Is bigger always better? Scientists explain the evolution of sperm size

Maybe size does matter.

This classic barroom debate has many interpretations when it comes to sex and desire, but scientists are now looking at it from an arguably more essential perspective: the size of sperm.

We’ve all seen some variation of this image in high school biology: A mass of tiny, microscopic swimmers — sperm — compete to be the first to fertilize a much larger egg and ensure the survival of the species.

But that “tiny” swimmer may not actually be so tiny, depending on the animal in consideration — and the environment that its sperm swam in. Specifically: the female reproductive tract.

According to research published Monday in the journal Nature Ecology & Evolution, one critically overlooked factor impacts the evolution of sperm length across the animal kingdom: where the sperm fertilizes the egg.

How they did it — The researchers searched the scientific literature for data on sperm size, ultimately creating a database of sperm records from more than 3,000 species from 21 different animal groups known as “phyla.”

This is the largest known database on sperm length and fertilization methods, including both vertebrates and invertebrates.

The study focuses on three classes of animals, depending on where and how their sperm fertilizes eggs during reproduction. The three groups are:

• External fertilizers: These animals — like fish — typically release sperm and eggs into an “external” watery environment, where sperm becomes diluted. Fertilization occurs in the water outside the female’s body.
• Internal fertilizers: Sperm is released into an “internal” environment inside the female reproductive tract. All birds, reptiles, and mammals, along with some amphibians and invertebrates, release sperm and fertilize eggs this way.
• Spermcasters: These animals — like barnacles — release sperm into an aquatic environment. A nearby female then takes up the sperm from the water into her body, where fertilization occurs.

The researchers compare how sperm evolution differs across these three different types of animals, revealing new information on how fertilization methods can impact the size of sperm.

What’s new – The study highlights four key findings showing how the environment in which sperm are released — known as “fertilization mode” — impact the size of sperm:

1. Sperm are shorter in external fertilizers and spermcasters
2. Conversely, sperm are generally longer in internal fertilizers
3. Sperm length evolves faster and with more evolutionary shifts in spermcasters and internal fertilizers — groups where fertilization occurs within the female body
4. A complex combination of postcopulatory — that is, after sex — forces affect the evolution of sperm length and diversity across animals

From this new research, scientists have discovered that female animals may play a larger role in shaping the evolution and wide diversity of sperm sizes than previously thought.

Scientists still don’t fully understand the evolution of sperm. But these findings paint a clearer picture of the way fertilization methods can change the length of sperm and determine whether successful reproduction will occur.

When Did Sperm Evolve?

Sperm has been around for a really long time.

A 2001 study found that the human sperm production gene, also known as the “Boule gene,” originated roughly 600 million years ago with the common ancestor of all animal life. This ancestor was likely just a “blob of cells” as National Geographic reported at the time.

Later research published in 2010 found that the Boule gene was widespread among the animal kingdom.

How Did Sperm Evolve?

Sperm shape, also known as sperm morphology, is extraordinarily diverse across the animal kingdom. In fact, sperm is the most diverse type of animal cell on planet Earth.

Pretty much all sperm has a head, a middle piece, and a tail (flagellum) that enables the sperm to swim. But beyond that, sperm shape and length can wildly vary among and within different species. The head of human sperm is rounded and smooth, but some rats’ sperm have hooked heads and different tail lengths. Certain birds even have corkscrew-shaped sperm.

Scientists have devised different hypotheses to explain why sperm has evolved differently across species. The new study discusses a hypothesis that focuses on fertilization mode, or the environment in which sperm fertilizes an egg.

Other theories on sperm morphology focus on how competition between sperm and the size of animals impact the length of sperm:

• Longer sperm often compete with each other to fertilize the egg
• Sperm competition typically occurs when a female mates with several males to increase the likelihood of successful fertilization from multiple sperm
• Sperm competition is still widely debated in humans

Basically, there is a trade-off between the size of the sperm and the number of sperm produced, according to the Nature study. For example, when external fertilization occurs in aquatic environments, the size of the sperm matters less for successful reproduction than the quantity of sperm that gets produced.

However, it’s a different story for animals where sperm fertilizes the egg inside the female’s body.

Larger female mammals, such as elephants, have bigger reproductive tracts. A 2015 study found that in these bigger animals, sperm can get lost in the reproductive tract and may never reach the egg. When sperm is longer, less sperm will be produced. Therefore, it’s not advantageous for males in these species to produce larger sperm.

Instead, researchers learned that large male animals, like elephants, tend to produce a greater quantity of smaller sperm to increase the chances of successful fertilization.

Research on sperm competition can explain why elephants evolved to produce shorter sperm compared to smaller animals rats, which likely have longer sperm because the females’ reproductive tracts are not as big.

How Does Sperm Size Affect Fertility?

The latest Nature study finds significant differences in sperm length across animal groups, depending on fertilization mode.

Among animals that fertilize internally and externally, amphibians and mollusks tended to have longer sperm.

The average length of sperm across all animal groups surveyed in the study was 420.3 micrometers for internal fertilizers and 61.3 micrometers for external fertilizers, showing how the mode of fertilization really does impact sperm size.

For comparison, human sperm averages 50 micrometers in length. On the other hand, fruit flies produce enormous sperm relative to their body size, clocking in at 5.8 centimeters in length. Fruit flies employ a strategy known as “sperm gigantism” to increase the likelihood of successful reproduction. For the fruit fly, longer sperm is more successful sperm.

In humans, it’s unclear how sperm size affects fertility since sperm morphology is highly subjective. However, previous research has found that men with wide variations in sperm length— as opposed to a consistent sperm size — produced sperm that could not swim as well.

It’s also not clear whether the size of the testes impacts the size of sperm. A 2014 review of sperm in different species found no significant relationship between testes' size and length of sperm, but another study published in 2019 found a correlation between larger testes and longer sperm in birds.

However, bigger may not necessarily be better when it comes to sperm. Ultimately, each animal has evolved sperm that is designed to maximize the chances of fertilization — and ensure the survival of its species.

Abstract: Evolutionary biologists have endeavoured to explain the extraordinary diversity of sperm morphology across animals for more than a century. One hypothesis to explain sperm diversity is that sperm length is shaped by the environment where fertilization takes place (that is, fertilization mode). Evolutionary transitions in fertilization modes may transform how selection acts on sperm length, probably by affecting postcopulatory mechanisms of sperm competition and the scope for cryptic female choice. Here, we address this hypothesis by generating a macro-evolutionary view of how fertilization mode (including external fertil-izers, internal fertilizers and spermcasters) influences sperm length diversification among 3,233 species from 21 animal phyla. We show that sperm are shorter in species whose sperm are diluted in aquatic environments (that is, external fertilizers and spermcasters) and longer in species where sperm are directly transferred to females (that is, internal fertilizers). We also show that sperm length evolves faster and with a greater number of adaptive shifts in species where sperm operate within females (for example, spermcasters and internal fertilizers). Our results demonstrate that fertilization mode is a key driver in the evolution of sperm length across animals, and we argue that a complex combination of postcopulatory forces has shaped sperm length diversification throughout animal evolution.