Tusko the elephant was swollen and sore, but recovering at the Oregon Zoo Sunday after a dangerous surgery to remove his infected tusk that ended before veterinarians had hoped.

Oregon Zoo

Veterinarians work on Tusko the elephant during surgery to remove an infected tusk.

Zookeepers said they were delighted when Tusko started eating Sunday, a sure sign he was bouncing back and adjusting.

They worked on Tusko for five hours, an hour longer than they had expected. They stopped the surgery because if the elephant had stayed under anesthesia any longer, there was the possibility he wouldn’t wake up, veterinarians said.

More: Details on Tusko's recovery

“…So we left a little fragment of his tusk,” said lead veterinarian Mitch Finnegan. “It’s still up inside his head…we’re going to have to come back at a later date to remove it.”

Slideshow: Tusko's surgery, step-by-step

Slideshow 2: Tusko roams the Oregon Zoo

 VIDEO: RAW: Zoo veterinarians operate on Tusko

 VIDEO: KGW report on Tusko's surgery

There’s also a chance the piece could loosen and come out on its own in time. If the extra surgery is needed, it won’t happen for about two years, zoo staff said.

The hardest part of the surgery was putting Tusko under anesthesia. Prior to the operation, staff had to train him to lie down on a waterbed durable enough to hold his 13,000-pound frame.

Tusko was able to stand up on his own following the operation, though it took a couple of tries. He also had something to eat. He had fasted for 24 hours before the surgery.