The Nature of Rats
Rats are social animals. That is just their nature. In the wild, there is never a rat, only a hoarde of rats, a plague of rats, a sea of rats, or just ratS.
Rats have a social structure most closely resembling that of human beings. It can be a loose hierarchy with the strongest dominating, but generally it is just a bunch of animals of both sexes sometimes greedily hoarding and fighting for their own survival and sometimes sharing and working together to meet their goals. They share parenthood responsibilities and care for their sick or injured. They stay together for warmth, for safety, and just for companionship.
Domestication never changes the basic nature of an animal. Rats still strongly bond to one another and an owner. They bond so much that when a companion dies, a rat will very often become withdrawn and uninterested, sometimes refusing to eat for a few days. Just imagine what it would seem like for you to live with a sister or brother or just a friend your whole life, knowing no one else, and then one day she/he dies...
Now, since rats DO have a social structure like people, it is not too far-fetched to consider this scenario. Imagine a young man or lady who is moved away from her parents and siblings just after school age or so. She moves to an apartment or even a mansion with a pet of some sort. She can hug and talk to this pet, but they don't speak the same language. She has plenty to eat and lots of board games and music and other amusements. However, she has no telephone or internet or TV or any way of ever seeing or talking to another human being for the rest of her life. Can a person live like this? Maybe a few very rare ones fit the hermit lifestyle. Should a rat live like this?
Why a Lone Rat?
People have a lot of reasons for deciding they want a single pet. I have heard most of them, and nearly all of them are not good enough for depriving a rat of probably the most important thing in its life.
"If I get two, they will bond with each other and ignore me." This is simply not true! Often this remark comes from people who have owned dwarf hamsters, mice, or gerbils (all social animals, like rats). However, unlike rats, these are animals that probably won't "bond" to you to begin with! They are not as dog-like in their bonding as rats are. The truth is, with rats, if you have one or ten, they will rush to be with you whenever you are near. A companion will likely help them become more comfortable with you and the surroundings much sooner, which only strengthens the bond. The only reason a single rat may seem to want to be with you more than two or more is that she is so bored and desperate for any kind of touch that she just begs for it.
"Two males will fight, so it's better to keep them single." Another commonly held (and too often appearing in print) misconception. While male mice tend to be quite territorial and will often fight brutally, male rats fight no more than females, provided they grow up together and/or are introduced young. In some cases, a pair of males gets along even better than a typical pair of females.
"I have time/money for only one." I'm afraid this one is counter to your belief. Rats need companionship all the time, even when they sleep. Do you have the time to be that companion? To hold your rat and carry her with you all day? What about midnight to 4 am when she is most active and needs to play? As for the money, two rats need fewer toys and less coddling. As idealistic as this may sound, they just need you and each other. As for the food and cleaning, the cage doesn't need to be any bigger for two rats, needs cleaned only as often as before, and one more rat may mean $5 or $10 more for food for the year.
"My parents won't let me have more than one." Go get them. Bring them to the computer. Show them this page and any other reliable rat resource. They are probably concerned about time, space, and money issues. Let them know the facts.
"My lease/landlord allows me only one pet." Unless you can sneak another one in or talk to him/her about it, that can't be helped. Go to the alternatives/options section.
Afflictions of Lonliness
Rats that live alone often live shorter lives. They get sick more easily and take longer to recover from surgery or disease. In fact, anyone familiar with rats will always recommend to NOT segregate a rat after any surgery. The stress of loneliness is that much greater a risk than having stitches picked at.
A lonely rat will often act bored, depressed or nervous. She may be unresponsive. She may chew cage bars incessantly or dig frantically at cage corners. She may even revert to a rat version of self-mutillation by chewing her nails, plucking her own fur, or chewing on her own tail.
If you still are set on getting a solitary rat, there are options out there.
Due to bad breeding, bad raising, or just a simple personality quirk, there are rats who are too aggressive to their own species (but often just sweet as can be with people) to live with other rats.
Often a male will bond to only one brother, and when that brother dies (sometimes too young...), he will refuse or attack any rats that an owner tries to place him with. These rats are most commonly males and can be found at rescues or even at a breeder (who may be trying to find the rat a home where he can be by himself). If you choose to get this kind of rat, you need to understand he will be more work than a social pair would be. He will need a lot of toys (a rag to cuddle seems to make them feel more comfortable) and a lot of attention.
Other than this, you may come to realize that a rat is not the right pet for you. A social animal such as a rat may just not meet your needs in a pet. Perhaps a Syrian Hamster (the largish ones often orange in color) is for you. They are, by nature, solitary animals. Although their requirements and personality are nothing like a rat's, you may consider them a perfect small pet once you do some reading about them. For some other small pets to consider, look at this page.
Caring for the Single Rat
If you do have a rat that is kept alone for one reason or another, you will have to take on some care considerations that you would not need to for a pair or group. One thing you will need to do is for a very close bond to him and have him with you a LOT. Carry him on your shoulder while you do house work and allow him several hours of supervised playtime in a rat-safe room. Hold him on your lap or let him explore while you watch TV or movies.
Play with your rat on his level. By petting him, you can imitate social grooming. Most rats really enjoy petting, and being touched is essential to a rat, especially if he doesn't have contact with other rats. You can play with him by tickling his sides and stomach. Your rat may play back by pouncing your hand or giving you a playful nip (not a bite, but a nip, like a kitten would). If you can sit cross-legged, he can curl up and sleep on your lap, feeling your warmth, or on your shoulder or the back of your neck.
Provide lot's of toys to keep him interested and busy when you are asleep or at work or school. Wooden bird toys and bells that can hang in the cage are fun and inexpensive. There are also some "rat toys" on the market such as Bordom Busters. You may want to consider getting a wheel. Young rats and females often enjoy running. Get one with a solid running surface and at least one side enclosed, if possible, for safety and privacy. Also provide boxes and tubes and other things to hide in. A hammock, bed (like a ferret bed), or blanket of some sort will also provide some comfort and security. On cold nights or if he gets sick, you may want to put a hot water bottle or heating pad (on low and with a folded towel on it) under part of the cage. Also, keep the cage in a room where there is human activity and traffic. Rats like to feel like they're part of the action.
Your rat may be able to have a playmate sometimes, whether it is another rat or a guinea pig or rabbit. Be very cautious about having your rat with another animal and always supervise. Rats, especially singly kept ones, can be territorial, so allow them to interact with other animals ONLY on neutral ground outside their cage.
Remember, that without a cagemate, your rat will bond to only you and/or another person they get to interact with. If you go on vacation, he could become lonely or depressed, so you may have to try to make arrangements to bring him along or have a friend who he knows and trusts and who can give him lots of attention petsit for you.