Females are generally more agile and active than males, although you'll find that the rats haven't always been told this. They remain smaller than the males and have softer fur. Their major health problem is with mammary tumours, although if you source your rats from a reputable breeder you will be able to ask them about the incidence of mammary lumps in their lines. These tumours can be operated on and removed fairly easily, and contrary to what some people will tell you, they don't always come back straight away. Adult females will generally weigh between 300 and 550 grams, depending on their breeding and past nutrition.
Male rats are slower and more laid back than females, but this is just the general rule and there are many exceptions. Male rats also tend to scent mark, which means they often leave a small drop of urine on you just to say that you belong to them. Adult males will weigh between 400 and 8oo grams, depending on their breeding and past nutrition.
Male rats have very obvious testicles by the time they're old enough to go to a new home. If you can't be sure which sex a rat is, it's probably female. It's always best to check for yourself when taking on new rats - don't take someone's word for it.
Female at four days old
Male at four days old
Female at eleven days old
Male at ten days old
If you're collecting rats from a breeder you may not be able to choose which of the babies you have, but if you do, begin by offering your hand to the rats to see which ones choose you. If at a pet shop, ask if you can hold the rats to see how well socialised they are. For a first time owner I would avoid choosing any rat that shows obvious signs of aggression or is very nervous. Check that their eyes are bright and clear, their fur is in good condition, and hold them against your ear to check for signs of respiratory disease. Do not feel obliged to take a rat you are not happy with. It's better to upset someone than to take on a sick or aggressive rat that you weren't planning on.
When you bring your new rats home you may find that their poo is very loose and smelly at first, either all the time or when you take them out of the cage. This is a stress reaction and will settle down once they feel more at home. Young rats who have been on a high protein diet may also have a stronger than usual smell for a while.