A Cincinnati Zoo and Botanical Garden worker’s thumb was partially amputated after a bonobo was attacked during morning feeding Friday, the zoo said.

The employee was stabilized at a hospital and was being treated, the zoo said in a statement Friday afternoon.

The bonobo went through a protective barrier while the primate care worker was giving food and medicine to the animals during morning rounds, the zoo said.

It was unclear if the primate came into contact with the worker or if the injury came from the barrier.

The zoo stressed that it happened “behind the scenes” in its Jungle Trails habitat, which it said was home to 10 bonobos last year.

“The incident took place backstage and no other staff, guests or animals were involved or injured,” the statement said.

There was no breach and no workers were inside the core habitat, the zoo said.

The Jungle Trails habitat was closed for the day but was scheduled to reopen Saturday, a zoo spokesman said.

“The zoo is examining the details to understand what led to the incident,” the zoo said in its statement.

The zoo did not identify which animal was involved in the incident.

Gilda, an 18-year-old bonobo, gave birth to a boy on Sept. 8 at the facility, the zoo said. On September 30, the zoo video posted of the pair making their first public appearance in their habitat.

The bonobo is a primate closely related to chimpanzees.

The International Union for Conservation of Nature and Natural Resources lists the bonobo as an endangered species, with a population in decline due to development, civil unrest and mining in its native Democratic Republic of Congo.

The nonprofit Bonobo Conservation Initiative said reliable population estimates are difficult to obtain. “There are only 15,000 bonobos left today,” it continued his website.

Bonobos, often organized around female leadershipthey are considered more peaceful, intelligent and empathetic than other primates. Because they are the last great apes discovered by humans—they were officially designated as a species in 1929—researchers are still making discoveries and hope to make more.

Key among the potential insights is what bonobos can teach researchers about humans. Primates share 98.7 of their DNA with humans, making them our closest relative in the animal kingdom, according to the study. WWF.