(CNN) — Most people learn about “the birds and the bees” when they’re young, but sex in the animal kingdom is not for the faint of heart.

For every peacock fanning out its dazzling feathers to attract a peahen, there’s another creature doing something strange, or downright deadly, in the name of passing down its genes.

This Valentine Day, meet the animals with love lives more complicated than yours.

Lovestruck marsupials on a die-hard mission

Related to the Tasmanian devil, the northern quoll is a small carnivorous marsupial that is the subject of a biological mystery. The males die after a single mating season, and no one has known why.

Now, researchers have put forth an explanation in a recent scientific paper: Male quolls are so sex-crazed that they die of exhaustion.

The quoll isn’t alone in kicking the bucket after one mating season — the phenomenon is a reproductive strategy called semelparity, present in animals such as salmon and praying mantis.

Joshua Gaschk, a doctoral candidate at the University of the Sunshine Coast in Australia, and his colleagues outfitted wild quolls in northern Australia with trackers to study how they move. (Getting a tiny tracker backpack on each animal proved difficult — Gaschk described the quoll as “a feisty animal that bites really hard.”)

After releasing the quolls, the researchers recaptured them to reclaim the tracking devices 42 days later — a time period that overlapped with the quolls’ mating season. The study team noticed something surprising: The male quolls had been moving far more than their female counterparts and resting only 7% of the time, compared with 24% of the time for females.

“Essentially, they’re trying to cover large distances to find more mates, and they’re doing so at the cost of their recuperation and resting period,” said Gaschk, the lead author of the study published in the journal Royal Society Open Science. Driven by sexual frenzy, this behavior might be the answer to why male quolls die after one mating season, while females survive for up to four, according to the researchers.

The tracker data appears to explain the die-off of male quolls, but competing theories disagree as to why an animal would evolve semelparity. Some scientists have posited that it frees up resources, while others have suggested having one or both sexes die after one mating season helps ensure they’re “all in” when it comes to passing on their genes.

Male anglerfish are the ultimate clingers

To survive without the resources of sunnier, shallower waters, deep-sea creatures have evolved into, to paraphrase Charles Darwin, endless forms most freaky. Take the anglerfish — you might remember it from “Finding Nemo,” with its bioluminescent lure and gnashing teeth. What you might not know is that that fishy villain must have been female.

That’s because in deep-sea anglerfish, the male is nothing more than “a little sperm-filled bag that responds hormonally to the ripened ovaries of the female,” said Ted Pietsch, a professor emeritus at the University of Washington’s School of Aquatic and Fisheries Science. “The females do all the hunting and the feeding. The males, their whole, sole purpose in life is to find a female.”

The male, which in some species is just one-sixtieth the length of a female, has oversize eyes and nostrils, the better to find a mate with. Once a male locates a female, he latches on with pincerlike teeth. Sometimes a male anglerfish will only hang on long enough to release his sperm and fertilize the female’s eggs. In other cases, the male sticks around.

“The longer he stays attached, the greater chance there is of the tissues of the male and the female fusing,” Pietsch said. “The actual cells meld, it’s like being soldered.” The partners sometimes even share a bloodstream, which helps sustain the male. Pietsch said anglerfish might have evolved this bizarre mating strategy to survive in the depths where food is scarce.

For years, scientists only knew these fish from deceased specimens brought up from the depths. Portugal’s Rebikoff-Niggeler Foundation published in 2018 the first footage of a live anglerfish couple, drifting in the deep off the coasts of the Azores in the North Atlantic Ocean. The diminutive male is hitched to the female’s belly, barely noticeable among her trailing illuminated tendrils.

Fire beetles win mates with toxic goo

You might be hoping for chocolates this Valentine’s Day. Female fire-colored beetles have a different gift in mind: poisonous goo from the head of a potential suitor.

Male Neopyrochroa flabellata beetles are attracted to a chemical called cantharidin. “Males eat the stuff like candy,” said Dan Young, a professor of entomology at the University of Wisconsin Madison. “They then sequester it away in their bodies, and they then transfer it to females when they copulate.”

A male entices a potential mate by secreting a cantharidin-laced substance from a gland in his head for females to sample. “She approaches the male head-on and literally tastes the stuff, and she’s basically asking the question, ‘Can you put out? Do you have enough cantharidin to satisfy my desires?’ And if he doesn’t, she won’t mate with him,” Young said.

Having eaten the goo, the female beetle yields to his advances and receives much more cantharidin in the male’s sperm package that she’s then able to secrete in coating on her eggs, making them unpalatable to predators. (The toxin, also called Spanish fly, has been used as an aphrodisiac, but it can also be deadly to humans in high concentrations.)

Though produced by blister beetles in other parts of the world, cantharidin doesn’t seem to be available in the wild where N. flabellata lives. “I wonder if there’s another compound out in nature that, from the point of view of the beetle, looks like cantharidin, and they’re getting faked out,” Young said.

While scientists attempt to solve the mystery of the beetles’ love of (and adaptations seemingly suited for) a substance they can’t produce or find in nature, in the meantime check out this earworm of a TikTok tune dramatizing N. flabellata’s bizarre mating behavior.

Snails swap mating roles and shoot ‘love darts’

For countless species, sex is not a simple male-female binary, with 30% of all animals in some way hermaphroditic when insects are excluded. Joris Koene, an associate professor of ecology at Vrije University in Amsterdam, studies the reproductive practices of land and freshwater snails, which he described as “male and female at the same time,” with each individual capable of producing both eggs and sperm.

Producing large cells like eggs takes up a lot of resources, but having each individual snail prepared to take on either mating role provides a key benefit. “As soon as you encounter another adult individual, it’s a potential mating partner,” Koene said.

When a pair of eligible snails meet, they sidle up to each other and get busy. During mating, the sperm-providing snail extends its penis out of a hole in its head called the genital pore and pokes it into its partner’s genital pore to transfer sperm. Some of the transferred sperm goes on to fertilize an egg, but the rest of it is diverted to the recipient snail’s digestive tract.

To help encourage more of that sperm to be used for fertilization instead of food, the sperm-donor snail has another trick up its sleeve.

Love darts are tiny spikes, made of the same calcium crystals as snail shells. During mating of some species, the snail providing sperm shoots its partner with these darts. The darts are coated in mucus containing proteins that act on the musculature of the recipient snail’s reproductive tract, ultimately leading more of the sperm to stay put instead of being moved to the digestive system.

And here’s a nice twist to the myth of Cupid’s arrow: “The amount of sperm that gets stored (for fertilization) depends on the quality of how well this individual can shoot its dart,” Koene said.

All-female lizard species procreate by themselves

Sexual reproduction provides genetic diversity that can lead to a healthier overall population, but it can be hard to pull off, especially when mates are scarce. When sex isn’t an option, some animals turn to an unusual method of asexual reproduction.

In parthenogenesis, or “virgin birth,” a female is able to fertilize her own eggs with a recombination of her own DNA. For some creatures, including zebra sharks, it can be a “Hail Mary pass” when no mates are available.

But a handful of animals, including several species of whiptail lizard, have evolved such that there are no males at all, and parthenogenesis is the only reproduction possible.

“There are some species that reproduce asexually but the males still serve some role, whether that’s to trigger ovulation or to trigger fertilization, but in these lizards, they don’t need the males for anything,” said Sonal Singhal, an associate professor of biology at California State University, Dominguez Hills.

These species of whiptails are 100% female.

The lack of sexual reproduction in whiptails doesn’t mean they don’t exhibit sexual behavior, though. Scientists have observed some species undertaking “pseudosexual” activities like female-female mounting, which may help promote ovulation.

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