:

• The solid line is the loop her satellite tag actually logged — out across the Pacific, past the dateline, and back to the point where the tag fell silent.
• The dashed line is only the final ~1,100 km home. No instrument recorded it; we know it happened solely because she was sighted on the beach. So it is drawn as inferred, never as a measured path.

That is the same rule that draws Nicole's photo-ID return dashed and the never-tagged humpback's whole path dashed: a solid line means a tag truly recorded this, and a dashed line means we know it happened, but no one tracked the route between.

What she adds to the cast

Phyllis brings a whole new behaviour to the fleet — the vertical ocean. She is the deep-diver: where the others measure their journeys in horizontal distance, an elephant seal's life is also measured in the hundreds of metres it descends, over and over, into the dark. Her story is a reminder that even the best tags tell only part of the tale, and that much of what these animals do happens far below where any map can follow.

See it on the map

Switch on the Animal Journeys layer on the Ocean canvas to follow Phyllis yourself. The chevrons show her direction; the solid line is the loop her tag recorded, the dashed leg is her unrecorded path home, and each glowing waypoint opens the story of that stage — from the day she left Año Nuevo to the morning she reappeared on the beach with a new pup. She joins Nicole the great white, Adelita the loggerhead turtle, the record humpback whale and E7 the godwit as part of a small, growing fleet of famous tagged animals, each one real, named, and drawn from published, citable science.

Sources: UC Santa Cruz / Daniel Costa Lab elephant-seal program, Año Nuevo (Rachel Holser, Daniel Costa), as reported by UC Santa Cruz News, "The mighty Phyllis returns after record-shattering swim" (8 February 2017). Diving behaviour: Le Boeuf, B. J., Naito, Y., Asaga, T., Crocker, D., & Costa, D. P., "Prolonged, continuous, deep diving by northern elephant seals," Canadian Journal of Zoology 67:2514–2519 (1989).