The cellular processes of sexual reproduction, namely meiosis and syngamy, are more time consuming than mitosis, which is type of division involved in asexual reproduction. Because there is a mixing of parental genes, favorable genes may be lost. Courtship leaves both parties vulnerable to predators because courtship displays are often conspicuous and elaborate. Both parties have a chance of catching a disease or parasites from their mate. Depending on the population density, an individual may find it difficult to find an available mate. The cost is doubled, because both parents suffer the cost for every one offspring produced.

Sexually reproducing populations evolve at a much faster rate than asexual reproducers due to gene recombination. Sexual reproduction increases rates of DNA repair, elimination of deleterious mutations, and accusation of favorable homozygous traits. Favorable traits will become prevalent and unfavorable genes will be selected out of the population at a faster rate.

Sexual selection favors male giraffes with longer necks because they are more likely to win a fight with another male, either over a desirable territory, dominance ranking, or directly for the female. Longer necks are favored because males will be able to fight with their heads while keeping their body undamaged if the opponent has a shorter neck. This is analogous to boxers with longer arms being generally more successful, because they can reach their opponents, but their opponents cannot reach them. Differential reproduction and survival has selected for the genes that influence neck length. Female necks have increased in length as a side effect of intrasexual selection as the genes were passed down. Males have proportionally longer necks than females because evolution directly selects for male neck length, while female neck length is only a byproduct.

Intersexual selection

Any male who displays a costly trait or ritual is a high quality male, and would make a good mate. Low quality males would not be able to bear the cost of such displays, and so the female can be reasonably certain her offspring will inherit "good genes" from the paternal line.