Difficult to breed for. Serious birth defects can result from the tailless gene. Also, since the tail helps with temperature regulation, they are much more susceptible to heatstroke . Variegated - Usually means a hooded rat with splotces of color on its sides. Can mean any spotted pattern, including Dalmatian.
Rats grind their teeth when relaxed and happy or very stressed.
Frequently accompanied by a sort of eye-bulging, which can be scary when you first see it, but simply results from the vibration of facial bones. Most common in pink-eyed rats. Poor eyesight makes them sway their heads back and forth to aid in depth perception. Usually a sign of annoyance. Heatstroke - Norway rats, being native to more northern climates, are very susceptible to heatstroke.