The strain of rat used in these experiments, the Sprague Dawley, is commonly used to study cancer because it has a higher baseline incidence of cancer and grows solid tumors faster than other strains [1].
Interestingly, it’s the same strain that was used in the now-discredited study purportedly showing that RoundUp causes cancer [2].
1. https://en.wikipedia.org/wiki/Laboratory_rat#Sprague_Dawley_...