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(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.
A tiny male anglerfish clings to the larger female fish's belly. Once a male locates a female, he latches on with pincerlike teeth.
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."