Rats show affection to both other rats and humans by grooming them, licking them, and playing with them. When a rat is happy it will grind its teeth together back and forth in a process called bruxing. Sometimes during bruxing their eyes will boggle in and out. It looks strange, but it means your rat is very content. It is also the process rats use to wear their teeth down.
Aggression
Rats who are aggressive are often rats who were not properly socialized when they were young, or may even have been mistreated by humans. An aggressive rat will not be afraid of humans and will often run directly towards them with intent to bite. Their bites almost always break the skin, and sometimes they will latch on and not let go. A rat who is aggressive will puff up his fur and run along the side of his cage when a human is present in the room, often hissing, huffing, and puffing up their fur to warn them not to come near. Aggression is mostly caused by hormones in male rats, though genetics can also play a role. Neutering a male rat can greatly reduce aggressive tendencies.
Some rats are not aggressive to humans, but will be aggressive to other rats. This is most often seen in males, but females can also exhibit this behaviour. Usually this kind of aggression is not seen until the rat is around 4 to 6 months of age. Aggressive rats will turn on their cagemates, showing excessive displays of dominance and not ceasing to fight when the other rat submits. They will bite hard and draw blood, sometimes injuring or even killing another rat. This kind of aggression can also be dealt with by neutering, but it doesn't help in 100% of the cases. A rat that shows this kind of aggression should live alone. Since he cannot get along with other rats, and chooses to be this way, he will be perfectly happy living by himself.
Dominance
Some rats, even though they may be perfectly tame and friendly, have a dominant personality and constantly show it by dominating other rats in their cage. Dominant rats will push other rats over to expose their belly and groom them furiously and sometimes harshly. They will walk right over top of another rat and urinate on them, marking them as subordinate. They will steal food, making sure they get to eat first and they sometimes will guard their food so other rats cannot eat it. They can become very pushy, shoving rats out of the way while they are drinking just because they can. Some dominant rats will exhibit a kind of teasing or bullying behaviour, nipping or grabbing and pulling at the skin of a more submissive rat. When two rats with dominant personalities are put together they will often fight if one of them doesn't want to submit.
Eating
Rats love food, and most of a wild rat's time is spent finding it. A pet rat has the luxury of a constantly full food bowl, but many of their wild behaviours still exist. A rat will often remove all of the food from his food bowl and hide it somewhere in his cage, often in several different places. When offered a treat, they will almost always snatch it quickly and run away with it. Rats feel vulnerable when they are eating, and therefore choose to do it in a place that they feel safe. Some rats will not accept treats when they are outside of their cage with no safe place to eat it.
Rats will also steal food from their cagemates, and it can escalate into quite the battle. Rats don't understand the concept of sharing and most rats will attempt to fend for themselves, dragging even the largest pieces of food away. If another rat attempts to take it, they will quickly grab the food and pivot so they are facing away from the other rat. If the other rat is persistent, they may pivot around quite a few times.
Rats will also eat their own feces, which is a perfectly normal behaviour and is necessary for proper digestion. Because the rat's metabolism is so high, food moves quickly through the digestive system and doesn't have time to relinquish all of its nutrients, so rats will eat their feces to send it back through the digestive system again.
Estrus
A female rat comes into heat every 4-5 days starting around 6 weeks of age. During her heat, which lasts about 6 to 12 hours, she will be much more active, jumpy, and sometimes even nippy. If touched on the hindquarters by a human hand or another rat, she will jump forward quickly then stop in her tracks, sometimes wiggling her ears back and forth. This is how a rat 'flirts' with a male, teasing him to come chase after her. Some females will arch their back, lift their tail, and vibrate their ears when touched on the hindquarters, displaying what is called the lordosis position, the position they assume when a male tries to mate with them. Some female rats will show absolutely no sign at all that they are in heat and some will be very obvious about it.
Exploring
Most rats love to explore, and given an area where they have never been before, they will check it out curiously but cautiously. They will often move slowly at first, sniffing about to detect any dangerous or delicious smells before moving on. Rats usually move along walls or furniture, feeling safer that way instead of running through the center of a room and leaving themselves vulnerable to predators from all sides. Rats will climb almost anything and can jump several feet straight up to reach a ledge. They will poke their heads into anything their head can fit through, and sample anything that may seem delicious, usually giving it a test bite to make sure that it is edible. If a noise or another being startles them, they will run for the closest safest place, often back towards their cage or under furniture.
Some rats, usually the shier ones, will hardly choose to explore at all, even if their cage doors are left open. It may take a while for them to feel safe in a certain area, and until they do they won't chance leaving the safety of their cage.
