Tenders at the LEO Zoological Conservation Center in Greenwich, Conn., said they were surprised to find giant anteater Armani had given birth to baby Archie one morning in April.
They said she had been separated from her mate, Alf, since she gave birth to their daughter, Alice, in August.
Male anteaters are known to commit infanticide, so keepers had moved Alf into a separate enclosure at the time, only reuniting the couple recently, the Greenwich Times reported Friday.
The gestation period for giant anteaters is six months, which didn't give Armani and Alf enough time to reproduce again, conservation staff said.
"It's a bit of a mystery," said Marcella Leone, founder and director of the center.
She said the pregnancy may have been the result of delayed implantation, in which a fertilized egg remains dormant in the uterus for a period of time.
Stacey Belhumeur, special survival plan coordinator for the North American population of giant anteaters, said that was unlikely.
"My guess is they thought they had him separated," Belhumeur said. "We've seen incredible feats of breeding success. We've had animals breed through fences."