Difficulties must be overcome by animals that achieve reproductive success with separate sexes. Reproductive timing must allow for male and female gametes to be available at nearly the same time. Sensory structures and hormonal controls must be precise in both parents. Seasonal cues and behavioral patterns must evoke a suitable response in both sexes. It is a challenging task to find and recognize a potential mate of the same species.
Males spend much energy in performing elaborate courtship rituals.
Fertilization also comes at a cost with separate sexes. External fertilization in water requires large numbers of gametes. Internal fertilization requires an investment in elaborate reproductive organs, including the penis, to transfer sperm to the female. Energy is set aside for nourishing some number of offspring. Those eggs with little yolk must develop larval stages quickly.