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. To analyse how fast the

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eiveplus.com/blog/the-secret-of-fertile-sperm/">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. '

Frequently Asked Questions

Does sperm size affect male fertility in humans?

Research shows that within humans, bigger sperm aren't necessarily faster or more effective at fertilisation. What matters more for male fertility is overall sperm health, including motility, morphology, and sperm count. If you're concerned about fertility, the NHS recommends seeing your GP after 12 months of trying to conceive, or sooner if you're over 35.

What does a NHS semen analysis test for?

An NHS semen analysis examines sperm count, motility (how well they swim), and morphology (shape and size). According to NICE guidelines, this test is one of the first investigations offered to couples who have been trying to conceive for over a year. Your GP can arrange a referral, and results typically take a few weeks depending on your local NHS trust.

Can sperm quality be improved naturally when trying to conceive?

Yes, lifestyle changes can positively impact sperm quality. The NHS recommends maintaining a healthy weight, reducing alcohol intake, stopping smoking, and avoiding tight underwear to keep the testicles cool. A balanced diet rich in antioxidants and zinc may also support healthy sperm production.

How do fertility clinics in the UK measure sperm speed?

UK fertility clinics, including those regulated by the HFEA, use computer-assisted sperm analysis (CASA) to measure sperm velocity and movement patterns. This technology, also used in the research discussed above, provides detailed information about how individual sperm swim, helping clinicians assess male fertility more accurately.

Why is my partner's sperm morphology abnormal and does it matter?

It's completely normal for a large percentage of sperm to have abnormal shapes – even fertile men typically have only 4-14% normally shaped sperm according to WHO guidelines. Whilst morphology is one factor assessed during fertility investigations, the NHS considers it alongside other parameters. Around 1 in 7 UK couples experience fertility issues, and abnormal morphology alone doesn't necessarily mean you won't conceive naturally.

How long is the waiting time for NHS fertility treatment in the UK?

Waiting times for NHS fertility treatment vary significantly depending on your local Integrated Care Board (formerly CCG) and can range from several months to over a year. Eligibility criteria and the number of funded IVF cycles also differ by area, so it's worth checking your local ICB's policy. Your GP can provide specific information about waiting times and funding criteria in your region.

Does the way sperm swim affect IVF success rates?

Sperm motility is an important factor in fertility treatment success. For couples undergoing IVF or ICSI at HFEA-licensed clinics in the UK, embryologists select the healthiest-looking sperm with good forward movement. If sperm motility is poor, ICSI (where a single sperm is injected directly into the egg) may be recommended to improve your chances of successful fertilisation.