Southern rockhopper penguins, Eudyptes chrysocome, mate for life, but these spiky birds also spend their entire winter seasons at sea without their mates. Despite being separated by hundreds of kilometers, males and females reunite with the same mate every year during breeding season, according to new findings published in Biology Letters this week. A pair spends less than a quarter of the year together.

Penguins of this serially monogamous species spend day and night with their partners during breeding season: that’s about 20 to 30 days for courtship and egg laying, followed by two to three days for incubation. They’re together only at night during chick rearing, which can last 70 days, and during the weeks they’re molting, they may or may not see each other at all. Then they migrate to their feeding habitats over the winter from April through October. We’re not sure if they maintain any contact with their mates during that time, or if go their separate ways to sex-specific niches. 

To investigate, a team led by Jean-Baptiste Thiebot from the National Institute of Polar Research tracked 10 rockhopper pairs from a colony on New Island, one of the Falkland Islands in the South Atlantic Ocean. The researchers mounted little 6 gram (0.2 ounces) geolocators on the penguins’ legs after they finished molting in March and April of 2012. 

The team managed to retrieve loggers from 16 birds when they returned to breed in October of that year. These included seven pairs from the previous breeding season that remained together for this new season. (Two of the females’ partners didn’t return, so they mated with different males.) To get additional information about what they were eating (and thus where the birds were) while they were away, the team also took blood samples to measure the ratios of carbon and nitrogen isotopes. 

During the winter, rockhopper partners were located 595 kilometers (370 miles) apart on average. One of the pairs was separated by 2,500 kilometers (1,550 miles) in June when the female moved to the Argentine Basin. Most dispersed over the Patagonian Shelf. 

What’s peculiar is how males and females from different pairs could very well be found in similar wintering habitats: the spatial distribution of both sexes largely overlapped based on the biochemical markers in their blood cells. Females distributed over a wider area and tended to head for warmer (more northerly) waters than their partners. But overall, they didn't head for male or female-only spots. Male and female penguins overlapped, even though partners didn't.

So couldn’t they have just stayed together? The researchers think it has to do with timing: females stayed at sea 12 days longer than males. They leave the colony six days earlier and return six days later. A different timing of migration onset between sexes, the authors write, may cause the partners to migrate independently while at sea – and to reunite afterwards at their nest only. Why they can’t leave and come back at the same time, however, remains unclear.