Zookeepers of a zoo in Japan believe they have finally figured out how a female gibbon got pregnant despite living alone in her cage.

In February 2021, staff at the Kujukushima Zoo & Botanical Garden in Nagasaki were baffled when Momo, a 12-year-old white-handed gibbon, gave birth despite not having a male companion.

On Jan. 31, the zookeepers announced a theory on how the female gibbon became pregnant.

Following a DNA testing of the baby gibbon, the zoo learned that the infant’s father is Itō, a 34-year-old male gibbon who was held in an enclosure adjacent to Momo’s.

“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 superintendent of the zoo, told Vice.

Although there is no surveillance footage for verification, the zookeepers believe that Momo and Itō were able to mate through a tiny hole measuring about 0.3 inch in diameter in a steel plate between their enclosures. 

The gibbons reportedly took turns going on display in the morning and afternoon in an exhibition area in front of Momo’s cage. 

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

The baby ape currently weighs around 4.4 pounds and is growing healthily under Momo’s care.

According to Yamano, the zoo hopes to move Itō in with Momo and their offspring.

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

“It is a precious life born into the world, we will continue to take good care of him and hope that he will live a healthy long life,” Hideki Hisano, the deputy director of the zoo, told CNN.

White-handed gibbons are among the smallest apes and are endangered primates native to Southeast Asia. Gibbons have loud singing voices and can swing from branch to branch at speeds of up to 35 miles per hour.