Rats are bisexual by nature and females will have group orgies with each other when in heat.
Male's balls can take up 10% of their entire mass!
There was a study where they put females in 'lingerie' and accustomed the males to it to the point they only saw the lingerie wearing rats as suitable mates.
Rats can give into peer pressure and eat things they don't like if everyone else is eating it.
Rats cannot burp or vomit.
With the amount of people killing pest rats by poison, they are creating bigger, smarter, poison resistant mutant rats.