[tombert]

I don't know about Japan, but in the US rats are also often used because they're basically the biggest mammal that has almost no regulation on what you do to them. Experimenting on, for example, a monkey might be closer to something human, but they're not legally classified as pests, so doing things with them requires a lot more paperwork and might not be approved.

Definitely not saying it shouldn't be that way; I'm kind of glad that we have some level of ethical rules for handling animals.

[magduf]

You're missing one of the other huge factors in why mice and rats are used for experiments: lifetime. These animals (esp. mice) have very, very short lifetimes by human standards, so you don't have to wait around for decades to see how something affects the animal over its whole lifespan. This is especially useful for aging research: it doesn't take very long for mice to become "elderly", so you can test out anti-aging experiments on them. Doing this on humans, even if it were totally legal, would be very slow because it'll take you 70+ years to grow a human in a lab to old age. Mice can only live a few years at most.

[tombert]

Yeah, and unlike something like a fruit fly, which also has a short lifespan, mice are mammals, so they are still fairly close to us in the animal kingdom, relatively speaking.