The Evolution of Sperm

Does size matter? When talking about sperm, recent research suggests that it might. According to a study published in the journal Nature Ecology & Evolution, the size of sperm may be affected by where it fertilizes the female’s egg.

All sexually reproducing species create sperm to fertilize eggs. But it turns out the size of sperm can range dramatically because they are among the most diverse cells in animals, including humans. To test how sperm size changes over time, evolutionary biologists compared the behavior of sperm in external fertilizers with those of internal fertilizers.

External fertilizers include animals like fish, which release sperm and eggs directly into water to be fertilized outside of the body. Internal fertilizers include mammals, birds, and reptiles that release sperm inside a female reproductive tract. Researchers found that sperm are generally larger and evolve faster in internal fertilizers than external fertilizers. This means that the environment in which a sperm is deposited may affect how it evolves. It further suggests that female animals may play a significant role in this evolution.

Sperm Size and Fertility

Part of the comprehensive screening we do on all sperm donors is a semen analysis that includes testing for morphology, or the percentage of healthy, moving sperm in a sample. Among other factors, healthy sperm has a smooth, oval head, a well-defined cap, and no visible deformities on the neck, midpiece, or tail of the sperm. While abnormally shaped sperm can fertilize an egg, some studies have found that a high percentage of such sperm may be associated with low sperm count and possibly male infertility.

However, we still don’t know enough to determine whether or how much sperm size affects fertility since morphology can range significantly even within a single sample. In addition, abnormalities in a semen sample do not necessarily mean a male is infertile. Men with poor semen analysis can still fertilize an egg and get a woman pregnant, it just may take longer.

If you have questions about sperm health or how our donor screening works, Our Donor Coordinators are here to help.