What Spider-Like Creatures Reveal About the Evolution of Fatherhood

KEY POINTS

• Study found fatherhood evolved through multiple pathways in harvestmen species.
• Researchers analyzed decades of data and thousands of iNaturalist observations.
• Male egg guarding evolved independently or replaced maternal care in some species.
• Citizen science data helped researchers expand records of parental care behaviors.

In the animal world, mothers usua lly take care of their young. Fathers rarely do. But a group of spider-like creatures called harvestmen is helping scientists understand why some males evolved to protect their offspring and how fatherhood developed over millions of years.

A new study published in the Zoological Journal of the Linnean Society found that maternal and paternal care did not evolve in the same way. By combining nearly 30 years of field research with thousands of observations shared by citizen scientists on iNaturalist (a global online platform where people upload photos and observations of plants and animals), researchers discovered that fatherhood followed multiple evolutionary paths, while motherhood appeared to evolve in a much simpler pattern.

The findings not only shed light on one of biology’s long-standing questions but also show how members of the public are helping scientists make discoveries that once would have taken years of fieldwork.

Why Fatherhood Is Rare in Nature

Across the animal kingdom, females provide most parental care because they invest more energy in producing eggs and raising young. In many species, males leave after mating and play little or no role in caring for offspring.

Harvestmen tell a different story. These spider-like arachnids include many species in which fathers stay behind to guard their eggs until they hatch, making them one of the few animal groups where paternal care is common. That unusual behavior has turned harvestmen into an important model for scientists trying to understand how fatherhood evolved.

“It’s very rare in nature, paternal care, and this behavior evolved many times independently,” said Glauco Machado, the study’s lead author and a researcher at the University of São Paulo. “By looking at harvestmen, we can explore questions related to the factors that led to the evolution of this behavior.”

More than 6,900 species of harvestmen have been identified worldwide. Although they make up only a small share of arthropods, they account for more than half of the independently evolved examples of paternal care known among arthropods, making them one of the best groups for studying how fatherhood evolved.

Motherhood and Fatherhood Followed Different Paths

To understand how parenting evolved, researchers combined decades of published studies with photos and observations uploaded by people around the world through iNaturalist, a platform where users record wildlife they encounter.

From 1936 to 2025, scientists had documented parental care in only 80 harvestman species.

Using iNaturalist, the team more than doubled that number, adding 62 new records from citizen scientists. Machado said searching the platform took only a few days, compared with years of visiting museums or conducting fieldwork.

The larger dataset allowed researchers to trace the evolution of parental care for the first time.

They found that maternal care always evolved from species that showed no parental care.

Fatherhood, however, was different.

Male egg guarding developed in two separate ways. In some species, fathers began caring for eggs even though neither parent had done so before. In others, males took over the role after females had already been guarding the eggs.

Researchers believe the second path may have been driven by sexual selection, where females preferred males that were already protecting eggs because it increased the chances of their offspring surviving.

The findings suggest that fatherhood did not emerge through a single evolutionary process but evolved under different conditions depending on the species.

Citizen Scientists Played a Major Role

One of the study’s biggest discoveries was not about harvestmen but about the growing role of citizen science.

Instead of relying only on museum collections and field expeditions, researchers searched iNaturalist, where people upload photos of plants and animals from around the world.

Those public observations dramatically expanded the amount of information available.

“It’s a tremendous source of information that can improve the speed with which we accumulate biological information,” Machado said. “I would never be able to do this by visiting museums around the world. It would be very expensive and very time-consuming.”

Even so, the researchers stressed that experts remain essential. Correctly identifying species, determining whether the parent guarding the eggs is male or female, and separating true parental care from similar behaviors still requires trained taxonomists.

Why the Findings Matter

The study offers new clues about one of evolution’s biggest questions: why some fathers stay while most do not.

By showing that fatherhood evolved through different routes than motherhood, the research challenges the idea that parental care follows the same evolutionary pattern in every species.

The findings could also help scientists studying parenting behavior in other animals, including insects, frogs, and reptiles, where both maternal and paternal care occur.

Beyond the biology, the research highlights another important trend. With millions of wildlife observations now available online, citizen science platforms are helping researchers answer complex questions faster than ever before.