These "exotic" behaviors are almost always the result of sexual selection. This evolutionary pressure occurs when one sex (usually females) chooses mates based on specific traits, or when members of one sex compete for access to the other.

: In one of the most extreme cases of sexual dimorphism, the tiny male bites into the much larger female and eventually physically fuses to her body, losing his organs until he is essentially just a source of sperm.

: These hermaphroditic creatures engage in "penis fencing." They attempt to stab each other with their bifurcated reproductive organs; the loser is "inseminated" and must take on the energy-draining role of the mother. Argonaut (Paper Nautilus)

: This small marsupial engages in a two-week "mating frenzy" where males mate for up to 14 hours at a time until their bodies literally shut down from stress.