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Monday Article #76: Motility in Reproduction


Reproduction takes place in many different ways. The most common divisions of reproduction are internal and external fertilization. This article will mainly focus on internal fertilization in human beings.

Generally, external fertilization is less efficient compared to internal fertilization. However, why is this so? The simple answer is that most gametes in external fertilization do not fuse to form a zygote. Instead, many gametes are washed away by tides or eaten by predators. The number of eggs released during the mass spawn is relatively high. However, the ratio of the number of fertilized eggs to the number of eggs released is low, illustrating the low efficiency of external fertilization.

For internal fertilization, the male usually transfers sperm into the female reproductive tract during copulation. In fact, the sperms are motile because they have a tail. Therefore, in the human body, sperms can easily swim to their target - the secondary oocyte. Because they are not exposed directly to an external environment, the chances of human sperm getting destroyed are significantly lower. Additionally, 40 million and 1.2 billion sperm cells are released in a single ejaculation. This number is even higher than the average number of eggs released during a mass spawn, which is about 50–200 million eggs in regular intervals. In an ideal situation, most of the sperm will be able to swim to the secondary oocyte, and one “winner” will successfully fertilize it. Then, a tough membrane is formed around the zygote to ensure that other sperms do not penetrate the egg. 

Figure 1: Structure of Sperm

However, if this is the case, why is infertility so common among males? Even if one sperm was faulty, there are still billions of other sperm cells that are available and viable, right? In the upcoming passages, we will be discussing a factor that affects sperm motility- the fluid that contains the sperm.

How Sperm Motility is Related to Physics

The tail (flagellum) of the sperm does not simply push the body but uses a rhythmical beating with pushing and fraying over a 4-beat pattern. As the sperm flagellum beats and propels the sperm forward, it interacts with the surrounding fluid. The flagellar beats generate a flow of fluid around the sperm, which, in turn, generates hydrodynamic forces that contribute to the overall propulsion of the sperm. The interaction between the flagellum and the surrounding fluid is complex and can vary depending on the fluid's viscosity and elasticity. 

Generally, sperms come together in groups. This phenomenon is known as the clustering of sperm. Scientists have tested sperm clustering in two different types of medium. 

Figure 2: The sperm clusters in different mediums.

This is where we can bring Physics into the equation. The movement of sperm is related to Stokes' Law because sperm cells are tiny particles moving through a fluid medium, which in this case is typically semen. Stokes' Law helps us understand the forces acting on these sperm cells as they swim through the semen.

Figure: Stoke’s Law equation

According to Stokes' Law, the drag force experienced by a particle (in this case, a sperm cell) is directly proportional to its velocity, size, and viscosity of the fluid. 

As the sperm swim, they experience resistance or drag force from the surrounding semen. This drag force can be influenced by the viscosity of the semen and the speed at which the sperm are moving. If the semen has a higher viscosity, it means it is thicker or more "sticky," and the drag force on the sperm will be greater.

The size of the sperm also plays a role. Generally, smaller sperm cells will experience less drag force compared to larger cells because the drag force is directly proportional to the radius of the particle, as per Stokes' Law.

Did you see the contradiction?

As aforementioned, sperms are more motile and easily form clusters in a thicker medium. However, it was also revealed that sperms experience more drag force in higher-viscosity fluids. So, how can this be?

Sperm cells have evolved to swim effectively in fluids with a certain level of viscosity, such as those found in the female reproductive tract or mucosal surfaces. The hydrodynamic environment plays a critical role in sperm motility, and the viscosity of the surrounding medium directly affects their ability to generate thrust and propulsion. The coordination within a cluster can enhance the overall motility and swimming efficiency of the sperm cells within the cluster. However, if the cluster becomes too large or disorganized, it may hinder the collective motion and decrease the motility of individual sperm cells.

When sperm swim through a fluid medium, they experience flow resistance, which is the force that opposes their movement. In a low-viscosity medium like water, the flow resistance is minimal because the fluid offers little resistance to the movement of the sperm. As a result, sperm may struggle to generate enough thrust to propel themselves forward efficiently. Their flagellar movements can be less effective in low-viscosity fluids, leading to reduced swimming speed and limited forward progression.

Sperm cells rely on the thrust generated by their flagellum to move through the fluid. In a higher-viscosity medium, such as the cervical mucus or the fluids in the female reproductive tract, the flow resistance is increased. This increased resistance allows the flagellum to push against the fluid more effectively, generating a stronger propulsive force. The viscosity of the medium facilitates the transfer of mechanical energy from the flagellum to the surrounding fluid, resulting in more efficient propulsion and better forward movement of the sperm.

The viscosity of the medium also affects the fluid shear forces experienced by sperm. Fluid shear refers to the frictional forces exerted on the sperm by the flowing medium. In a low-viscosity medium, the fluid shear forces may be relatively weak, reducing the interaction between the sperm and the fluid. This can make it challenging for the sperm to maintain their motility and navigate through the fluid. In contrast, in a higher-viscosity medium, the fluid shear forces are more significant, providing the sperm with a better grip on the fluid and aiding their motility.

In a nutshell, sperm cells have evolved to swim effectively in the specific hydrodynamic conditions found in the female reproductive tract. The female reproductive tract secretes fluids with varying viscosities that create an optimal hydrodynamic environment for sperm movement. The viscosity of these fluids allows sperm to overcome flow resistance, generate thrust, and navigate through the complex reproductive tract to reach the site of fertilization.

Thickness of Semen

This is affected by other factors in the body’s hormones and homeostasis mechanism.

Figure: Mindap of what affects the viscosity of semen.

In simple terms, this is one of the root causes of infertility in males.


Because of Stoke’s Law, we understand that sperm motility is interlinked with drag force. However, correlation does not equal causation. Drag force does not directly allow sperms to swim faster. While drag force may initially impede the movement of sperm, the interplay between the propulsive forces generated by the flagellum and the opposing drag force determines the net movement of the sperm. 

It is with the hope that in the future, further research can be conducted to better grapple with fertility issues that arise among modern humans, especially by connecting different fields of science together because strength always lies in diversity. 


Article prepared by: Christabelle Johneva Lee, Member of MBIOS

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  1. Jack J. Lee. (2020). Human sperm don’t swim the way that anyone had thought. [online] Available from: [accessed: 19 May 2023]

  2. Ishimoto K. (2018). Hydrodynamic Clustering of Human Sperm in Viscoelastic Fluids. [online] Available from: [accessed: 19 May 2023]

  3. Dr. Tom Crawford. (2020). How Do Sperm Swim? [online]. Available from: [accessed: 19 May 2023]

  4. Marita Espejo Catena M.D., M.Sc., Ph.D. (2017). Diagram of Sperm Cell Tail. [online]. Available from: [accessed: 19 May 2023]


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