Rats use their tails to balance and also to help maintain body temperature. The rat has specialised blood vessels, which control the amount of blood flowing to its tail. Blood flowing into the furless skin of the tail carries heat, which is then radiated out of the body.
The rat can direct anywhere between 0.1 and 10 per cent of its cardiac output to the tail. Under ideal conditions, it can get rid of up to 20 per cent of its metabolic heat in this way. Conversely, restricting blood flow to the tail reduces heat loss in cold weather.
It is interesting to note that the length of the rat's tail is partly determined by the temperature of the environment in which it is raised.
Radiating heat through the skin plays a major part in regulating body temperature in humans as well.
Observing three rats suggests that the long tail — usually around the same length as the body — is used for balancing. Although rats cannot grasp objects with their tails, they do seem able to control the muscles along the tail's length with considerable precision.
If a rat is running along a narrow surface such as a rope, the tail constantly flicks from side to side to steady them, in the same way tightrope walkers at the circus use a pole to help them stay on the wire. A cat walking along a fence does the same thing with its tail.
When rats are eating, they perch on the edge of the food bowl, which is about 20 cm from the floor of the cage, grasping it with their back paws and eating with their front paws.
In this position their rear ends stick out quite a long way over the side of the bowl so that it looks as if they should fall off backwards.
They manage to stay balanced by curling their tails under the bowl. This moves their centre of gravity below its back paws, keeping it balanced. The rat's centre of gravity is lower too, so it is more stable and less likely to fall.
As an experiment, a food bowl was moved to about 10 cm above the floor of the cage, which is about the length of a rat's tail. Instead of curling their tails under the bowl when they are feeding, the rats stiffened their tails and pointed them straight down so that they touched the floor, propping themselves up with this "third leg."