do macs suffer from fragmented hard drives like windows machines do?

Well, my title says it all. Do Macs need defragging? So far I haven't heard anything to that affect, but my mac is a little slow starting up and I'm wondering if this might be the cause. Considering I haven't seen any defragging tools for macs I'm assuming it's not an issue. But, I'm wondering if doing a clean install of the OS and then running timemachine would be a suitable fix. What do you think?

David.