Paternal care is uncommon among several wild animal species, even among arthropods—the invertebrates that include insects, arachnids, and crustaceans.
Now, in a recent study in the Zoological Journal of the Linnean Society, harvestmen—also known as daddy longlegs—account for more than half of all independently evolved cases of paternal care currently documented among arthropods.
The news insights were made possible by a recent citizen science effort that significantly increased the available data about these paternal behaviors in arthropods, all within days.
Built for Fatherhood
Harvestmen are one of the most diverse arachnid groups, with more than 6,900 described species. Despite making up just 0.6% of arthropod diversity, they show more independent origins of paternal care or egg guarding than any other arthropod lineage. This concentration of paternal care makes harvestmen a key group for studying the evolution of this behavior.
Led by Glauco Machado at the University of São Paulo, the research team combined almost thirty years of fieldwork with a review of iNaturalist, a public website for sharing nature sightings. From 1936 to 2025, published studies had found parental guarding in only 80 harvestman species. However, the iNaturalist search added 85 new records: 78 showing parental care and seven showing no care, more than doubling the number of harvestman species with available behavioral data.
Machado was surprised by how quickly the platform produced results. “It’s a tremendous source of information that can improve the velocity with which we accumulate biological information,” he said. “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.”
The Evolution of Fatherhood
With the expanded dataset, the researchers were able to map parental behaviors onto a newly constructed evolutionary tree encompassing 165 species across nine families. This represented the first reconstruction of how both maternal and paternal care evolved within the superfamily Gonyleptoidea. The findings challenge the assumption that caregiving behaviors follow a single evolutionary pathway.
The analysis found that maternal care evolved only from ancestors that did not provide care, a pattern also seen in insects. Paternal care, however, appeared in two ways: once from ancestors with no care, and once from species where females already guarded eggs. This difference suggests that separate evolutionary pressures shaped the development of parental care in harvestmen.
Machado suggests that sexual selection may have driven the evolution of paternal care from maternal care. In this scenario, which the researchers describe as enhanced fecundity, females favor males that are already tending eggs.
“It’s very rare in nature, paternal care, and this behavior evolved many times independently,” Machado said. “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.”
The evolutionary tree also showed that parental care does not always persist across lineages. Several groups appear to have lost caregiving behavior over evolutionary time. Some harvestmen now use easier methods, like hiding their eggs or burying them under debris, to protect them without having to stay and guard them.
Limitations of Citizen Science
Machado emphasizes that experts remain essential to this process. It takes specialized knowledge to correctly identify species, tell the sex of the guarding animal, and tell real parental care apart from similar behaviors such as mate-guarding. “I think taxonomists’ role in modern science is more important than ever,” he said. “We cannot preserve a species that doesn’t have a name. And names are provided by taxonomists.”
Additionally, animals that are actively guarding eggs are more likely to be photographed than those not showing any behavior, which could make active parental care seem more common than it really is.
What Comes Next
Machado views this approach as a model for future research. More than half of the new records came from individuals without formal training in arachnology, and he anticipates similar results as researchers apply iNaturalist to other animal groups for which parental care remains poorly documented.
“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,” he said. The next mystery solved through citizen science may not involve arachnids at all.
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.