Two male Humboldt penguins cautiously guard the entrance to their cave in the zoological park in Bremerhaven, northern Germany, where they are foster dads to a six-week-old penguin chick. (Focke Strangmann/Associated Press)Keepers at Germany's Bremerhaven zoo couldn't get two penguin parents to take care of their egg, so they're trying an experiment — they gave the egg to a gay male penguin couple.

The biological parents "always rolled the egg out of their nest, they kicked it out again and again," zoo veterinarian Joachim Schoene said.

"Then we made the decision not to give it up and instead try to give it two fathers."

So far, the experiment has been a success. The two foster dads incubated the egg for 30 days until it hatched and have continued to care for the newborn chick.

The male penguins, named Z and Vielpunkt, are one of three same-sex pairs of Humboldt penguins at the zoo. That means almost a third of the zoo's 20 penguins who have attempted to mate exhibit homosexual behaviour. Same-sex penguin pairs have also been observed at zoos in Japan and New York.

The behaviour is not considered unusual because homosexuality has been well documented in the animal kingdom.

Scientific observation has shown that most sexual encounters between giraffes are homosexual. Male bottlenose dolphin calves often form homosexual bonds and exhibit bisexual behaviour when they're older. And female Japanese macaques, or snow monkeys, form monogamous relationships with each other that last from days to weeks.

All of these behaviours have been observed in the wild and do not appear to have been influenced by a scarcity of available mates of the opposite sex.

A similar experiment was performed at New York City's Central Park Zoo in 2004. Two male chinstrap penguins named Roy and Silo incubated an egg together and raised the chick, called Tango, when she hatched.

A children's book written about the New York penguins called And Tango Makes Three has been the book with the most requests for removal from libraries in the United States over the past three years, according the American Library Association.

Z, Vielpunkt and their adopted chick have proven a hit with visitors to the Bremerhaven zoo.

With files from The Associated Press