were children, McCollum captured some rats from a horse barn, but they were too vicious and wild. He replaced them with twelve young albino rats with some domestication that he bought for $6 from a Chicago pet store. His rat colony, started in January 1908, was the first in America used for nutrition research. Early error McCollum decided that some diets failed because they lacked "palatability". He thought that if a diet tasted good, and the animals ate more food, then nutrition would be adequate. In 1911 Osborne and Mendel criticized McCollum's hypothesis and his data, when they found that plant protein diets needed to be supplemented with protein-free milk. Rats would then eat purified diets without flavorings. Further, Mendel and Osborne suggested in their papers that McCollum had been less than careful in his experiments. Embarrassed, McCollum acknowledged his error. Discovery of vitamin A He was still responsible for the care of the heifers until 1911, and the care and feeding of his rats fell to a new volunteer, Marguerite Davis, a home economics-turned-biochemistry student who looked out for them daily, unpaid for five years. She earned $600 for her sixth and final year. Davis helped McCollum develop "the biological method for the analysis of food", and she co-authored a number of papers. Among them was "The Necessity Of Certain Lipins In The Diet During Growth" in 1913. They fed rats pure casein, carbohydrates (lactose, dextrine, and/or starch), with a little agar-agar and a mixture of six or seven salts. They substituted lard or olive oil for some of the carbohydrates for a group of rats. For 70 to 120 days, their rats grew, and then they stopped growing. They still appeared to be healthy, except the females did not have enough milk to nourish their young. They successfully restored about thirty rats to normality, after these rats had reached the stage of growth suspension, by adding a small amount of extracts of egg or butter. Their paper included charts for five rats, illustrating their weights over time compared to a normal growth curve, meant to show McCollum and Davis's "almost invariable success in inducing a resumption of growth after complete suspension for a time". One chart showed their results with a fat-free diet. They became convinced that without a substance in the egg or butter extract, rats could not grow, even though they appeared to be healthy. They