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All s***m perform the same basic job: They fertilize egg cells. But in a new study, researchers have figured out that size matters, and it's largely the female that pushes s***m to be big or small.

S***m cells come in a huge variety of sizes. For instance, the parasitoid wasp Cotesia congregata produces little swimmers that are less than one-thousandth of a centimeter long, while fruit flies make s***m with 2.3-inch (6 cm) tails that coil tightly to fit inside their tiny bodies.

In the new study, the researchers set out to determine how s***m size varies among species and what might be driving the differences.

"We have all these studies that show evidence of natural selection pushing s***m size in various species to be either bigger or smaller, but we wanted to take more of a zoomed-out view and look for trends across species," said lead author Ariel Kahrl, a postdoctoral researcher in evolutionary biology at Stockholm University.

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Kahrl and her colleagues examined data from 3,200 species and discovered a governing principle that determines s***m size in a species: Females with small reproductive tracts drive the production of bigger s***m, and the need to spread s***m far and wide shrinks s***m across evolutionary timescales.

Here's why. For the most part, animals use two modes of sexual reproduction. One group — which includes mammals, insects and birds — are internal fertilizers that carry eggs inside their bodies. External fertilizers, by contrast, eject their eggs into the environment and hope for the best. Commonly, these species live in water, like fish and sea urchins. In both modes, tons of s***m are competing in a battle royal for the prize of fertilizing the egg, but the challenges of each mode exert incredible evolutionary pressure on s***m size.

"We found that external fertilizers tend to have really small s***m because they have to make a ton of it to reach the eggs," Kahrl said. External fertilization requires ejecting a cloud of s***m, typically into water. As the s***m spread, they become diluted, so the best strategy would be to produce as many s***m as possible to maximize the chance that at least one will reach an egg. Because an animal has a limited amount of energy to use for making s***m he can't afford to make them any bigger than they absolutely have to be.

It's a completely different situation for the internal fertilizers. "We think that for internal fertilizers, the female's reproductive tract influences the way s***m fight each other," said study co-author John Fitzpatrick, an assistant professor of biology who is also at Stockholm University. In internal fertilization, the s***m work in a tight space, so reproduction becomes less of a treasure hunt and more like a game of king of the hill. In this situation, bigger may be better for shoving other s***m out of the way, regardless of whether they came from the same father or different potential fathers.

"Some of these species make huge s***m, and if you're making enormous s***m, you don't make that many of them," Kahrl said. "These males coil up their s***m like a ball of yarn and pass it along."

In addition to internal and external fertilizers, the researchers examined a rarer third reproductive mode, called s***mcasting. S***mcasting is like a combination of internal and external fertilization; for example, a river mussel might eject s***m into a stream, and that s***m would ride the currents until it is picked up by a stationary, filter-feeding female.

"With s***mcasting, you have this dilution effect because the s***m are ejected into the water, but when the s***m enter the female, they evolve rapidly under the same types of pressures that we see in internal fertilizers," Fitzpatrick told Live Science. The s***mcasters, though, have smaller swimmers, similar in size to the s***m of external fertilizers, likely because ejecting s***m into water incentivizes making more of them, forcing them to be small. But once those s***m are taken up by the female, the biggest s***m tend to win.

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Despite being internal fertilizers, humans don't have monster s***m. Instead, human s***m measures a modest 0.002 inches (0.005 cm) long, well within the range seen in external fertilizers. That's because animals with bigger bodies have reproductive tracts that allow the s***m to spread out similarly to the way external fertilizers' s***m do.

The smaller the reproductive tract, the larger the s***m. And for a fruit fly, it's as cramped as it gets. "Fruit fly s***m is 20 times the length of the animal's body," Kahrl said.

One new baby is a handful — but what about two or more? One in 30 parents have their hands full with twins, and nearly one in 10,000 juggle triplets or more, according to U.S. data from the Centers for Disease Control and Prevention.

