London (CNN)A same-sex penguin couple will raise an adopted "genderless" chick for the first time, a London aquarium announced Tuesday.
The four-month-old Gentoo penguin chick will not be named or characterized as male or female, Sea Life London said.
The chick is being raised by two female penguins Rocky and Marama, who were given the egg to relieve the penguin's birth mother of the pressure of raising two chicks.
It will be "the world's first penguin to not have its gender assigned," Sea Life London said.
Experts at the aquarium usually give newborn penguins gender-based names, but in this case "decided it was more natural for the chick to grow and develop into a mature adult as genderless which is normal in the wild until they mature," the centre said in a press release. The chick has been fitted with a purple tag on its wing so it can be identified.
"While the decision may ruffle a few feathers, gender neutrality in humans has only recently become a widespread topic of conversation, however, it is completely natural for penguins to develop genderless identities as they grow into mature adults," said General Manager Graham McGrath.
"What makes us really proud at the aquarium is the success of Sea Life London's Gentoo breeding program and the amazing job of same-sex penguins Rocky and Marama who took the chick under their wing and raised it as their own," he added.
Rocky and Marama were given the responsibility of raising a chick after showing themselves to be a "close couple" for the past six breeding seasons and proving that they could create nesting conditions to raise a chick, the aquarium said.
The penguin may later be incorporated into Sea Life London's breeding program, "depending on the gender its biology determines," the aquarium added.
Dr Gemma Clucas, Postdoctoral Research Fellow at the Cornell Lab of Ornithology, said that male and female penguins displayed little difference beyond biological characteristics, which required close and "difficult" observation to distinguish.
"The two sexes in penguins behave very similarly to one another,"she told CNN. "They look almost identical to one another. Behaviorally, they act very similarly as well, particularly in terms of reproduction -- both males and females invest pretty equally in raising their chicks."
Rocky and Marama are not the first same-sex penguin parents -- in August, two male king penguins at a zoo in Berlin adopted an egg. Other zoos in London, Sydney and New York have also had same-sex penguin couples.
Gentoo penguins have "near-threatened" status in the IUCN Red List of Threatened Species, as a result of climate change affecting their feeding and nesting habits, Sea Life London said.