Why Are Men So Bad at Making Good Sperm Cells?

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Dec. 5 2012 3:52 PM

Terrible Swimmers

Why are men so bad at making sperm cells?

Human sperm.
Human sperm stained for semen quality testing in the clinical laboratory.

Photo by Bobjgalindo/Wikimedia Commons.

The sperm count of French men dropped 32 percent between 1989 and 2005, and the number of normally-shaped sperm dropped 33 percent. Abnormal sperm morphology is quite common: In many men, only 10 to 15 percent of sperm is appropriately shaped, depending on the standard applied. Why are men so bad at producing regular sperm?

Because sperm are expendable. Men produce millions of sperm every day and ejaculate about 300 million at a time, so evolutionary pressures haven’t punished this inefficiency. Artificial selection, however, shows that higher success rates can be achieved. In bulls, which are selected for their ability to generate viable sperm, between 70 and 75 percent of sperm are morphologically sound. Stallions have a slightly lower success rate, but still outperform men.

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Genetic differences account for some of the variability in sperm quality. Of the genetic mutations that are known to cause infertility in mice, 75 percent affect only males. Researchers speculate that more genes may be involved in spermatogenesis than oogenesis, or perhaps egg production has more genetic safeguards to prevent mutations from undermining viability. This makes sense from an evolutionary perspective: Women release only between 400 and 500 eggs in their lifetimes, so it’s much more important that each one be viable.

Spermatogenesis is also a lengthy, complicated process, with many opportunities for errors. When you damage a skin cell, for example, neighboring skin cells divide to replace the lost tissue. The process takes a matter of hours or a couple of days at most. By contrast, sperm cells are made from stem cells, which have to launch a long series of changes in daughter cells, including replacing all the cytoplasm and making a flagellum. Human sperm cells are about 60 days in the making.

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Brian Palmer writes about science, medicine, and the environment for Slate and the Natural Resources Defense Council. Email him at explainerbrian@gmail.com. Follow him on Twitter.

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