Time 0 - We start with one pair of babies

January - The pair of babies grow up

Febuary - The pair has a pair of babies. Now we have two pairs

March - The first pair has another pair of babies. The second pair grows up. Now we have three pairs

April - The first pair again has another pair of babies. The second pair has its own pair of babies. The third pair grows up. Now we have five pairs

May - The pair once again has another pair of babies. The second pair another pair of babies. The third has its own pair of babies. The fouth pair grows up. The fith pair grows up.