Fear
Any rat, no matter how tame, can show signs of fear, though it is most often seen in the unsocialized or untame rats. A rat shows fear by the classic 'fight or flight response', where they will either bite at the source of their fear, or run away from it. When a rat runs in fear, they will go to a small place where they can hide such as their nest box in their cage or a corner if nothing is present for them to hide in. They will then turn around so they are facing the object of their fear to make sure it is not coming any closer. A cornered rat that is fearful will often bite, as it is fearing for its life.
When handling a rat that is fearful, it may urinate or defecate on your hands. These 'fear feces' can also be seen when attempting to give a rat a bath, or taking it into some other situation where it is frightened. With gentle handling and environmental conditioning, a fearful rat may get used to its surroundings and not be so afraid.
Gnawing
Rats love to chew, and their powerful incisors that are constantly growing allow them to chew through almost anything, given enough time. Anything within reach of the rat is fair game for chewing, and hammocks or plastic toys will often become full of holes very fast. Even though the rat doesn't need actual objects to chew on they do enjoy gnawing. Rats can wear their own teeth down by grinding them back and forth against each other which makes a chittering noise and sometimes causes their entire body to vibrate a bit. This is called bruxing, and rats often brux when they are happy and content. Sometimes their eyes will boggle in and out while they brux.
Grooming
Rats are very clean animals and groom themselves several times a day. They will also groom their cagemates as a form of social interaction as well as cleanliness. They will start by licking at their front paws, wetting them, and using their paws to clean off their heads, then twist around and use their mouth to lick and nibble at their back, sides, and stomach. Rats can also groom as a form of dominance called power-grooming where a dominant rat will push another rat down so his belly is facing up, then groom him very quickly.
One area rats often neglect to clean is their tail. If their tail becomes dirty you can easily wash it with a tiny bit of baby shampoo and a soft cloth or toothbrush.
Rats will also groom humans as a form of affection. They will lick at your skin, nuzzle into your hair, and sometimes bite your fingernails or scrape their teeth against your skin. If your rat is grooming you too roughly, make a loud squeak to let him know he is hurting you.
Marking
Rats, both male and female, mark their territory with urine. Males usually do this more than females, but it depends on the personality of the rat. They will let out a few drops of urine as they walk around in their cage or outside of it, and will often urinate on humans in small drops as well. Neutering can reduce the frequency of urine marking but it will never completely eliminate it. The best you can do is have some paper towels handy for when your rat is out of its cage.
Mating
Rats will only mate successfully when the female rat is in heat. The male will usually try even when she isn't, but she won't let him unless she is in heat. When mating, the female rat will display the flirting behaviour mentioned above, darting forward, flapping her ears, and seeing if the male will chase her. After several attempts, she will arch her back, lift her tail, and allow him to mount her. Mating itself only takes a few seconds at most, and afterwards the male will retreat from the female and groom his genitals before trying again shortly. He may not actually ejaculate on the first mating attempt; most of the time it takes a few matings to result in a pregnancy. However, a rat can get pregnant after only one mating and it is never a good idea to allow males and females to play together.
After many matings, the male will often grow very tired and fall asleep.
Same-sex rats will also display mating behaviour, usually as a display of dominance or to release sexual tension. It is most often seen in males who are trying to dominate their cagemates, and in females who are in heat with no male around.
Observing
Rats enjoy watching the world around them, and whether in their cage or outside of it they can often be found observing their surroundings, either from up high where they have a good vantage point or from somewhere safe and hidden if they are more shy.
Sometimes when observing, rats will sway from side to side on their front legs. This is commonly seen in red and pink eyed rats and 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.
Playing
Rats enjoy playing, both with each other and with humans, well into their old age. Younger rats are the most playful, with babies hardly sitting still at all. Rats will chase each other around their cage, play-wrestle, and play-fight. They will squeak if the play gets too rough to let the other rat know they are getting hurt. Grooming can also be a sort of play, where the rats chase each other around then furiously groom each other, sometimes pushing the other rat down to groom his belly, known as 'power-grooming'.
Rats will also play with humans in much the same way. They will chase your hand around, nip at it, and may even try to wrestle with it. If you are gentle, you can push the rat onto its belly and tickle its stomach much in the same way another rat would do, just make sure you let the rat do the same to you! If your rat is running away when you try to pet him but then quickly runs back, he is trying to initiate play. They also may nip a little while they are playing, just let out a squeak to let him know he is playing too rough.
