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.

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

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.