A young black howler monkey died Sunday, Oct. 7, at the Smithsonian's National Zoo.

Loki—born March 22, 2010, to first-time parents Chula (her mother) and Pele (her father)—was euthanized "because she was ill from complications of metabolic bone disease," the zoo said.

The zoo described the disease as a vitamin D, calcium and phosphorus imbalance. It is most often caused by lack of sunlight (ultraviolet light), lack of dietary intake of vitamin D and/or the body’s inability to properly metabolize these compounds.

"For five days, Loki was treated in the hospital with injectable vitamins, iron [and] calcium and exposed to direct sunlight while receiving around-the-clock nursing care," the zoo said. "A blood transfusion was provided from [her] father as well. Despite the combined efforts of keepers, nutritionists, technicians and veterinarians, the monkey grew weaker. The consensus decision was made to humanely euthanize Loki on Sunday morning."

The skylights in the Small Mammal House where Loki lived are made of "UV-transmissible glass, which allows the UV to penetrate the animal enclosures in order to provide sunlight. This is important for young animals especially as they wean. This monkey was in the weaning process as she was entirely dam-reared and just starting to eat solid foods," according to the zoo.

National Zoo staff regularly use light meters to measure the intensity of ultraviolet light in the animals' living areas. "Zoo experts are now examining the accuracy of the light meters used. Staff are implementing immediate husbandry changes, where appropriate, such as rotating animals to outdoor enclosures to allow direct sunlight exposure and evaluating diets for at-risk animals," the National Zoo reported.

The National Zoo has two pairs of howler monkeys on exhibit.