 McCollum determined that he must find out what was lacking in their purified diets, and that he needed to experiment on animals with a short life span, finally deciding on rats.Rats are omnivorous, and cheaply fed, matured quickly, and "had little sentimental and no positive economic value".The dean of the college of agriculture would not support experiments with what he feared was vermin, but Babcock, who was retired, understood the rats' importance and told McCollum to proceed.