A colony of penguins at Punta Tombo, Argentina.

Researchers from Japan and Argentina think they have solved a strange penguin mystery. For years, thousands of magelha penguins have been washed up on the beaches of northern Argentina, Uruguay and southern Brazil. Striking: three-quarters of the dead washed-up penguins are female. Their swimming route explains the difference with the mortality rate of males.

By placing a device on the legs of the animals, the researchers mapped out the swimming routes and swimming depths of individual penguins in detail. The penguins breed in the south, in Argentina, Chile and the Falkland Islands, but swim to the north every year for food. The females, according to the research in Current Biology, seek their food in the north and closer to the coast than the males. This northern area – near Rio de La Plata, the sea inlet of Buenos Aires – Is dangerous for the animals. They encounter there earlier on polluted water, shipping and fishing. The researchers think that the penguins catch less fish, get entangled in nets or accidentally get caught.

On the list of the IUCN the penguins are listed as almost threatened, the number of magelheng penguins has been running for years. The researchers are particularly worried about the fact that female in particular are lashing to death on the beaches, because the male/female ratio in the population can become oblique. Magellanic penguins breed in pairs, so for their survival a healthy balance between the number of males and females is extra important.

Females dive less deeply
The fact that the female penguins end up in the north, according to the researchers, is because the females are smaller and lighter than the males, and therefore less deep for food. They look for shallower water, closer to the polluted coast. In that less deep water they can better keep their bodies at temperature. The females also end up more to the north because they can swim less well than the males, causing the current to carry them higher.

Interesting research, according to pool biologist Maarten Loonen of the University of Groningen. “They have shown that there is a difference in the distribution between males and females.” According to the associate professor of Arctic ecology, this phenomenon also occurs in other animal species, such as ducks. Nevertheless, according to him, it has not yet been shown that more females than males die from magelha penguins. “Males probably die just as often, but further away from the coast, their carcasses are eaten by scavengers. An interesting follow-up study seems to me whether there are really more males in the breeding colonies than females.”

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