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Most people have at least a passing familiarity with sperm. Like, you’re probably picturing a tadpole-like thing with a head and a wiggly tail.

And that's a pretty good approximation of most mammalian swimmers. Sperm across the animal kingdom all have basically the same job: getting to the egg and fertilizing it. But they have the most diverse shapes and sizes of any cell type.

That’s because competition doesn't end with finding a mate — sperm duke it out inside reproductive organs. And that can lead to some really weird adaptations! In fact, while researchers have described a lot of strange sperm, we don’t always have full explanations for how they came to exist.

So here are 9 kinds of sperm that might make you think twice about what you know about reproduction. All sperm are small — you need a microscope to see them. But that doesn’t mean they’re the same size across species.

For instance, human sperm is about 50 micrometers long, while porcupine sperm averages around 28. But the current record holder for longest sperm is the tiny fruit fly. In one species, sperm have been measured up to 5.8 centimeters long.

Sure, most of that length is the tail. But still, that's over 2000 times longer than that porcupine sperm and about 20 times the body length of the actual fly! Over the years, scientists have measured sperm in lots of different animal species.

And they’ve found that sperm size doesn’t scale with the size of the animal that’s making it. In fact, the opposite seems to be true: in general, smaller animals produce bigger sperm. In larger animals, the females have longer, more voluminous reproductive tracts.

So that means there’s a greater risk that sperm might not make it to an egg, or even get lost in a sea of sperm from other males. According to the sperm dilution hypothesis, the best strategy in this case seems to be making a whole bunch of small sperm to compete. But smaller animal species seem to produce longer or bigger sperm.

Smaller female reproductive tracts make for smaller battle arenas where sperm can push each other around. In the case of female fruit flies, scientists have found that part of their reproductive tract called the seminal receptacle has evolved to be long and coiled in certain species. So sperm cells have coevolved to match.

The females essentially tilt the playing field towards healthier males who have the nutrients and energy to make longer sperm. They’re able to successfully beat out the competition and fertilize more eggs. So, I guess, size does matter for them.

There’s also a lot of variation in sperm shape. Human sperm, for instance, has three main parts: There’s the head, which contains all the genetic material and a packet of enzymes that helps the sperm penetrate the egg. Then, there’s the midpiece.

It’s packed with mitochondria — cellular structures that are basically tiny engines. And that energy powers the third part: a tail that propels the sperm forward. Human sperm, and a lot of mammalian sperm, has kind of a conehead.

But another common sperm shape is a corkscrew head, which sometimes extends through the midpiece. A wide variety of animals have this helical sperm — like, many birds, some beetles, centipedes, and frogs. And even though this twisty shape evolved multiple times, scientists aren’t exactly sure why.

One hypothesis is that it might help with movement, letting the sperm swim forward like tiny turning drill bits. But not all sperm swim! Some species of worms called nematodes have sperm without tails.

Their sperm crawl kind of like amoebas do, using pseudopodia, or fake feet. In amoebas, coordinated groups of microfilaments made of the protein actin extend the pseudopodia, which stick to a surface and then retract to drag their cells along. Nematode sperm crawling looks really similar.

But their pseudopodia are formed by filaments made of major sperm protein instead. By comparing the ways these proteins work, scientists have learned more about different mechanisms cells use to move. And they think these sperm might be better off crawling because nematode fertilization is a tight squeeze.

In nematodes, fertilization occurs in a sperm holding chamber called the spermatheca near the ovary or ovaries. To get there, nematode sperm need to make their way past the uterus. But as they crawl there, they’re working against a tide of already-fertilized eggs that are moving down into the uterus, like bowling balls through a chute.

So maybe an army crawl just works a little better than a flutter kick. Sometimes those army-crawling nematode sperm get a bit overzealous, though. In one nematode genus, some species have a lot of sperm competition.

And scientists think that it benefits those males if their sperm get as close to the ovary as possible, to try and intercept an egg. This favors adaptations that lead to faster, more vigorously crawling sperm — although scientists aren’t quite sure what those adaptations are right now. But that intensity can be unpleasant for their partner.

If the sperm overshoot their mark, they might enter the ovary, or even bust out of the reproductive tract into the body cavity and wreak havoc. So female reproductive tracts have co-evolved to be tougher and withstand the roughhousing. But sometimes, things get confusing in a warm, dark compost pile, and nematodes from two different species accidentally mate.

In a series of experiments, scientists found that if a species with lots of sperm competition mates with a species with less competition, there can be a mismatch between sperm and reproductive tract. In one such cross-species matchup, the aggressive sperm entered the ovary 90% of the time and broke through the female reproductive tract 7% of the time. So… ouch.

Other sperm competition involves some tricky tactics outside the body too. For instance, some flatworms are simultaneous hermaphrodites, which means they have functional male and female reproductive organs. They have sex by reciprocal mating, where the partners take turns donating and receiving sperm.

