They can draw their testicles up inside them if they are afraid, but will not do this for a long period of time. A good rat breeder or staff at a good pet shop will find it easy to tell which sex baby rats are. Both male and female rats make great companion animals, although they have different characteristics. Approximately once every five days a doe will be in heat for around twelve hours. This usually happens in the evening.
Bucks are larger and more laid-back than does.
Their coat is coarser and has a slight musky smell to it. While they are as affectionate as does, they are much lazier, and when left free-range will often curl up in a corner or on your lap. This is not a usual occurrence and should not be confused with the normal rough and tumble of adolescent rats. This condition can usually be cured by having the rat neutered, and his hormonal levels and behaviour will soon return to normal. The National Fancy Rat Society has a list of vets that have experience in dealing with rats.