Every year, thousands of Magellanic penguins get stranded along the coast of South America – but puzzlingly, about 75% of those that get stuck are female. Now scientists say they have worked out what is behind the gender imbalance: the females migrate further north than males.

Magellanic penguins finish breeding in Patagonia in February, and during the subsequent winter months head north, reaching as far as Brazil, in search of anchovies. But every year thousands become stranded, with many airlifted to safety onboard military aircraft.

Writing in the journal Current Biology, researchers in Japan and Argentina report how they attached tracking devices to eight male and six female penguins in 2017 and tracked where the birds went after they left the breeding grounds of Cabo dos Bahía in Argentina and began the migration north in April.

Previously it was unknown whether males and females took different paths or not.

“Although some assumptions are made, the exact reason for the female-biased stranding has been unknown due to the lack of information on their behaviour outside the breeding season, despite the growing focus on the conservation of this species,” said Takashi Yamamoto, a co-author of the report from the Institute of Statistical Mathematics in Japan.

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The results from the study offer a clearer picture. While the authors note that only a small number of penguins were tracked, the females generally travelled further north, with many reaching waters off the coast of Uruguay. The males, meanwhile, predominantly stuck to waters off the Argentinian coast. The study also found males typically dived deeper than females: about 59 metres compared with about 35 metres.

The authors offer a number of reasons why males and females may head to different areas – including to avoid competition for food – and why they dive to different depths – the females’ smaller bodies may restrict the depths they can reach.

They also suggest the different sexes may seek waters of different temperatures, with the lighter females preferring warmer, shallower surroundings. The females may be more susceptible to being swept along on sea currents, they add.

While researchers are still trying to understand why penguins end up stranded, Yamamoto and colleagues suggest those that end up further north may have used more energy swimming or catching dispersed prey, leaving them exhausted. Climate change, pollution in the area or injury from fishing equipment could also play a role. Reports of strandings have noted that many penguins are unwell or exhausted, with some also washing up dead.

“In addition, plastic remains have been reported in the stomach of beach-washed Magellanic penguins,” said Yamamoto, adding that a skewed sex ratio could potentially lead to a population decline.

While the findings may not help prevent strandings, Yamamoto said it could help with conservation of the species in the face of growing human-generated hazards. “If we do not consider any conservation action, such as marine zoning, I expect that the number of stranding individuals will increase.”