Researchers who studied Magellanic penguins for 30 years off the coast of Argentina, using satellite tracking, found that at least one couple remained loyal to each other during a 16-year period.
The long-term devotion endured despite each going its separate way during winter feeding trips that took them thousands of miles apart.
“It is unbelievable how far Magellanic penguins swim — and each breeding season they come back to the same nest and to the same partner,” said Pablo Garcia Borboroglu of the National Research Council of Argentina.
It was earlier thought that penguin bonds lasted between five and 10 years, and were often cut short by the unexpected death of one of the partners during migration.
Magellanic penguins live only around the Falkland Islands and southern South America, with Argentina having the highest population of 900,000 breeding pairs.
Another 800,000 couples are believed to live along the coast of Chile.
But their numbers have plummeted over the past two decades from what environmentalists say is oil pollution, climate change and dwindling numbers of the fish on which they feed.