Zookeepers Say They’ve Solved the Mystery of How a Gibbon Got Pregnant by Herself

Momo the gibbon was kept in a cage by herself in a Japanese zoo. But life found a way.
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Momo (left) gave birth to her child two years ago, puzzling zookeepers. Photo: Courtesy of Kujukushima Zoo & Botanical Garden

For two years, a zoo in southern Japan had been puzzled by a mystery: How did Momo, a gibbon kept alone in her cage, get pregnant?

The 12-year-old white-handed gibbon lived by herself and was never joined by a companion. Some of her neighbors are males, sure, but their cages are separated by sturdy bars and jagged chicken wire fencing. It was inconceivable to the zookeepers that they could have mated through the two layers of barriers. She gave birth in 2021 to a yet unnamed male gibbon with black hair and white fur trimming around its face.


But with the help of DNA tests, zookeepers in Nagasaki prefecture have identified the baby gibbon’s father. And they say they’ve figured out how the ape’s parents met.

After enlisting a researcher to analyze the DNA in stool and hair samples collected last year from Momo, her child, and four potential fathers, the Kujukushima Zoo & Botanical Garden on Tuesday announced the identity of the father: Itoh, a 34-year-old agile gibbon.

“It took us two years to figure it out because we couldn’t get close enough to collect samples—she was very protective of her child,” Jun Yamano, the zoo superintendent, told VICE World News.

But that still left one burning question unanswered—if she never had direct contact with Itoh, how did she get pregnant in the first place?

The zoo has no hard evidence like surveillance footage, but Yamano said the apes’ point of contact was probably a hole in the wall measuring nine millimeters in diameter.

At the zoo, Momo and Itoh take turns going on display in the morning and afternoon, in an exhibition area right in front of Momo’s cage. The two spaces are separated by a partition—a perforated board—that supposedly prevents the apes from mingling. 

But life found a way.

“We think it’s very likely that on one of the days that Itoh was in the exhibition space, they copulated through a hole,” Yamano said.

Such mating habits are unheard of, he said. Normally, the zoo pairs gibbons after a series of trial and error to familiarize the apes with each other.


In the wild, gibbons select their mates based on physical appearance, social behavior, and vocalization, such as the complex songs they sing. The zoo didn’t say what, if anything, Itoh did to woo Momo.

Since the mystery has been solved, Yamano said the zoo hopes to move Itoh in with Momo and their child.

“They have to get used to each other first. But hopefully they live together as one family,” he said. 

To prevent unwanted pregnancy, the zoo has replaced the perforated partition board with a barrier with no openings, Yamano said.

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