All science related courses are dominated by males in my school, my grade has aprox 30 males and 5 females doing physics, 20 males and 6 females in chemistry, 8 males and 0 females in engineering studies.
I dont think it has to do with brain power or the level of difficulty of the subjects, i think that females just lack the urge to want to understand why/how something works, which is what the science courses all are about, thus they might find the science subjects extreemly boring and stay clear from them in subject sellection. Well at least my sister is like that, she dropped physics because she found it boring