Rats at Petsmart

Question
My last rat passed away in May and I currently am not a rat owner, I just have a general question. Why are the rats displayed at Petsmart only males? Is this because many people who do research would usually rather own a female and Petsmart only displays males for the "impulse buyers"?  

Answer
PetSmart purposefully avoids accidental litters by carrying ONLY males or ONLY females depending on the store. Usually it's by region; for instance in my general area we have 5 petsmarts within a 40 mile radius and two are female and three are male -- it's based on the store number. All of their animals (excepting birds) are the same gender. It's actually a really good program, for the duration of my employment we only ever had the occasional female come in with our all-male shipments -- which is far better than stores that carry both genders.