The zoo said the monkey, named Loki -- and female, though initially thought to be male -- was ill from complications of metabolic bone disease.
In a media release, the zoo explained that staff are checking to see if Loki's indoor enclosure is to blame, since metabolic bone disease can be caused by a lack of sunlight:
The skylights in the Small Mammal House 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. Because the amount of UV light penetration can vary depending on many factors, staff regularly use light meters to measure the intensity of the UV light. 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.
According to the 1995 paper "Vitamin D and Primates: Recurring problems on a familiar theme," "captive primates housed indoors with little access to ultraviolet light have historically been susceptible to metabolic bone disease," with mother-reared infants being especially vulnerable.
"We had one pair of howlers in the outdoor exhibit and kept this family indoors as that was a better husbandry choice with the baby," zoo spokesperson Pamela Baker-Masson told The Huffngton Post. "Obviously the staff didn’t realize that the UV was a problem or they would have managed the monkeys differently."
Loki received medical care for five days in the zoo's animal hospital, where she was exposed to direct sunlight. She was also treated with injectable vitamins and received a blood transfusion from her father. But, according to the zoo's release, "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."
Loki was the first baby born to mom Chula and dad Pele, who are among the two pairs of howler monkeys on display at the zoo. Baker-Masson told HuffPost both parents are being watched closely and zookeepers have noticed that Chula is "more sedate and not moving around as much. This indicates that she is aware that the baby is gone."
Chula is also "spending more time with the male," Baker-Masson said. "Typically, males are not involved in rearing the young. The fact that she is spending more time with the male indicates that she is returning to more normal behavior."
Here is Loki with her mother, in the weeks after her March 22, 2012 birth.