Citizen science data from iNaturalist reveals evolution of parental care in harvestmen

A Father's Devotion: How Daddy Longlegs Redefine Parental Care in Nature

A major new study has revealed that paternal care in harvestmen—the arachnids often called daddy longlegs—is far more common and evolutionarily complex than previously thought. By combining nearly three decades of fieldwork with data from the citizen science platform iNaturalist, researchers have doubled the known cases of parental care in this group, painting a new picture of how such behaviors evolve.

The research team added 62 records of parental care from iNaturalist in just two days. Previously, scientific literature had documented this behavior in only 80 species between 1936 and 2025.

The study, published in the Zoological Journal of the Linnean Society, was led by a scientist from the University of São Paulo. For the first time, the team reconstructed the evolutionary history of both maternal and paternal care within the superfamily Gonyleptoidea. Their findings reveal a dynamic, non-linear pattern: parental guarding behavior has repeatedly evolved, disappeared, and re-emerged across the harvestmen evolutionary tree.

• Maternal care showed a clear pattern: It evolved only from a state of no care, a route that is also observed in insects.
• Paternal care followed two distinct paths: It could evolve directly from no care, or it could evolve from maternal care.
• A Hypothesis for Paternal Evolution: When paternal care evolved from maternal care, the authors hypothesize it may be driven by sexual selection. They term this the 'enhanced fecundity' hypothesis.
• An Evolutionary Anomaly: Harvestmen comprise over 6,900 species (just 0.6% of all arthropod diversity), yet they account for more than half of all independent origins of paternal care found in animals—a remarkably rare trait in nature.

Lead author Glauco Machado emphasized the significance of fathers that care for their young:

"It's very rare in nature, paternal care, and this behaviour evolved many times independently. So, by looking at harvestmen we can explore questions related to the factors that led to the evolution of this behaviour. In many species where males care for the offspring alone, the caring activity is a sexually selected behaviour, which means that females prefer males that are caring for the eggs."

Machado also highlighted the revolutionary role of citizen science and taxonomic expertise:

"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."

He added a crucial warning about the foundation of all biodiversity science: "I think taxonomists' role in modern science is more important than ever. We cannot preserve a species that doesn't have a name."

The authors acknowledge a sampling bias: it is inherently easier to record instances of parental care than instances of no care. However, they argue the study effectively fills major gaps in our knowledge regarding the presence and absence of this behavior.

Broader Context: This research showcases the immense power of platforms like iNaturalist. By enabling scientists, especially those in the Global South, to tap into a global network of georeferenced observations, citizen science can massively accelerate data collection, drastically reduce costs, and make large-scale evolutionary research feasible. Machado notes this approach has strong potential for application to other animal groups where maternal and paternal care are observed.