Question about pterosaur crests

["Paul Cambridge" <pcambridge@explorer.org>]

Can anyone tell me why the larger pterosaurs like Pteranodon and others 
bothered to develop large head crests? I know theories abound about mating 
display purposes, but if an animal is working so hard to LOSE pounds, i.e. the 
hollow tubular bones and such, wouldn't it be self-defeating to have these 
crests? It must have been a pain to get of the ground in the first place being 
of that body size.