FARGO - Poor Butler. The river otter at the Red River Zoo has been without his sister and constant companion for several weeks after she died following surgery.

See video from otter exhibit opening in 2013

Lisa Tate, the zoo's executive director, said Sunday that staff has been spending extra time playing with him and engaging in various enrichment activities designed to stimulate his mind. "Since we've put water in the pool, he's doing quite well."

Tate said she's working with other zoos to find another playmate for Butler. Otters are sociable animals and it's especially important for Butler to have a companion because he was raised with another otter, she said.

Annabelle, his sister, was recovering from spay surgery when she died, according to a flyer posted at the zoo's popular Otter Rapids exhibit. An examination of her body revealed an undiscovered lung disorder had hampered her ability to breathe and the stress of general anesthesia was too much for her. She never awoke.

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The zoo said the disorder is analogous to Pulmonary Alveolar Proteinosis in humans. PAP has not been observed in other species so vets did not screen for it before the operation.

Annabelle and Butler were pups when a man found them in spring 2012 along the Sheyenne River in West Fargo and brought them to the zoo. Butler was injured and zoo staff didn't know if he would survive.

Bottle-fed by staff, the two otters grew to healthy adulthood, entertaining visitors at their exhibit, which allowed them to be viewed above and under water.