Zookeepers at the Parken Zoo in Eskilstuna said the couple, Tarzan and Jane, were found to be caring for their new arrival Monday morning, The Local.se reported Thursday.
"We couldn't be sure if she was pregnant," zookeeper Louise Nilsberth said. "It's not easy to tell but we had our suspicions since she had quite a round belly."
Officials said it is rare for gibbons in their 40s to have offspring.
"In human years that means they are in their 70s or 80s," Nilsberth said. "Gibbons usually live until they are 25-30 years old. We don't know of any other cases in the world like this."
Nilsberth said zookeepers do not want to stress the animals with invasive research into their fertility.
"It would be quite stressful for the animals to do research as such," Nilsberth said. "But it is so rare that we do intend to look into it."