Which seems pretty cooperative... but at least one species doesn’t really play fair. Frequently after mating, one flatworm will lean down and try to suck the sperm out of its own female reproductive tract opening to get rid of it. Maybe it was happy enough to fertilize that other flatworm’s eggs to pass on its genes.

But it’s saving its own precious eggs for a higher quality mate, or just saving its energy. But because of this two-timing behavior, the sperm has a trick up its sleeve too. This flatworm’s sperm has two stiff, backward-facing bristles on the head that act like barbs on a fish hook.

They snag in the opening and make the sperm difficult to suck out — so it sticks around and fertilizes those eggs. Other sperm use teamwork, because, y’know, it makes the dream work. In certain species of mice, for example, sperm form large swim teams to get to the egg.

These sperm have a hook on their head that can grab onto the head or tail of fellow sperm. So they make a kind of disordered train-like structure. Some scientists wanted to put deer mice sperm trains to the test, so they created a mathematical model and also tested the swimmers under a microscope.

And they found that the sperm trains don’t necessarily swim faster, but they do swim straighter than a solo sperm. That’s because the wobbly movement of one sperm cancels out the wobbles of its partners — kind of like all the hands on a Ouija board planchette. So they take a straighter path to the egg spand get there sooner.

But if too many sperm link up, the train gets a derailed and they start working against each other. In fact, according to that study, the ideal number for a deer mouse sperm train is a lucky. Now, we tend to think of sperm as short-lived.

In humans, for instance, if sperm make it to the fallopian tubes, they may bind with the walls and wait for a egg to be released. But they only hang around for about 5 days max before they get broken down. That’s because sperm are stripped down DNA delivery packages.

They don’t have the usual cellular machinery that allows most cells to make proteins and repair themselves for a while. But in some species, sperm cells live a long time — much longer than the males that produced them. In some insects, like leaf cutter ants and honeybees, the queen usually has one very brief flurry of mating early in her life and then goes off to found a new colony.

During the nuptial flight, a female honeybee mates with 7 to 15 drones in just a couple hours — which sounds like a whole lot of love, but it’s a little… deadly. Males only mate once, because when they ejaculate, their endophallus gets ripped off, which kills them pretty quickly. The queen stores millio ns of sperm in her spermatheca, and can use it for the rest of her life — 3 or 4 years later.

In honeybee queens, scientists have found that they nourish the sperm as if it were part of their own body. The fluids inside the spermatheca provide the stored sperm with the proteins they need for repair and maintenance. Plus, there are antioxidant enzymes that may protect sperm from damaging oxidative stress, so they have less need to repair themselves in the first place.

Sounds pretty cozy in there! But if all this sperm is chilling in a spermatheca together, things aren’t all fun and games. The battle continues inside this chamber.

In one species of leafcutter ants, scientists found that if you expose one male’s sperm to another male’s seminal fluid, the sperm died more quickly. They think that proteins in the seminal fluid poison a rival’s sperm. But even with this sperm competition, it probably benefits the queen ant to have her offspring sired by more than one male.

She doesn’t want to put all her eggs in one basket, so to speak. More genetic diversity is good! Plus, those sperm need to last her for potentially two decades, so she can’t have them all poisoning each other.

So the researchers discovered that the queen provides an antidote. When they exposed one male’s sperm to seminal fluid from another male and added fluids from a queen ant’s spermatheca, all the negative effects were neutralized. Sperm competition can definitely lead to odd sperm adaptations, but lack of competition can also lead to some weirdness.

In species that are more monogamous, sperm of different males aren’t usually in direct competition. This means that quality control may not be such a priority. Take humans.

We’re generally pretty monogamous compared to many species. And, according to some estimates, about 90% of the sperm in human ejaculate are deformed — they might have two heads, two tails, pin heads, or other things. And this is totally normal.

But, take comfort, because we don't have the worst sperm. That title probably goes to the naked mole rat… because apparently everything about these rodents is weird. Naked mole rats are eusocial animals, which basically means they live in really tight-knit, cooperative colonies.

Only one female in the colony reproduces and she chooses from one to three males to mate with for a lifetime. This means that there’s almost no sperm competition. Scientists have found that only about 7% of naked mole rat sperm are normal and functional — most have a deformity.

And only up to 15% of a naked mole rat’s sperm can even swim. Despite sounding bad, this wonky sperm is honestly just practical. Producing perfect sperm is costly in energy — so why bother if they don't need to be perfect to be successful.

Thanks for watching this episode of SciShow, which is a Complexly production! If you’d like to learn even more about sperm in humans and all things sex, check out one of our sister channels, Sexplanations, over at youtube.comsexplanations. [ Outro ].