Sperm Evolution Becomes Supercharged Only When They Swim Inside Females

Sperm size varies dramatically among different animal species. But why is sperm size so variable when they share the same job — to fertilize eggs? In a new article published in Nature Ecology and Evolution, researchers from Stockholm University show that animal sperm evolution becomes supercharged only when sperm swim inside females.

Sperm are the most variable cell type known, ranging in size from 0.002 millimeters in a freshwater rotifer to nearly 6 centimeters in a fruit fly. Explaining why sperm are so variable has been a major focus in evolutionary biology over the last 100 years because sperm, no matter from what organism, always have the same function: to fertilize eggs.

“Researchers usually try to explain sperm diversity by focusing on how sperm compete to fertilize eggs or how females choose which sperm fertilize their eggs,” says Ariel Kahrl, a researcher in the Department of Zoology at Stockholm University.

“But it turns out that there is a missing piece of the puzzle — the location where sperm and eggs meet can also influence sperm size.”

To examine how the location of fertilization influences sperm evolution, the researchers compiled data on sperm size from more than 3,200 animal species — ranging from corals to mammals, including humans — and classified each species based on where sperm and eggs meet.

“In species with internal fertilization — like mammals, birds and insects — sperm fertilize eggs inside the female’s body, while in species with external fertilization — like sea urchins and many fish species — sperm and eggs are released into the water and fertilization happens outside of the female’s body,” explains Ariel Kahrl.

The researchers found that sperm were on average six times longer and changed size more rapidly in animals that use internal fertilization compared to sperm from animals that use external fertilization.

Ariel Kahrl. Credit: Aaron Reedy

“When sperm are released externally, selection keeps sperm size small to allow males to produce a lot of sperm,” says Rhonda Snook, a professor in Zoology and an author of the paper. “But when sperm are transferred to the females in internal fertilizers, males may compete better with bigger sperm and females may prefer to fertilize eggs with bigger sperm.”

The researchers also examined a third form of fertilization found in invertebrates called spermcasting, where sperm are released externally and then filtered out of the water by females where they then fertilize eggs inside the female.

Dr John Fitzpatrick, Wallenberg Academy Fellow, Department of Zoology, Stockholm University. Credit: Magnus Bergström/Knut and Alice Wallenberg Foundation

“Spermcasting represents a mix of internal and external fertilizations, which gave us the opportunity to see what part of the fertilization process influenced sperm evolution,” says John Fitzpatrick, an associate professor in Zoology at Stockholm University and the senior author of the study.

In spermcasters, the study found that sperm were small, like external fertilizers, but evolved rapidly, like internal fertilizers.

“Our results clearly show that interactions between sperm and females help generate the tremendous diversity in sperm size we see in animals today. The greater the potential for interactions between sperm and females, the faster sperm evolve,” says John Fitzpatrick.

Since humans are internal fertilizers, does this mean that men have supersized sperm? It turns out this isn’t the case; human sperm are about the same size as animals that release their sperm into water.

“In animals with large bodies, like humans, sperm are diluted inside the female’s reproductive tract. From the sperm perspective, it doesn’t matter if dilution occurs inside a female or in the ocean — dilution keeps sperm small. It’s only when sperm are confined in small spaces within the female that sperm become supersized,” explains Ariel Kahrl.

Reference: “Fertilization mode drives sperm length evolution across the animal tree of life” by Ariel F. Kahrl, Rhonda R. Snook and John L. Fitzpatrick, 21 June 2021, Nature Ecology and Evolution.
DOI: 10.1038/s41559-021-01488-y

More about the study

The idea that fertilization environment influences how sperm size has been around for more than 60 years. But researchers haven’t been able to test this idea throughout animal evolution. In the article, researchers from Stockholm University compiled the largest database on sperm morphology ever assembled and show that sperm size increases and changes rapidly when sperm operate inside the female’s body.

Cell BiologyEvolutionGeneticsPopularReproductive BiologySpermStockholm University
Comments ( 17 )
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  • Edson

    Didn’t you know that sperm illustration is outdated?
    The tail motion is not 2dimansinal it’s 3dimensional, a spiral.

  • Taylor

    6cm in a fruit fly? 6cm? That’s about the size of a pinky finger

    • Mike O'Neill

      Yes. It is strange, but correct. The sperm is much longer than the fruit fly itself.

  • Raven

    I think you got those sperm sizes wrong folks–6 centimeter sperm would not fit inside a fruit fly.

    • Mike O'Neill

      No, it’s actually correct. Even though a fruit fly is only a few mm in size, its sperm is actually about 6 cm long.

      Your skepticism is understood though, because that giant relative size is indeed strange.

  • Dennis

    Poor female fruitfly…
    Average size of a fruitfly is 2-3 millimetres. According to this article fruitfly sperm is 20-30 times larger than the fly itself 😉

  • Joel Weichsel

    Perhaps the driving context is the opportunity for physical competition among tightly packed allo sperm vs the lack of such opportunity if not so tightly packed.

  • Jo

    The article is correct fruit fly sperm is 6cm longer, much longer than the fruit fly itself but much thinner and stringier.

  • Julia Holland

    You have stated that fruitfly sperm is 6 cm. This must be a error as a dilated cervix for passage of a human infants head is 10cm. An entire fruirfly is not 6 cm. Proofread please

  • Don

    Vow, sperm of the male knows what kind of environment is waiting for it. Does sperm have a brain?

  • Heather

    Looked pretty updated to me?! (In regards to that 1st comment).
    Thank you for your article.

  • Andrew

    It’s true! From Wikipedia: “Males of this species are known to have the longest sperm cells of any organism on Earth—an impressive 5.8 cm long when uncoiled, over twenty times the entire body length of the male”

  • Nick

    The fruit fly sperm and other thing are mixed up lol.

  • Jamie

    Wait wait wait…maybe my math is off…but a 6cm sperm….for a fruit fly…..hate to see the zygote.

  • BC

    The length of the sperm of the fruit fly is 6 CM, but the diameter is much smaller. A little research goes a long way.

  • Nilda.

    Nice….Everything comment is truly. But the people writing the new not look well and not writing Exactly that the DR.said.