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Why are more female than male Magellanic penguins stranded in South America every year?

  • Thousands of Magellanic penguins become stranded every year along the coast of South America, from northern Argentina all the way down to southern Brazil, and are unable to make it back to their breeding grounds in Patagonia 1,000 miles or more away.
  • Scientists have observed that the penguins that get left behind are three times as likely to be female as male. But, due to a dearth of data on the penguins’ migratory habits, it could not be determined why there was such a gender-based discrepancy to the strandings.
  • New research published this month in the journal Current Biology sheds new light on the situation, however, finding that, among other behavioral differences, female Magellanic penguins are likely to venture farther north than their male counterparts — and that by doing so, they’re more likely to run into the kinds of trouble that can leave them stranded.

Thousands of Magellanic penguins become stranded every year along the coast of South America, from northern Argentina all the way down to southern Brazil, and are unable to make it back to their breeding grounds in Patagonia 1,000 miles or more away.

Scientists have observed that the penguins that get left behind are three times as likely to be female as male. But, due to a dearth of data on the penguins’ migratory habits, it could not be determined why there was such a gender-based discrepancy to the strandings. Because a disproportionate number of females are being removed from the breeding population, this could have serious consequences for the viability of the population as a whole.

New research published this month in the journal Current Biology sheds new light on the situation, however, finding that, among other behavioral differences, female Magellanic penguins are likely to venture farther north than their male counterparts — and that by doing so, they’re more likely to run into the kinds of trouble that can leave them stranded.

Takashi Yamamoto of the Institute of Statistical Mathematics in Tokyo led a research team that used animal-borne data recorders to track the migratory and diving habits of 14 Magellanic penguins — eight males and six females — during their non-breeding period in 2017. The penguins spend most of their wintering season at sea, where they are at risk from a range of manmade threats.

“Anthropogenic threats have been considered to threaten wintering Magellanic penguins along the coasts of northern Argentina, Uruguay, and southern Brazil; these include water pollution caused by oil development and marine transport as well as fishery-associated hazards, such as bycatch and depletion of prey species,” Yamamoto said in a statement. “Our results suggest that the northward spatial expansion likely increases the probability to suffer these risks, and particularly so in females.”

A Magellanic penguin. Photo Credit: Takashi Yamamoto.

Yamamoto and team found that females not only tended to range farther north than males, but that they also don’t dive as deep during the winter.

“The Magellanic penguins finished breeding in late February, commenced their migration throughout April and returned to the breeding colony between mid-September and mid-October,” the researchers write. “During the wintering period, females predominantly utilized areas to the east of the Río de la Plata Estuary (<36°S), whereas males were mainly concentrated in areas between the north of the Golfo San Matías and Mar del Plata (ca. 38–41°S).”

On average, males were distributed at distances ranging from 268 to 1,023 kilometers from their Patagonian breeding colony, while females were at distances between 371 and 1,202 kilometers. Males also reached average depths of about 60 meters, whereas females were found to reach average depths of just 35 meters.

The researchers theorize that these behavioral differences might be related to the fact that the males are larger and weigh more than the females: thus females might ranger farther afield as a means of avoiding competition for food; they might have a harder time negotiating the northward-flowing Patagonian Current along the continental shelf; or they might simply prefer foraging in warmer waters at lower latitudes and shallower depths than males.

Despite the insights the study affords, there is still a pressing need for more data on the migratory patterns of Magellanic penguins. Yamamoto said that his team’s findings point to “the necessity of gaining a better understanding of the long-term spatial utilizations of species throughout their annual cycle, including any differences within a population, in order to facilitate dynamic and adaptive conservation practices.”

Noting that juvenile penguins have been observed to become stranded more often than adults, Yamamoto added that he and his colleagues would like to next track the movements of juveniles, from birth until they breed for the first time. “Information during this period is totally missing,” he said.

Magellanic penguins. Photo Credit: Takashi Yamamoto.

CITATION

• Yamamoto, T., Yoda, K., Blanco, G. S., & Quintana, F. (2018). Female-biased stranding in Magellanic penguins. Current Biology 29(1). doi:10.1016/j.cub.2018.11.023