The dolphin has one that is spiral-shaped and filled with trap doors. The ring-tailed lemur’s disappears, and the opossum has three.

Welcome to the wild, weird and woefully ignored world of animal vaginas.

There are vaginas that look nothing like vaginas, such as the spider monkey’s pseudo penis or the hyena’s 8-inch clitoral tube out of which she pees, copulates and gives birth.

“Biologists love penises,” writes Rachel Gross in “Vagina Obscura” (Norton), noting that the vagina has typically been given short shrift by animal researchers. “They’re also easy to study. Penises kind of hang out.”

The field has suffered from the long-held belief that vaginas were mere passive receptacles — a bias that extended to female animals in general. Gross calls this “vagina neglect.” Mount Holyoke biology professor Patricia Brennan, who has studied vaginas for nearly two decades, calls it “the copulatory black box.”

The 2014 study “Genital Evolution: Why Are Females Still Understudied” found that 49% of papers published in the last 25 years only studied male sex organs, compared to 8% that focused solely on female anatomy. “We argue that understanding genital evolution is hampered by an outdated single-sex bias,” the paper reads.

In the last decade, however, there’s been an overhaul of the study of animal genitals, as long-established beliefs aren’t standing up to scrutiny.

Vaginas are finally having their day in the sun — especially in the rollicking new book “Bitch: On the Female of the Species” (Basic Books) by Lucy Cooke, out now.

“In the natural world female form and role varies wildly to encompass a fascinating spectrum of anatomies and behaviors,” writes Cooke, an author, TV personality, zoologist and self-proclaimed explorer. This new understanding of female anatomy “redefines not just the female of the species, but the very forces that shape evolution.”

Grandfather of evolutionary biology Charles Darwin wrote that “genitals do not concern us” and largely avoided the topic. Female animals were then viewed as less evolved versions of males — smaller, more immature, less aggressive. Females upheld the Victorian standards of the era, or “the coy female,” as Darwin described. Textbooks on animal behavior wrote about the “reluctant female and the ardent male.”

Amidst mounting evidence to the contrary, Colombian biologist Patricia Brennan started studying vaginas.

She began with the mallard. After finding a spiral shaped, wormlike penis in blue-gray birds called tinamou in Costa Rica (when 97% of birds have no external genitals), she turned her attention to the duck. When she discovered the fowl’s huge corkscrew-shaped phallus and the disturbing sex act that came along with it, she nearly fell off her chair. It made her wonder — what did the corresponding vagina look like?

She found that they were just as elaborate, filled with byzantine crevices and blind alleys. During regular copulation, the female anatomy works counter-clockwise to the male’s. When copulation is forced — which it often is — a female duck’s elaborate anatomy protects them with tooth-like cavities.

Brennan’s duck work “transformed scientific thinking and rehabilitated the female from passive victim to active agent of her own evolutionary destiny,” writes Cooke.

But duck sex was just the “gateway drug.” Other animals followed, including the dolphin which has strikingly similar spiral shaped sex organs. “Convergent evolution with a duck!” Brennan said to Cooke. “It’s nuts!”

Like ducks, female dolphins can shift their body and force a penis into a blind alley. “Females have evolved creative ways to control the insemination of their eggs, even when males are more powerful, numerous or forceful,” Cooke writes.

The battle of the sex organs often has wide evolutionary implications. When male mosquito fish evolve longer genitals, for example, the females’ brains actually grow bigger “to outwit their aggressors.”

Eventually, Brennan set herself a new goal to compile the world’s first physical library of animal vaginas.

“So many vaginas, such a short life,” Brennan told Cooke.

Her investigations revealed how adaptable the organ is: for example, the female privates of the spiny dogfish change shape, becoming more asymmetrical in pregnancy to accommodate pups. But nothing is quite as remarkable as the opossum, which has a surprise vagina that disappears “like a secret door” after giving birth. Opossum also have two uteri, two ovaries, and two other vaginas.

There are female moles with “balls,” called ovotestes, that consist of both ovarian and testicular tissue. The ovaries produce eggs and the testes make testosterone, but not sperm, to beef up the mole’s aggression and body mass. These parts of the body shrink and expand according to need and breeding season.

Then there are all the fake phalluses. Madagascar’s Fossa, which Cooke describes as a “puma with a shrunken head,” actually grows an “internal bone” to look like a male penis, even exuding a yellow liquid on its underside. The pretend penis disappears when the fossa is old enough to mate. Researchers believe it evolved to “protect them from unwanted attention from older males and territorial females.”

The ring-tailed lemur also has a pseudo-scrotum and an extended clitoris that is nearly identical to a lemur penis. They could, if they wanted, “write their name in the snow,” joked one lemur researcher.

Genitals evolve faster than any other body part, Cooke writes, and Brennan and other researchers believe that females actually play a greater role in genital natural selection than males, thereby acting as more active agents of evolution than any of the earlier, male-centric researchers could have imagined.

Another interesting finding of Brennan’s is the dolphin’s “enormous clitoris” that is “dense with erectile tissue and blood vessels and shaped remarkably like a human clitoris.”

Though researchers have long witnessed sexual behavior in dolphins outside copulation — rubbing clitorises against other dolphin’s noses, for example, or against objects on the sea floor, no one had gone the extra step to study the function of the clitoris.

Brennan and colleagues dissected dolphin clitoris samples and found webs of nerve endings and spongy tissue that enabled swelling in areas easily accessible to the outside. Interestingly, dolphins are far from the only animal with clitorises — all mammals have one.

Everyone knows about the penis-fencing bonobo male monkeys, but what about the huge “cantaloupe-sized” clitoris of the bonobo female? Some bonobo researchers believe that the clitoris — in a front-facing position like a human’s — “facilitate[s] mutual stimulation with other females.” Bonobos engage in genital-to-genital rubbing with other females, and will often choose this act over sex with males.

These animals are engaging in sex acts outside of the need to make babies. Sex, therefore, “serves richer and more complex purposes than solely the transfer of sperm from one party to another . . . It can be used to strengthen friendship and alliances, make gestures of dominance and submission, as part of social negotiations,” writes “Vagina Obscura” author Rachel Gross.

For decades, female sex outside copulation was dismissed as aberrant, along with female aggression. Lots of researchers “looked the other way if they encountered philandering females.” The reality is that only 7% of species are sexually monogamous.

A female lion will mate up to 100 times a day with multiple male suitors during heat. Chimpanzees border on “nymphomania, especially when ovulating” — and animals like orangutans and marmosets have sex throughout their cycle when there’s no chance of getting pregnant.

The lopi antelope are a great example of how bias has clouded research. When Darwin noticed the female’s powerful horns, he dismissed it as a “waste of vital power.”

Boy, was he wrong. Females are not only randy, they’re sexual aggressors. During their short fertility window, “it’s not uncommon to see males collapsing from exhaustion as the demands of the female get too much for them,” writes Cooke.

Females duke it out for the finite sperm reserves and will even charge a “top stud” to stop the act of copulation with another female. As a result, the male takes on the “female’s traditionally choosy role in order to conserve precious sperm.”

 “Females aren’t destined to be passive and coy, evolutionary afterthoughts just waiting to be dominated by males,” writes Cooke. Learning about the vast differences in female form and function “leaves me empowered by the boundless possibilities of the female experience.”

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