That's what they're wondering at the Los Angeles Zoo, where the eight-year-old chimpanzee gave birth Sunday.
Keepers say they didn't know she was pregnant and aren't sure how she got that way.
CBS News Correspondent Vince Gonzales reports that all three of Yoshiko's eligible mates have had vasectomies, according to zoo officials.
That leaves one older male, 44-year-old Toto, who has never shown any interest in the opposite sex, and two baby males thought to be too young to be monkeying around.
But the zoo staff believes it's more likely that one of the vasectomies failed.
The surprise birth was discovered early Sunday when an assistant keeper arriving for work spotted Yoshiko cradling her offspring, since dubbed Toshi.
"The baby looked like she had just been born," Mark Atha said. "It was a total shock."
The baby weighs two pounds, about average for a newborn chimp, and is nursing well, the zoo said.
The zoo has tried many birth control methods on the chimpanzees in the past including the pill, hormonal implants, and Norplant. Each time, though, the chimps kept having babies.
"They are just determined to breed no matter what," said Atha.
When the chimp is older, keepers plan to use DNA testing to determine who the father is.