Photograph by Darlyne A. Murawski, Nat Geo Image Collection
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These anglerfish don't look like they're mating, but that's exactly what's happening. The male attaches to the female below her tail and begins to disintegrate until he's nothing but testes.

Photograph by Darlyne A. Murawski, Nat Geo Image Collection

Sneaky Animal Sex—Are They or Aren’t They?

From corals to cichlids, some animals have sex you never even see.

Animal sex is dizzyingly varied, and some mating methods make us wonder if we’d be able to tell whether we’re looking at a love scene, a fight, or just a friendly “Hello!”

Check out these animals. Would you be able to tell what they were up to if you caught them in the act?


Terri Ducay photographed these millipedes in central Thailand and wondered, “[Are they] trying to kill each other or making love?”

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Are these millipedes fighting or mating?

Paul Marek of the Department of Entomology at Virginia Tech says love wins, because “the male’s head is more to the left of the frame.” He points out that the insect's seventh segment has modified legs, called gonopods, which are used to transfer the sperm to the female.

While the gonopods are on the seventh segment, the millipede’s sperm actually originates on the second segment. To transfer the sperm from the second to the seventh segment, the male bends his body so that his gonopods touch the openings where the sperm comes out, in effect “charging” them with sperm. Then he can insert the sperm-filled gonopods “into the female’s genital opening on her second segment.”

If human sex involved such acrobatics, there might be fewer of us.

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These giant millipedes in Costa Rica are mating, not fighting.


Barnacles are stationary crustaceans and hermaphrodites, yet all but one species—the gooseneck barnacle—require sexual intercourse to reproduce. So how do they achieve their goal?

“I think it's pretty obvious what mating barnacles are doing, but judge for yourself,” says Diane Kelly, an evolutionary biologist at the University of Massachusetts. We’d have thought they were just tapping one another and saying “Hello!” but those long appendages are the barnacles’ penises reaching out to fertilize someone. (Related: Barnacles Can Change Penis Size and Shape.)

Oysters and Corals

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When coral spawn, they release both eggs and sperm into the water at once in a fireworks-like display.

For some marine animals, you could mistake mating for fireworks or a balloon release at an underwater party.

Oysters and coral, for example, “spawn simultaneously, so it looks like they're all spewing fluid into the water at once,” Kelly says. They’re shooting both eggs and sperm into the water, and a chance meeting of the two means offspring.


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The male tarantula weaves a sperm pouch that he places in the female's abdomen in a reproductive act that looks more like a dance.

Tarantula mating looks more like a dance, squabble, or gift exchange than a reproductive act. In the western desert tarantula, the male weaves a sperm pouch and uses syringe-like parts of appendages near his mouth to place it in a pouch in the female’s abdomen. This behavior is the same for all tarantula species, says Shakara Maggitt, entomologist at the University of Georgia Cooperative Extension Program. The unromantic transaction—which is risky for the male, who has to worry about a surprise attack from the female—can produce around 1,000 spiderlings if successful. (See video: Tarantula mating game)

Deep Sea Anglerfish

A mating pair of deep sea anglerfish might not look like a “pair” of anything—more like a female with an extra fin. That’s because their mating consists of the tiny male biting into the large female and immediately disintegrating until he becomes little more than a sperm supply. (Related: Five Gross and Amazing Ways Animals Deliver Sperm)

African cichlids

Many cichlids are mouth brooders, meaning they keep fertilized eggs in their mouths to hide them from predators.

In the species Pseudorrenilubrus multicolor, the female mouth broods the eggs, and fertilization takes place there and in the open water.

A 2010 study details how the female scoops the eggs into her mouth as soon as she releases them. The male has spots on his anal fin that look like the eggs, which he flashes to the female while releasing his sperm. The female then snaps at the spots, which allows some eggs to be fertilized in her mouth—though some external fertilization is also possible.

We suppose it looks like a game of keep-away…which is the opposite of what sex really is.

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