But how unusual is humans' ability to bear multiple young at once, and how often do other mammals have twins, triplets and quadruplets?

In many animals, multiple babies are the norm rather than the exception. For example, the average dog gives birth to five puppies in a litter, according to a 2011 study published in the journal Theriogenology. So, what's the difference between a litter and twins? "It really just comes down to number," said Charles Long, a reproductive physiologist at Texas A&M University. Whereas a set of twins is by definition two babies, litter-bearing animals almost always have more.

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A female animal has a litter when she releases multiple eggs. When fertilized, these eggs produce multiple embryos. (Identical twins are the exception — but we'll get to that later.) When a woman releases two eggs, we call the resulting offspring fraternal or non-identical twins. (Three eggs results in fraternal triplets, four in fraternal quadruplets. You get the picture.) Whether they're littermates or fraternal twins, these siblings share about half of their DNA — the same as any other sibling pair.

Litter-bearing animals tend to give birth to more than three babies, on average, Long said. But the difference between litters and twins ends there. Technically, there's no difference between a litter of five and a set of fraternal quintuplets — aside from the fact that human quintuplets are exceptionally rare. In 2017, the Centers for Disease Control and Prevention reported only 49 quintuplet and higher order births out of 3,855,500 total births.

On the other hand, some animals rarely give birth to litters, and instead nearly always bear two young with each pregnancy. In these animals, two babies born at the same time are called twins, not littermates. However, these are usually fraternal, not identical twins. Sheep, goats, and deer all regularly give birth to fraternal twins, Long said.

But what about identical twins?

These genetically identical siblings form when a single fertilized egg splits in two. They're rare in humans: about three to four in 1,000 human births result in identical twins, according to the U.S. National Library of Medicine. And as far as scientists can tell, they’re less common in other animals. Veterinarians have identified identical puppies only once, in 2016. In that rare instance, a veterinarian performing a C-section on an Irish wolfhound happened to notice that two of the puppies shared a placenta. A genetic test confirmed that the two puppies shared all of their DNA. But scientists usually aren’t on the lookout for identical twins in other animals, which may be why they appear so uncommon.

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In one animal, identical siblings are the rule, not a rarity. Armadillos always give birth to identical quadruplets. In other words, female armadillos ovulate one egg that subsequently splits into four once fertilized. Scientists aren't sure why this happens, or why it's unique to armadillos, Long said. One possible explanation: having identical quadruplets prevents inbreeding. Armadillo siblings can't possibly mate with one another, so they're forced to venture outside of their underground burrows to find mates.

And other animals may give birth to identical twins at higher rates than humans realize. "If a sheep has twin males or twin females, we don't go and test them to see if they're identical," Long said. The uncanny resemblance shared by identical twins isn't a helpful clue, he added. "You know, sheep look alike."

"This is the first creature, the first animal that we're aware of that actually grew a skeleton," Wilby said. Its te****le structure hints that A. attenboroughii likely fed on plankton and protists, which would make it the earliest known predator in the animal kingdom.

A. attenboroughii shares many core characteristics with Cambrian fossils of Medusozoa, a group that includes modern jellyfish and other animals that transform into free-swimming, bell-shaped creatures for part of their life cycle. "That's what leads us to believe that it is a Medusozoan," Wilby said. While the fossil might not look like a jellyfish at first glance, it's important to note that, for part of their life cycle, neither do Medusozoans. For a chapter of their lives, the animals anchor themselves to the seafloor to reproduce asexually. During this life stage they resemble anemones — and A. attenboroughii.

If A. attenboroughii is indeed a member of Medusozoa, it would belong to a broader group of organisms known as the cnidarians, which also includes corals, sea pens and sea anemones. Prior to the new study, fossil evidence suggested that the basic "blueprint" for cnidarians didn't emerge until the Cambrian period. However, "what we're able to show here is that, at least 20 million years before that, the blueprint for cnidarians was actually set," Wilby said.

