Full marks for original thinking, Max, but there's another complication we need to get around, even if we allow the original bunny's 0.5 pair to be rounded down in perpetuity. In order to produce the Fibonacci series, the original female cannot reproduce again after she gives birth to her first pair of kittens. Does she die during birth of her offspring, do we need to introduce a well-timed trauma to her reproductive organs, or are we talking some kind of one-off lapine Divine conception here? Not so far-fetched for a rabbit society where everyone has eternal life? But perhaps this line of thinking has gone far enough.....