Big sperm don't always win the race
When females mate with more than one male, each one's sperm has to compete to get to her eggs. Until now, researchers had thought the fastest sperm would dominate. But in a new study, published in Evolution, a team of scientists have found that it is much more complicated than that.
'If you compare species with little sperm competition, like humans, with a species where there is a lot of sperm competition, like Bonobos, then the bonobos tend to have faster, bigger sperm,' says Dr John Fitzpatrick of the University of Manchester, lead author on the paper. 'But if you look within a species, you typically don't see this relationship where bigger sperm are faster.' Instead, the team found that bigger sperm – those with longer tails, called flagellum –were only the fastest in species with external fertilisation, such as fish or mussels. In animals with internal fertilisation, like emu, bigger sperm were often the slowest.
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To analyse how fast the sperm were depending on their size, the researchers took video recordings of sperm swimming and then broke them down frame by frame using high speed cameras. They then deconstructed these frames into a stack of still images which could be entered into a computer programme to focus on a single sperm at a time. They then used computer assisted sperm analysis – software also used in human fertilisation clinics – to assess sperm quality. 'In sperm with longer flagellum there's more thrusting force to move sperm forward, and there we found clear results. A shorter head and bigger tail means you swim faster if you're an externally fertilizing species, but if you have a bigger head and a smaller tail it seemed you swam faster if you were internally fertilizing,' Fitzpatrick says.
The researchers think that the reason bigger sperm swim slower in some internally fertilizing species is because they may be using a different strategy of moving slower but swimming for longer periods. 'In externally fertilizing species you need to be fast as sperm don't live long outside of the body, so they have to get to the egg quickly, whereas in internally fertilising species the sperm can hang around for longer. We think going slower is the sperm's way of trying to conserve energy and taking its time to reach an egg,' explains Fitzpatrick. 'This may reflect the different ways that selection acts to make a successful sperm in internally versus externally fertilizing species. '
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Shop Now →Does sperm size affect fertility in human males?
Research shows that within human species, larger sperm aren't necessarily faster or more fertile than smaller sperm. What matters more for male fertility is overall sperm quality, including motility (swimming ability), morphology (shape), and count—factors that fertility clinics in the US assess using computer-assisted sperm analysis (CASA). If you have concerns about sperm quality, the American Society for Reproductive Medicine (ASRM) recommends a semen analysis through a fertility specialist.
How do fertility clinics test sperm quality in the United States?
US fertility clinics use computer-assisted sperm analysis (CASA) software to evaluate sperm swimming speed, movement patterns, and morphology. This same technology was used in the research study mentioned above to analyze sperm at a microscopic level. The ASRM provides standardized guidelines for semen analysis that American clinics follow to ensure consistent and accurate results.
What is the difference between internal and external fertilization in humans?
Humans use internal fertilization, meaning sperm meet the egg inside the female reproductive tract rather than outside the body. This is why human sperm have different evolutionary pressures compared to species with external fertilization—they can survive longer in the reproductive environment and don't need to prioritize raw speed the way fish sperm do.
Can sperm live longer inside the female body?
Yes, human sperm can survive in the female reproductive tract for up to five days under optimal conditions, which is why the timing of intercourse around ovulation is important for conception. This extended lifespan means sperm don't need to be the fastest swimmers—they can use energy conservation strategies instead, which may explain why larger sperm aren't always advantageous in humans.
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"name">What factors should I focus on if I'm trying to conceive?
Rather than focusing on sperm size, prioritize factors that support overall reproductive health: regular ovulation, healthy hormone balance, adequate nutrition, and avoiding smoking and excessive alcohol. If you've been trying to conceive for over a year (or six months if over 35), the ASRM recommends consulting a fertility specialist who can perform comprehensive testing and recommend evidence-based treatments.
Does my insurance cover fertility testing and treatment in the US?
Insurance coverage for fertility services varies significantly by plan and state, as there is no federal mandate requiring fertility coverage. Some states have "fertility insurance laws" requiring coverage, while others don't. Contact your insurance provider directly to understand your benefits, and ask your fertility clinic about financial counseling services that can help navigate coverage options.
What nutritional supplements support fertility in women?
Research supports certain supplements for reproductive health, including myo-inositol (which helps regulate ovulation and hormone balance), vitamin D, CoQ10, and prenatal vitamins containing folic acid. While supplements can support overall fertility health, they work best alongside healthy lifestyle habits and medical care from a fertility specialist who can recommend what's appropriate for your individual situation.
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