How Harvestmen Shed Light on the Evolution of Fatherhood

Citizen scientists played a key role in helping researchers trace the development of parental care among harvestmen, a diverse group of spider-like arachnids. Their contributions through the iNaturalist platform supplied crucial field observations that expanded the available data significantly. The resulting study, featured in the Zoological Journal of the Linnean Society, demonstrates that behaviors involving parental guarding have arisen, vanished, and re-emerged repeatedly across the evolutionary timeline of these creatures.

Citizen science reveals the evolution of parental care

By merging nearly three decades of professional fieldwork with crowdsourced records from iNaturalist, an international research group led by a University of São Paulo expert more than doubled the number of known instances of parental care in harvestmen. This richer collection of information enabled scientists to map the separate evolutionary trajectories of maternal and paternal care within the Gonyleptoidea superfamily for the first time. The analysis revealed that parental guarding did not follow a straightforward linear progression. Instead, the behavior appeared independently on multiple occasions, disappeared in certain branches, and later resurfaced in others. Maternal care appears to have originated exclusively from ancestors that lacked any form of parental investment, a pattern already documented in various insect groups. Paternal care, by contrast, emerged through two distinct pathways: directly from species without prior parental care or from lineages where females had already begun guarding eggs. These findings point to differing selective pressures that influenced the rise of each type of care.

Researchers suggest that when paternal care developed from existing maternal care, it may represent a sexual selection mechanism termed enhanced fecundity, in which females preferentially choose males already engaged in egg protection.

Why harvestmen are ideal for studying fatherhood

With more than 6,900 recognized species, harvestmen rank among the most species-rich arachnid groups. Although they constitute only about 0.6 percent of total arthropod diversity, they account for over half of all independently evolved cases of paternal care recorded in arthropods. This makes them an outstanding model for investigating the origins of parenting behaviors. Lead researcher Glauco Machado noted that paternal care remains uncommon in nature yet has evolved independently many times. Harvestmen therefore offer an exceptional opportunity to examine the ecological and behavioral factors that promote such care. In numerous species where males alone tend the offspring, this activity functions as a sexually selected trait, with females showing preference for males already guarding eggs.

iNaturalist dramatically expanded the dataset

Citizen science initiatives enable individuals without formal scientific training to submit valuable observations that advance research. Volunteers worldwide have contributed to bird population monitoring, rediscovery of species thought extinct, and even interpretation of ancient cave inscriptions. After learning about successful applications of citizen science in ornithology, Machado's team adopted the iNaturalist platform, which supports georeferenced uploads of plants and animals from nearly any location. The impact was immediate. Between 1936 and 2025, peer-reviewed literature recorded parental guarding in just 80 harvestman species. iNaturalist contributions added 62 new records, more than doubling the previous total. The entire search process on the platform required only two days.

Citizen science speeds research worldwide

Machado emphasized that iNaturalist's primary advantage lies in the global accessibility of its records rather than sheer volume alone. The platform accelerates the pace of biological discovery by removing financial and logistical barriers associated with museum visits and extensive travel. A comprehensive search that once demanded weeks or months of international fieldwork can now be completed in roughly one week. This democratization of data proves especially valuable for researchers in the Global South who may lack resources for repeated museum expeditions.

Taxonomists remain essential

Even as citizen science grows in importance, the study underscores the irreplaceable role of professional taxonomists. Accurate species identification, determination of the sex of caregiving individuals, and differentiation between genuine parental care and related behaviors such as mate guarding all require expert knowledge. Machado highlighted that taxonomy underpins conservation efforts because unnamed species cannot be effectively protected. Names assigned by taxonomists therefore form the foundation for all subsequent biological research.

Study limitations and future research

The authors acknowledge potential sampling biases, noting that guarding individuals are far more conspicuous and photographable than species that provide no care. Despite these constraints, the expanded dataset helps address longstanding gaps in knowledge about which harvestman species exhibit parental care. With more than half of the analyzed records representing new discoveries, the researchers anticipate that citizen science will increasingly support comparative studies of maternal and paternal care across insects, amphibians, and many other animal groups.