1.At the end of the first month, they mate, but there is still one only 1 pair.
2.At the end of the second month the female produces a new pair, so now there are 2 pairs of rabbits in the field.
3.At the end of the third month, the original female produces a second pair, making 3 pairs in all in the field.
4.At the end of the fourth month, the original female has produced yet another new pair, the female born two months ago produces her first pair also, making 5 pairs.