Zookeepers were stumped when a female gibbon got pregnant while alone in her cage. 2 years later they say there was a tiny hole that she and a neighboring male used to copulate through.

• Momo, a white-handed gibbon, got mysteriously pregnant at a zoo in Japan in 2021.
• Momo was so protective of the baby it took years to identify the father, a zoo official told Vice.

Zookeepers at a zoo in Japan were stumped when a female white-handed gibbon named Momo got pregnant in 2021, even though she had her habitat all to herself. Two years later, the zoo said it wasn't an immaculate conception after all, but the result of a small loophole, so to speak.

The Kujukushima Zoo & Botanical Garden, located in Nagasaki, announced last week that DNA testing showed the father of Momo's child was Itoh, a male gibbon who was held in a separate enclosure.

"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.

Yamano explained to Vice that the zoo believes the two gibbons were able to mate due to an area adjacent to Momo's cage that both she and Itoh took turns occupying, whenever one was put on display for the public. The zoo discovered a tiny hole measuring 9 millimeters in diameter — or less than a centimeter — in the partition board that separated the display area and Momo's cage.

Though there's no footage to confirm how they did the deed, the zoo believes the gibbons must have mated while Itoh occupied the display area next to Momo, who was just on the other side of the hole.

Yamano told Vice that such mating behavior was unprecedented and that gibbons are usually paired more intentionally after being exposed to each other. Now they plan to move Itoh in with Momo and the baby and have replaced the wall with the hole.

The zoo did not respond to Insider's request for comment.

White-handed gibbons are small, endangered primates native to Southeast Asia. They are mostly monogamous, with breeding adult couples typically forming bonds that last for life. Gibbons also commonly live in groups of two to six that consist of the mating pair and their children.

The primates mate year-round, with the mother typically serving as the primary caregiver, though she may receive some help from the father. Gibbons tend to stay with their parents until they reach sexual maturity or about seven years.