This not only pushes back the evolutionary history of cnidarians but also provides hints about what animals must have come before them, Donoghue said. Prior research suggests that cnidarians and bilaterians — a group of animals that includes humans — split off from a common ancestor. If A. attenboroughii existed 560 million years ago, it’s possible that the split already occurred and the earliest bilaterians were already roaming the planet.

"The fossil's not just important for showing us, clearly, cnidarians are here — by implication, their sibling lineage must have also evolved by this time," Donoghue said.

Ask anyone what the fastest animal on Earth is, and they'll probably say the cheetah. But the focus on the speedy feline has stolen attention from other species that go much faster — some three or more times faster than the cheetah. Who are the overlooked speedsters of the animal kingdom?

To be clear, the cheetah (Acinonyx jubatus) is undeniably fast. And it is true that it's the quickest animal on land. With documented top speeds of 64 mph (103 km/h), the cheetah easily surpasses other swift animals, like racehorses, to take the title of world's fastest land animal. And some estimates of their top speed are closer to 70 mph (113 km/h), according to the Smithsonian National Zoo & Conservation Biology Institute.

A combination of leg length, muscle size and a long stride gives the cheetah the ideal body for running across land, said John Hutchinson, a professor of evolutionary biomechanics at the Royal Veterinary College in London. Plus, a 2017 model(opens in new tab) based on 474 land and marine species, ranging from whales to flies, demonstrated that speed is closely tied to size. Speed increases with size until you reach an optimum. Beyond that optimum, larger animals are slower because they require more energy to accelerate. A cheetah has the optimal medium size for speed, Hutchinson said.

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However, cheetahs are only the fastest animals on land over short distances. That's because they don't pursue prey at high speeds for long distances. Their hunting strategy is more about accelerating and maneuvering very quickly, according to a 2013 study in the journal Nature(opens in new tab). In essence, their endurance is limited. "Cheetahs, like most cats, aren't pursuit animals," Hutchinson said. No other land species can get to 70 mph, or even 64 mph, but the pronghorn antelope (Antilocapra americana) is estimated to reach 60 mph (97 km/h) and can sustain a speed of 45 mph (72 km/h) for miles, according to the book "Built for Speed: A Year in the Life of Pronghorn (Harvard University Press, 2003).

Once you include marine and avian animals, the competition really heats up. The dive speed of peregrine falcons (Falco peregrinus) has been recorded at over 200 mph (322 km/h), according to Guinness World Records. In fact, they may dive at speeds of 350 mph (563 km/h), though scientists haven't officially documented a speed that high.

A peregrine falcon on the Cantabrian coast of Spain hunts for prey.

A peregrine falcon on the Cantabrian coast of Spain hunts for prey. (Image credit: Javier Fernández Sánchez via Getty Images)
"Quite a few flying birds can go faster than a cheetah," Hutchinson said. The common swift (Apus apus) has been measured to fly 69 mph (111 km/h), and the white-throated needletail (Hirundapus caudacutus) is estimated to reach speeds of 105 mph (169 km/h), according to the National Audubon Society.

The ocean, too, holds an elite list of speedsters. Black marlins (Istiompax indica) have been clocked at 80 mph (129 km/h), according to Britannica, and the swordfish (Xiphias gladius) and sailfish (Istiophorus) can reach speeds of 60 mph (97 km/h) and 68 mph (109 km/h), respectively, according to data from the ReefQuest Centre for Shark Research.

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So, while the cheetah deserves its place among the fastest animals on the planet, it gets an undue share of the limelight. One reason for that, Hutchinson said, is that most animals' speeds haven't been studied thoroughly. The speeds of racehorses, cheetahs, greyhounds and camels have been measured carefully and repeatedly; researchers even verified that the animals were fully exerting themselves, he said.

But most other animals' speeds are just observations and estimates, Hutchinson said. They give us an idea of how quickly these animals move, but the estimates are "not good [enough] data for a nitpicky scientist," he said.