The Vitamin D, calcium, phosphorus balance is critical for developing bones, and MBD is common in birds that are being rehabilitated. (A young owl we received for rehabilitation had MBD and could neither stand nor perch, even though she was fully feathered, because of the improper diet she had received before we got her.)
Howler monkeys are hindgut fermenters, digesting the leaves they eat very slowly after they pass through their regular stomach into the hindgut, and lying on a tree branch in the sunshine aids with their digestion. Since our biologist suggested that we add a skylight to the cage during construction, Chiquito has gotten adequate sunshine. Lolita, however, has been in the house until this past week. Fortunately, spider monkeys are frugivores (fruit eaters) rather than foliovores (leaf eaters) like howler monkeys, and their digestion is very rapid so it hasn't been an issue. Lolita is also still nursing and her primary diet is Isomil, which is at least balanced for human infants.
Nevertheless, sunshine is vital for health, and Paul hustled out this morning to wash the skylight. I'm just worried that after seeing his limitations without a step ladder and extension ladder, Lolita might suspect she's adopted...