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5 Fascinating Animal Mating Behaviors

animal mating reproduction banana slugsWe'll assume you've gotten the birds and the bees talk, so without ado: Animals reproduce, usually (though not always) through sex. And while that might evoke scenes of cuddly coitus, à la March of the Penguins, procreation isn't always so pretty. It can be strange, scary -- even downright deadly. But behaviors that might seem bizarre in the boudoir are actually completely normal in nature. Today we bring you five fascinating animal mating habits and explain why natural selection might sometimes favor the kinky. 

Banana slugs:  They might look cute, but these bright yellow critters, which inhabit damp, coniferous forests along the north Pacific coast of the U.S., take tough love to a new level. They’re also enormously endowed. As an adult, the banana slug measures roughly 6 to 8 inches in length -- and so can its penis when erect. The organ emerges from a genital pore on its head.  Since slugs, like their snail cousins, are hermaphrodites, the banana slug also has female organs.

Banana slugs begin their tryst with some rough foreplay, lunging, biting, and hitting one another with their tails. Then they curl around each other, like two chubby, slimy commas, and insert their penises. Sometimes one partner gives sperm while the other receives, but usually they exchange sperm. The partners can remain enjoined for several hours. Normally, they then retract their organs and crawl on their merry way. But things can take a grisly turn.

In some cases, one slug chews off the other's penis, or both partners munch each other's organs. Some scientists believe this behavior, called apophallation, developed as a quick, last-ditch escape tactic in case the slugs get stuck. Others, like biologist Beth L.W. Miller, think that it might have evolved as a strategy to outcompete other slugs in passing on their genes.

Let's say one slug chews off its partner's penis, making that partner incapable of inseminating any other slug. That means the partner might not look for another mate, or if it does, perhaps it'll get rejected because it lacks a penis. In either case, the first slug ensures that its sperm, and no other slug's, fertilizes its partner's eggs. In the end, the first slug has a better chance of passing on its genetic material.

Still, Miller's research has yet to conclusively prove her idea, and the advantage that apophallation gives slugs that sever each other's penises remains unclear. While scientists continue to ponder banana slug penis-chewing, you can watch a video of this phenomenon on Miller’s website.

Image courtesy of Wikimedia Commons


argonaut paper nautilus animal matingArgonauts/paper nautiluses:  Argonaut octopuses drift through the open seas in tropical and subtropical regions. Females can reach 10 centimeters in size and bear a brittle, translucent shell, which is why argonauts are also known as paper nautiluses. Meanwhile, the male rarely measures more than two centimeters and lacks a shell. But don't let his diminutive appearance fool you. What he lacks in size, his penis makea up for in agility.

As with many octopuses, the male argonaut's third arm doubles as a penis, called a hectocotylus, which detaches during mating. But the argonaut's hectocotylus is autonomous, and once disembodied, it may travel independently to the female.  

argonaut paper nautilus animal mating hectocotylusThe hectocotylus embeds in the female's pallial cavity to deposit sperm and can remain there long after mating. Plugging the cavity in this way possibly helps blockade it from other hectocotyli, ensuring that the embedded appendage's "owner," and no other male, fertilizes the eggs. But scientists don't know for certain whether this really does give males a reproductive advantage.

Due to argonauts' elusive nature, few have been captured and kept in captivity, or even studied at sea, according to James Wood, a marine biologist and webmaster of the Cephalapod Page. As a result, we know almost no details about argonaut mating habits, but "we can make educated guesses based on anatomy," Wood said.

What becomes of the now penis-less male argonaut?  Fortunately, he sprouts a new hectocotylus every year.

Fun fact: French naturalist Georges Cuvier, who first noted the argonaut’s hectocotylus lodged in a female, mistook it for a parasitic worm.

Images by Julian Finn, Museum Victoria 

Animal mating reproduction New Mexico whiptail lizardNew Mexico whiptail lizard: New Mexico whiptail lizards don't need males to reproduce, thank you very much. Found in semi-arid regions of the U.S. Southwest and parts of northern Mexico, this whiptail species reproduces through a process called parthenogenesis. While embryonic development usually requires genetic material from sperm and egg, the parthogenetic New Mexico whiptail lays eggs that develop into daughters on their own, without sperm. Male offspring aren't viable, which means that all New Mexico whiptails are female.

