As with human beings, temperament in rats seems to have a genetic basis, enhanced by early nurturing experiences. One cannot necessarily take the offspring of two aggressive parents, and love and cuddle it into a friendly, submissive rat. The evidence suggests that aggression is passed as a trait from generation to generation, and may be modified, but not necessarily removed by early nurturing. Before being able to choose a suitable rat to breed from it is important to decide what the ideal pet rat's temperament should be. Whilst some of us might have personal preferences (in my case, a squishy, laid back buck comes close to perfection) it is clear that some traits are undesirable, and can make rat ownership more difficult, even miserable.

It is undesirable for a rat to show any aggression towards his humans. I think this would be generally agreed by all responsible rat breeders and certainly by all pet rat owners. Rat bites can be the result of foolishness on the part of the owner (for instance trying to handle a rat who is in extreme distress, fear or pain), but any rat that bites without reason or provocation has in my opinion a poor temperament, and should not be bred from. It is similarly undesirable for a rat to display excessive aggression towards other rats. The rat that is overly dominant and bullies all his cage-mates, can make life very difficult for them, and for his owner. All rat colonies establish a hierarchy, and some squabbles are necessary to enable status to be earned and maintained. However, if a rat is constantly bullying his companions, causing repeated injuries his behaviour could be considered overly aggressive.

Anxiety can also cause problems for the rat and owner alike. A really anxious rat will run and hide from everything and become overtly distressed in new and potentially threatening situations. His activity may be frenetic, and the frequent passage of loose 'anxiety' stools can even cause growth problems. An anxious rat may not always be at the bottom of the hierarchy, and may be on a constant quest to dominate the rats underneath him. Put together these anxiety 'symptoms' can equal a rat that is very difficult for the owner to handle, and who causes constant problems amongst the other rats in his cage.

Aggressive or dominant does can become fiercely protective of their babies, preventing the early handling that is so desirable in socialising young rats with their humans.

If as a breeder you seek to place your rats in other people's homes then you have a responsibility to seek to breed from those who have excellent temperaments, and display none of the above traits. The reason I have put this at the top of the list of breeders considerations when choosing a pair to breed from is because there is no point at all in producing healthy, show quality rats that are unhandleable, or live long, lonely lives because they will not live with other rats.