In fact they've been known to cannibalise each other. Well, that famous variant on the Fibonacci sequence, known as the Lucas sequence, can be used to model this. It goes 2 1 3 4 7 11 18 29 47 76 and so on, but like Fibonacci adding each successive two numbers to get the next.
For our rabbits this means start with 2 pairs and one eats the other, so now only 1. However that 1 then gives birth to 3. Of those 3, 1 gives birth to 2, but the other 2 don't give birth yet, so now we have 4. Next the 2 that didn't give birth last time now give birth to 3 each, while of the remaining 2, one eats the other, leaving us with a total of 7. And so on.