I've always suspected that rats do so well in Aus because they don't mind being upside down... (just kidding)...
Actually to be honest, black rats would be a better rat choice for hot climates. They are very much like our brown rats, but do better in the heat. Black rats were originally domesticated at the same time as brown rats, there were even a couple of color morphs including white and green. Then they pretty much evaporated from the fancy. Some folks have recently started to work with them again and even the wild ones tend to become really good companion animals. And the fact that they don't need around the clock air conditioning might be a nice benefit for people who live in tropical climates.