Males compete against each other for female attention, for access to mating opportunities, and the sperm of multiple males can compete to fertilise a female’s eggs. As such, male-male competition is a ubiquitous component of sexual selection. My thesis broadly examines how male-male competition for mates influences reproduction, life-histories, and, ultimately, produces sex differences in the behaviour of male and female animals. These varied topics are explored first through a comprehensive meta-analysis of sex differences in animal personality behaviours, then through focused experimental manipulations of contest experiences using male eastern mosquitofish (Gambusia holbrooki), robotic crabs to exploit female mate choice for courtship synchrony in fiddler crabs (Austruca mjoebergi), and finally male mate choice for dominant females in eastern mosquitofish (Gambusia holbrooki).