How does parthenogenesis work? Normally, a type of cell division called meiosis produces sperm and egg cells. Meiosis doubles a sperm or egg cell's chromosomes (which are composed of DNA) before the cell divides twice, resulting in four daughter cells, each with half the chromosomes of the original. The union of sperm and egg results in a cell with a full set of chromosomes, which then develops into an embryo. Asexual whiptails get their full dose of chromsomes by replicating their egg chromosomes twice, producing eight daughter cells. During meiosis, some of the chromosomes swap segments, so that the offspring aren't quite perfect clones of their mother.  

Scientists think that reproduction evolved as a means of diversifying the gene pool to help reduce vulnerability to disease. Still, parthenogenesis could offer an advantage if the mother thrives in her habitat, since her offspring, nearly identical to her, will also be well adapted.  

But the New Mexico whiptail -- which emerged as a hybrid of two sexually reproducing lizard species -- isn't the only animal that procreates this way. In fact, some 70 vertebrate species reproduce parthenogenetically, including Komodo dragons and bonnethead sharks. What makes the New Mexico whiptail unique is its participation in "pseudocopulation," where two lizards simulate sex as if one were male.   

While pseudocopulation isn't necessary for New Mexico whiptails to reproduce, it does seem to help. Researchers have observed that lizards that engaged in this playacting laid a larger clutch of eggs than those that didn't.

Image by Michael Benard, Natural History on the Web

Animal mating reproduction anglerfishAnglerfish: Think a profession of undying love on the first date sounds creepy?  You might not have heard of the male anglerfish.

Anglerfish roam the ocean at depths of about 6,000 feet. It's the female we're most familiar with -- her ghoulish, gaping mouth and dangly, luminescent lure. Females can measure up to three-and-a-half-feet long. Males are tiny in comparison, reaching a measly two-thirds of an inch in length. They live solely to seek and mate with a female.

As if plumbing the deep, pitch-dark sea for a mate isn't tough enough, male anglers have slim pickings, outnumbering females 30 to 1. Fortunately, they come equipped with highly sensitive olfactory organs that sniff out female pheromones. (Relative to body size, the male angler's olfactory organs are the largest of any vertebrate.) When a male finds a female, he opens his jaws and clamps down on her -- without ever letting go.

As soon as the male bites his mate, he releases enzymes that digest his mouth and eyes and cause his skin and even his blood vessels to fuse with hers. The male dissolves to not much more than a parasitic lump. His gonads, spared from absorption, release sperm whenever the female is ready to spawn. A female angler preserved in the Scripps Institution of Oceanography's library of fish in San Diego, California, shown above, has a male latched to her belly.

The anglerfish's relationship might sound nightmarish to the commitment-phobic but, in the end, both partners benefit. Because the male angler has a stunted digestive system, he needs to rely on a female for nutrients, which flows through their shared circulatory system. So the male gets fed, allowing him to swell up to six inches in length, and the female gains a readily available sperm supply.

Image by Marc Tule

Animal mating reproduction praying mantisPraying mantis: The praying mantis's lovemaking begins tenderly enough, with each partner stroking each other's antennae and engaging in ritual dancing. Once foreplay ends, the male mounts the female's back and begins mating. About a third of the time, she bites his head off.   

Meanwhile, the decapitated lover continues thrusting, and in fact, losing his head causes him to ejaculate faster. A mass of nerve cells, or ganglion, located in the male's abdomen controls these  copulatory motions, while the subesophogeal ganglion in his head inhibits these movements. Losing his head therefore allows his abdominal ganglion to generate thrusting motions uninhibited. 

Until recently, scientists thought that females always beheaded their mates. It turned out that the laboratory study that led to this conclusion used starving females. In the wild, not all females mate on an empty stomach. Of course, the more hungry a female is, the more likely she will cannibalize her partner. But the female of one praying mantis species, Mantis religiosa, found throughout the northeastern U.S. and Canada to the Pacific Northwest, must sever her partner’s head for mating to occur properly.

Biologists have proposed multiple theories for why natural selection has favored sexual cannibalism. Some, for example, suspect that females simply mistake their partners for prey. Others think that  serving himself to a female increases a male's chance of passing on his genetic material. His carcass could feed the female, helping to ensure that their offspring hatch, mature, and procreate, thereby transferring his genes to the next generation.

Those who find the praying mantis's sexual appetites more fascinating than frightening can watch a video of a fatal tryst below.


Image by iStockphoto/yumenoginga1227

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HS_Melissa_BLOGMelissa Pandika is an editorial intern at Sierra and a graduate journalism student at Stanford University. Her interests include environmental health and justice, urban environmental issues, and conservation biology. She has a soft spot for cetaceans.

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