Fertilization is usually portrayed as a microscopic race: millions of sperm cells competing to reach a single egg, with only one emerging victorious. A new study suggests that this familiar image is incomplete. In many arthropods—including insects, spiders, crabs and centipedes—sperm can join forces, forming coordinated groups that may improve their ability to navigate the female reproductive tract and influence reproductive success. The findings point to cooperation, rather than competition alone, as a major and repeatedly evolved feature of reproduction.
The research, conducted by evolutionary biologists at Syracuse University, the University of Siena in Italy and the University of Szeged in Hungary, examines a phenomenon known as sperm conjugation. During conjugation, individual sperm become physically associated with one another, sometimes forming organized chains, clusters or other cooperative structures. Although scientists first described these arrangements more than a century ago, they were long considered unusual biological curiosities. The new analysis indicates that sperm cooperation is much more widespread across arthropods than previously recognized and has evolved independently in multiple lineages.
The study’s authors reconstructed the evolutionary history of sperm conjugation by comparing sperm morphology across hundreds of arthropod species. They combined information from decades of anatomical and reproductive studies with evolutionary family trees, allowing them to map the appearance and disappearance of cooperative sperm traits over time. Their results indicate that conjugated sperm emerged hundreds of millions of years ago and was repeatedly gained, lost and regained in different groups. The findings also suggest that the common ancestor of all insects possessed conjugated sperm, making cooperation part of insect reproductive history from a very early stage.
A central feature of many cooperative sperm systems is sperm-associated material, or SAM. This membrane-bound substance can attach sperm cells to one another or create an external framework around them. SAM may have initially evolved to package, protect or transport sperm, but later became associated with more complex forms of collective behavior. In some species, it binds sperm into mobile groups; in others, it surrounds individual sperm cells in thick coatings. These different arrangements suggest that a shared biological material can support very different reproductive strategies.
The potential advantages of sperm conjugation remain under investigation, but the researchers propose several possibilities. A group of sperm may move more efficiently through the female reproductive tract than isolated cells, maintain coordinated motion or resist physical and chemical barriers. Conjugation could also help sperm deliver proteins, signaling molecules or other functional components to particular locations. In some cases, cooperation may protect fragile cells from degradation or improve their ability to remain viable until an egg is encountered. These benefits would depend on the species and on the structure of the reproductive tract, meaning that sperm cooperation is unlikely to have a single universal function.
The study also highlights how rapidly reproductive cells can evolve. Sperm are exposed to demanding conditions outside the body, where they must operate in a chemically complex and physically restrictive environment. Their form, movement and surface structures are shaped by interactions with the female reproductive system, as well as by competition among males and selection imposed by fertilization itself. Because these pressures vary widely between species, sperm morphology can change dramatically even among closely related animals. The repeated evolution of conjugation suggests that similar reproductive challenges may sometimes produce similar cooperative solutions.
One particularly unusual example comes from the invasive spotted lanternfly, an agricultural pest spreading through parts of New York and other eastern states. Lanternfly sperm do not form conjugated groups, but each cell is embedded in a substantial layer of SAM. Researchers do not yet understand how these sperm move while enclosed in the material, or precisely how the coating contributes to fertilization. However, the unusual arrangement could eventually offer a target for species-specific pest management. If SAM is essential for sperm survival or function, disrupting its production or organization might interfere with reproduction without relying on broad-spectrum pesticides.
The implications may extend beyond arthropod biology, although the researchers caution that direct applications to human fertility remain distant. Human sperm do not form the same cooperative structures described in many arthropods, but the broader principle is relevant: fertilization depends on interactions between sperm, the female reproductive tract and the molecular environment surrounding them. Studying how groups of sperm organize, communicate and respond to reproductive barriers could provide new perspectives on fertility disorders across animals. It may also help scientists understand why some sperm succeed while others fail, beyond simple measurements of swimming speed or cell number.
The next challenge is to observe these cooperative systems inside living reproductive tracts rather than on laboratory slides. Sperm behavior can change substantially when cells are removed from the body and placed in artificial fluids or on glass surfaces. The researchers therefore aim to determine how conjugated sperm move in their natural environment, which cellular structures control their coordination and what trade-offs accompany group formation. Cooperation could improve transport but reduce flexibility, or increase protection while limiting the ability of individual sperm to respond to local conditions.
Taken together, the findings challenge the idea that reproductive success is governed primarily by a contest among independent sperm cells. Across arthropod evolution, sperm have repeatedly developed ways to operate as collectives, and those arrangements have sometimes disappeared when environmental or reproductive conditions changed. The pattern shows that cooperation and competition can coexist even at the cellular level. By revealing how tiny reproductive cells combine forces, the study offers a broader lesson about evolution: biological success often depends not on acting alone, but on assembling the right form of cooperation at the right time.
Subject of Research: Evolution of sperm conjugation and sperm-associated material in arthropods
News Publication Date: Not provided
Web References: https://www.nature.com/articles/s41467-026-73950-z
References: Nature Communications
Image Credits: Courtesy of Romano Dallai, Department of Life Sciences, University of Siena, Siena, Italy
Keywords: sperm cooperation, sperm conjugation, sperm-associated material, arthropods, insects, evolutionary biology, fertilization, reproductive success, sperm evolution, spotted lanternfly
