Standing on hind legs while sniffing the air: The rat is looking for certain smells from other rodents, humans, food, and potential predators. This is a natural instinct.
Grooming (washing face or entire body): This is also normal. Rats groom themselves from head to tail several times per day.
Female rat crouching down, sticking her rear end up, lifting her tail, and flapping her ears back and forth: This behaviour, usually exhibited when the female is touched on her back end, means she is in heat. This is her presenting herself to be mated with.
"Play" bites (tiny nips that don't break the skin): A rat doing this to another rat or a human probably isn't trying to be aggressive; it is a part of normal play behavior. Rats can also nip at human hands if their hands smell like food, or they are so used to getting fed through their cage bars that they don't realize your finger is not food. Rats often bite at other rats to initiate play, sometimes pulling on their skin as well.
Wrestling with other rats playfully, or acting hyper when touched by a human, sometimes running away from their hand quickly then running back: This is also part of normal play behaviour. A rat darting away only to return in a second or two indicates that he wants to play with you. Treat your hand like another rat, and if your rat is tame enough he will let you 'wrestle' with him, gently pushing him to expose his belly so you can tickle him. Be warned, he will try to push your hand around as well and may give you a play nip or two. If he nips too hard, let out a loud squeak to let him know he has hurt you.
Licking your hand: Rats lick humans both as a sign of affection and a form of social grooming, though if your hands smell like food they may be trying to lick it off! Rats who give 'kisses' this way are often well-socialized and very tame.
Weaving the head back and forth and swaying on the front end: Commonly seen in red and pink eyed rats, this strange behaviour is the rat trying to judge depth perception. Rats have very poor vision and pink and red eyed rats can often see even worse. Moving from side to side like this helps them judge how far away something is in relation to them.
Submissive / Fear Behaviours
Backing away rapidly: A sign of fear, perhaps of another rodent or a human hand. Usually this animal will hide somewhere where it thinks it will be safe.
Assuming a ready stance (on hind legs, but crouched down with head pointed towards the aggressor and forearms extended but bent at the elbow): This rat is ready for anything. A rat assuming this stance and not moving is afraid, and will probably bite or run away if something comes near.
(While having its behind sniffed by another rat) Assuming a stance on three legs, the fourth being a hind leg which is lifted and pointed towards the other rat’s head: This rodent doesn't like the sniffing, but doesn't want to bite. It will use the lifted hind leg to kick at the other rat when it has had enough. This behaviour is usually observed when a male sniffs a female's genitalia to check for her receptiveness.
Dominant / Aggressive Behaviours
Male or female mounting a rat of the same sex: This rodent is trying to show its dominance over the one it is mounting. Males do this a lot more than females. They mount other males to tell them who's boss. Females will also often do this to each other while they are in heat.
Assuming a ready stance (see above): This rat is most likely one attacking another. This position allows the rat to bite or strike quickly and then retreat just as fast. It also allows it to dodge an oncoming attack from a retaliating opponent.
A stiff tail being rapped rapidly against the ground: This rat is very angry. This is usually in response to another's challenge to fight, or to tell another rat to get away. If the other rat does not leave or continues to bother him, it will likely instigate a fight.
Pushing another rat down, exposing his underside and grooming him fiercely: This is called power-grooming, and is a display of dominance. The submissive rat being groomed will often squeak in protest, but allow the more dominant rat to continue. If the other rat does not want to submit, it can result in a fight.
Making a hissing sound and puffing up the fur: this rat is showing aggression and will very likely bite if a human or other rat comes too close. The hissing almost sounds like a puff of air or huffing and can sometimes be easily mistaken for heavy breathing. An aggressive rat will often hiss while running alongside the cage bars when humans are present, warning them not to come any closer.
