The gray whales cruising along California's coast during their annual fall migration are well known to science. But there is another small, mysterious group of related whales off the Russian coast—the western north Pacific gray whale population—that researchers are just now beginning to track.

Researchers have long believed that this critically endangered western group has remained isolated from their eastern Pacific counterparts. But new research is changing that view, and has documented a female in that population who completed the longest migration on record for a mammal. 

The nine-year-old whale, named Varvara, swam from Sakhalin island, Russia, to Cabo San Lucas, Baja California, and back beginning in November 2011. She logged 14,000 miles (22,511 kilometers) during her 172-day trip,