The gray squirrel, which was introduced to Britain from the US in the 19th century, is now overrunning the countryside, out-breeding and out-eating its smaller British cousin, the red squirrel. The red squirrel only numbers 160,000 in the wild while the gray squirrel is up to 3 million and rising. Scientists are now experimenting with a fertility control agent to try to control the population and allow the indigenous red squirrel to make a comeback. Lethal control is currently the only way they have of controlling the gray squirrel population so an alternative method is favorable to many.