Psychology Revision HELP!

Hey all, I'm currently revising for my PSY3 re-sit for 11th June and I'm studying Relationships, Gender and Aggression.

I'm looking at my Relationships booklet and I've came across the Natural and Sexual Selection of the Evolutionary approach and I'm revising Sexual Dimorphism but I didn't wrote in my notes WHY many species of animals have this different look between the sexes.

If you could reply, as I'm very confused with my notes everywhere, it would be such a big help.

Thank you!