www.popularscience.co.uk website has been sent for review a book called Shapes by Philip Ball. It's an interesting book about shapes in nature - everything from why shells have a particular form to why a zebra's pattern looks like it does. Something the reviewer drew to my attention, was a fascinating observation about a creature for whom the idea of prime numbers isn't just a bit of abstract maths, but a matter of life and death.
Apparently there are some cicacada species whose life cycle tends to operate on prime numbers - say a 13 year or 17 year period. Most of their life they are tucked away safe underground, but once every 13 or 17 years they pop up to breed and are vulnerable. It has been suggested this is a self preservation thing. Typically the number of predators around go through regular peaks. Imagine your life cycle was 12 years rather than 13. Then you'd be particularly susceptible to predators with a 2, 3, 4 or 6 year peak, if that peak synchronized with your cycle. But with a prime number life cycle, a predator is much less likely to be able to synchronize.
As it happens, these cicacadas don't have any predators with appropriate cycles. But it has been suggested this is because said predators have died off/given up and moved to the South of France/etc. due to limited prey.
The only problem I have with this theory is that I'd expect it be more common than it seems to be... but I still love the idea that a species' existence could depend on prime numbers.