A 6-month-old black howler monkey at the National Zoo has been euthanized.

Loki, born on March 22, was euthanized Sunday because she was suffering from complications of metabolic bone disease -- a vitamin D, calcium and phosphorus imbalance. The illness is most commonly caused by a lack of ultraviolet light, a lack of dietary intake of vitamin D and metabolic issues, zoo officials said.

The young monkey was treated in a hospital for five days, receiving injectable vitamins, iron, calcium and exposure to direct sunlight. She received around-the-clock nursing care, and a blood transfusion was provided by her father. Still, officials said, she grew weaker.

Zoo staff members are making changes where appropriate, like rotating animals to outdoor enclosures to allow exposure to direct sunlight and evaluating the diets of at-risk animals, officials said. Also, experts are evaluating the accuracy of the light meters used to measure the intensity of ultraviolet light in the Small Mammal House.

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Howler monkeys are considered the loudest animals in the New World and live in Latin American rainforests, according to information on the zoo's website. The zoo has two pairs on exhibit.

Last month, a giant panda cub died at the zoo about a week after being born.

njagoda@washingtonexaminer.com