"If the abdominal cavity is then opened up the internal fat is also yellow, yet the musculatrue (flesh) and the organs unchanged in color and consistency. Here we are dealing with "yellow fat," a lipometabolism disorder...It occurs when a diet deficient in vitamin A is fed and leads to an abnormal oxidation of fatty deposits into yellow fat with a wax-like consistency. Similarly, the uptake of too much unsaturated fatty acids in the diet (such as rancid wheat germ) can lead to such changes... often these particular animals are well developed and quite fat."
"Not all animals in a particular group or colony are affected by this metabolic disorder, and therefore breeders generally do not believe that diet is at fault. Each animal reacts individually since physiological conditions vary from one chinchilla to the next. "Yellow ears" can be an externally visible sign of "yellow fat."
(pic is mine, not book's)
"As a preventative measure, breeders should make sure that their animals always get enough vitamins E and A. Usually the full vitamin requirements are NOT met by the principle diet alone and there will have to be vitamin supplements administered. If wheat germ is used, it is important to make sure that the grains are not rancid...storage in excess of eight to 14 days is not recommended. Moreover, the breeder has to make sure when he is feeding pellets that unsaturated acids have not developed during compaction of the pellets. This is of course only applicable if ingredients that can give off unsaturated acids when heated are being used in the manufacture of chinchilla pellets."
-- from Diseases of Chinchillas, by Helmut Kraft, copyright 1987 by TFH Productions
...a wealth of knowledge in these old books, definitely good to keep that inheritance of information for the benefit of pet chinchillas today.