At a Japanese zoo, the puzzle of how Momo, a female white-handed gibbon, became pregnant while residing solo, has finally been solved by the zookeepers.

An adorable baby gibbon was born to Momo, a resident at the Kujukushima Zoo & Botanical Garden in Nagasaki, after she became pregnant in 2021.

However, the staff at the zoo were left puzzled as to how the solitary female, who has her own enclosure, became pregnant.

The zoo, in a post on Instagram following the gibbon's birth, acknowledged that the identity of the father was still uncertain.

The zoo explained: "Just so everyone wants to know 'who's the dad' hasn't been found out yet because we haven't done DNA testing, but we found out the gender is male."

Was Momo's pregnancy due to immaculate conception in the gibbon world? No, not quite. Almost two years later, the zoo staff finally uncovered the cause of Momo's pregnancy.

Regarding the delay in solving the mystery, Jun Yamano, the superintendent of the zoo, explained: "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."

Initially, they conducted a DNA test on the baby and found that the father was Itoh, a male gibbon housed in a different enclosure.

However, love knows no limits, and the separation of their enclosures was not enough to prevent Momo and Itoh from being together.

According to Yamano's explanation to Vice, the staff believes that Momo and Itoh were able to mate through an area next to Momo's enclosure, which they both used in rotation while on public display.

The investigation uncovered a small hole, with a diameter of 9 millimeters, in the board separating the unit from Momo's enclosure.

Although the zoo was unable to obtain any video of the copulation, it's believed that the two gibbons mated while Itoh was in the display area and Momo was next door on the other side of the hole. A sort of gibbon "glory-hole," so to speak.

Yamano added that the mating and subsequent pregnancy were unprecedented at the zoo as the gibbons are typically paired together deliberately after being introduced to one another.

The zoo now intends to permit Itoh into the same enclosure as Momo and their baby and has also repaired the problematic hole in the wall.