Complex genitalia can result from different evolutionary mechanisms, though sexual selection is increasingly regarded as the primary force behind the evolution of genital diversity. Females respond to manipulating male strategies with behavioural counterstrategies to retain control over fertilization. ~ American evolutionary biologist Patricia Brennan et al
Nature has been inventive with genitals. They come in various shapes, sometimes with adornments.
Drakes have dicks shaped like corkscrews. Females have correspondingly long corkscrew vaginas, spiraling in the opposite direction.
Certain male ducks, especially mallards, are rare among birds in regularly being rapists. 40% of duck matings are forced.
The Lake Duck has the longest relative penis of vertebrates. Typically coiled when flaccid, the penis grows to half-duck length or longer when erect. This enormous spiny organ likely evolved in response to competitive pressure in these highly promiscuous birds: to the sperm of previous matings much like a bottle brush.
As an evolutionary counterstrategy, a female duck fends off sexual violence with maze-like genitals – featuring twists, pouches, and dead ends – that help her retain control of who fathers her offspring. Generally, there is such a poor fit between the male and female ducks’ genitals that a female must be relaxed and willing for mating to succeed.
The males of most birds forgo penises altogether. Instead, they have a cloaca: an opening much like a female’s. Mating is attained with a cloacal kiss.
Certain sessile barnacles pack the longest cock relative to body size. Among mobile animals, the deep-water greater hooked squid have the longest relative penis: as long as the mantle, head, and arms combined.
Many mammal penises, including mice, chimps, and felines, have keratinized penile spines. A cat withdrawing its penis rakes the walls of a female’s vagina, prompting ovulation.
Males of many insects have spiked penises that puncture and wound a female. Bed bugs and bat bugs bypass the vagina altogether. Instead, they pierce a female’s abdomen, directly inseminating the blood. Sperm then swim to the ovaries, guided by pheromones.
This has prompted counter-evolution. Female African bat bugs have paragenitals: a defense mechanism that limits damage by guiding a male’s sharp prick into a spongy structure.
Bat bugs are rabidly randy. A male may rape another male. Because of that, male bat bugs also evolved protective paragenitals.
Having a penis does not necessarily nix motherhood. Male seahorses and pipefish – not females – get pregnant.
Role reversal can be complete. In 4 closely related species of booklice, female possess a prickly penis (gynosome), while males have vaginas. The female is in charge of mating. A female inserts her erect gynosome into a male. Penal spines anchor her inside him. Unlike conventional penises, a gynosome is no squirter. Instead, it vacuums up a male’s sperm, which a female uses to fertilize her eggs.
Booklice can copulate for 70 hours non-stop. Besides sperm, male spermatophores provide nutrients. The female does not to let go until she has had a satisfying meal.
Female booklice packing penises is highly irregular, but not altogether unique. An extinct species of mite had a similar genitalia scheme, though without the anchoring spines.