For years, scientists have studied how animals protect and care for their young, but harvestman spiders have revealed a surprising twist. In many species, fathers—not mothers—take on the responsibility of guarding eggs. Researchers have used decades of field observations combined with citizen science data to uncover how this rare parenting behavior evolved. The discovery shows how online platforms like iNaturalist are changing the way scientists study the natural world.
The world of spiders is full of unexpected behaviors, but few are as unusual as the parenting habits of harvestmen. While many animals rely on mothers to protect their offspring, some harvestmen species have developed a different approach, with fathers taking the lead in guarding eggs, as per a report by Phys Org.
A new study published in the Zoological Journal of the Linnean Society has uncovered how this rare parental behavior evolved. Researchers combined almost three decades of field research with observations collected through the citizen science platform iNaturalist.
The international team, led by a scientist from the University of São Paulo, more than doubled the number of documented cases of parental guarding in harvestmen. The findings allowed researchers to reconstruct the evolution of maternal and paternal care in the superfamily Gonyleptoidea for the first time.
The study revealed that parental guarding has appeared, disappeared, and returned multiple times throughout the evolutionary history of these spiders, as per a report by Phys Org.
Why are harvestmen spiders unusual parents?
Harvestmen are one of the most diverse groups of arachnids, with more than 6,900 recognized species. Although they represent only a small portion of arthropod diversity, they account for more than half of the known independent origins of paternal care.
That makes them an important group for studying how parenting behaviors develop across species.
Lead author Glauco Machado explained, "It's very rare in nature, paternal care, and this behavior evolved many times independently. So, by looking at harvestmen, we can explore questions related to the factors that led to the evolution of this behavior. In many species where males care for the offspring alone, the caring activity is a sexually selected behavior, which means that females prefer males that are caring for the eggs."
Researchers found that maternal care evolved only from species that previously showed no parental care. However, paternal care followed a more complex path, developing either from no care or from existing maternal care.
The scientists suggested that when paternal care evolved from maternal care, it may have been connected to sexual selection. This idea, known as “enhanced fecundity,” suggests that caring males may become more attractive to females.
How did citizen science change the discovery?
One of the biggest breakthroughs came from using public observations. Citizen science allows people around the world to contribute information about plants and animals without needing professional training.
Platforms like iNaturalist have become valuable resources for researchers by collecting photographs, locations, and observations from nature enthusiasts.
Machado’s team was inspired to use iNaturalist after hearing about citizen science applications in bird research. The database helped researchers gather information much faster than traditional methods.
Between 1936 and 2025, scientific studies had documented parental guarding in 80 harvestmen species. This new research added many more records, with 62 observations coming directly from iNaturalist.
The researchers completed this expansion of data in only two days, showing how quickly citizen science can support biological discoveries.
Machado emphasized the importance of accessibility, saying, "It's a tremendous source of information that can improve the velocity with which we accumulate biological information. I would never be able to do this by visiting museums around the world. It would be very expensive, very time consuming, but here we conducted the search in only one week."
Citizen science is helping scientists overcome traditional barriers, especially for researchers who may not have access to expensive equipment, international collections, or large fieldwork budgets.
What does this reveal about evolution?
The study highlights that citizen science is not replacing expert research but expanding what scientists can discover. Researchers still need specialists, especially taxonomists, to correctly identify species and understand behaviors.
Machado explained, "I think taxonomists' role in modern science is more important than ever. We cannot preserve a species that doesn't have a name. And names are provided by taxonomists. So, it's very important."
The research also has limitations. Scientists noted that studying parental care can be challenging because it is easier to record examples of care than cases where no care happens. This can create sampling bias.
However, the researchers believe these studies are helping fill important gaps in understanding animal behavior.
With more than half of the records in the study being new discoveries, Machado hopes scientists continue using citizen science platforms to explore parenting behaviors across many animal groups.
"I think it's a very broad contribution for people that are working with insects, frogs, and all kinds of groups, animal groups, in which we have both maternal care and paternal care."
The study shows that even small observations from nature lovers around the world can contribute to major scientific discoveries, revealing hidden stories about evolution and animal life.
FAQs
Do male harvestmen care for eggs?
Yes, paternal care is surprisingly common in some species.
How did scientists find new data?
They used iNaturalist observations from around the world.
