Adult rats vary enormously in size. Average is about the size of a smallish grey squirrel, but the smallest does are not much bigger than hamsters - the biggest bucks as big as guinea-pigs. Normal weight is from about ½ lb to 1¼ lb for does and 1lb to 1¾ lb for bucks - but the biggest buck on record weighed 2½ lb.

Does tend to be playful, intelligent, active and fidgetty. They are prone to mammary tumours (harmless but expensive) - and their behaviour when in heat will expand your children's sex-education, possibly a lot faster than you had intended.

Bucks are usually more placid and cuddly with humans, but less entertaining to watch, more inclined to fight each other with serious aggression, and more likely to develop kidney problems.

Adults of either sex will usually accept babies being added to the existing group; but it is much harder to introduce adult bucks to each other successfully than it is with adult does. See section on social behaviour for advice on introductions.

Bucks are more likely to establish a lavatory corner and stick to it, making them easier to keep clean.

Bucks are more likely to scent-mark you/your sofa, either by urinating on the desired object or by rubbing the scent glands in their sides along it.

Serious aggression towards humans is very rare in both sexes: but it is more likely to occur in a buck than a doe.

Although bucks are more likely to get into serious fights than does, on the very, very rare occasions when a fancy rat actually kills another rat (I've only heard of 5 or 6 cases in the whole of Britain in 20 years) the killer is almost invariably a doe.

Bucks are more likely to be aggressive towards other pets such as cats; does are more likely to tease them ragged and to become sexually fixated on them.

Bucks tend to form deeper attachments to both people and places. This makes them very loving, but such an attached buck may become depressed and snappy if for some reason he has to be re-homed.