A photograph uploaded to iNaturalist may look like an ordinary snapshot of a strange-looking arachnid, but thousands of such images have helped scientists uncover a major evolutionary story. By combining more than 24,000 citizen-science photographs with field and laboratory observations, researchers have more than doubled the number of harvestman species for which parental care is known. Their findings reveal that caring for eggs evolved repeatedly—and that paternal care followed more than one evolutionary route.
The study, led by Glauco Machado of the Institute of Biosciences at the University of São Paulo, examined parental behavior in harvestmen belonging to Gonyleptoidea, a diverse superfamily of Opiliones found mainly in the Neotropics. The results, published in the Zoological Journal of the Linnean Society, provide one of the most detailed examinations yet of how egg guarding evolved in these animals. The research also demonstrates how wildlife photographs collected by the public can answer questions that would otherwise require decades of fieldwork.
Harvestmen are commonly called daddy longlegs, although they are neither spiders nor closely equivalent to the long-legged flies that share the same nickname in some countries. They are arachnids, but unlike spiders, they do not produce silk and do not possess venom glands used to subdue prey. Their bodies usually consist of a compact, fused structure bearing eight legs, and many species are active on the forest floor, tree trunks and leaf litter. Their biology is still poorly documented, particularly in tropical regions where new species and unusual behaviors continue to emerge.
In many gonyleptoid harvestmen, females deposit clutches of eggs on surfaces such as soil, vegetation, logs or specially constructed structures. Some species abandon the eggs after laying them, leaving embryos exposed to predators, parasites, fungi and environmental fluctuations. Others exhibit parental care, with a male or female remaining near the clutch and defending it. The guarding individual may attack predators, remove debris or fungi, maintain the condition of the eggs and sometimes protect newly hatched young. Although both sexes can provide care in different species, male care is especially significant because paternal investment is comparatively uncommon across the animal kingdom.
The researchers first turned to iNaturalist, a platform where users upload photographs and observations that can be identified and reviewed by a global community. Images showing harvestmen in contact with egg masses, positioned beside clutches or occupying characteristic nesting sites provided clues about reproductive behavior. In some cases, the photographs captured unmistakable evidence of care: an adult standing over eggs or defending a nest. In others, images showing an adult-free clutch supported the conclusion that a species lacked parental care, provided the observation was interpreted alongside biological and taxonomic information.
From the citizen-science records, the team documented the presence or absence of parental care in 53 species. They then combined those findings with new observations gathered by 19 researchers working in five Latin American countries. Together, the records produced information on parental behavior in 85 species, many of which had never before been included in an evolutionary analysis of care. The expanded dataset allowed the scientists to construct a phylogenetic framework—a family tree showing relationships among species—and map parental behaviors onto it.
The evolutionary pattern was striking. Maternal care appeared to have evolved only from ancestors that did not provide parental care. In other words, the available evidence did not support a transition in which egg-guarding mothers directly gave rise to species in which females abandoned care. Paternal care showed a more complex history. It evolved in some lineages directly from ancestors with no parental care, but it also arose from lineages that already possessed maternal care. This suggests that the evolutionary shift from mothers guarding eggs to fathers taking over the task may be possible under particular ecological or reproductive conditions.
A male harvestman of the genus Quindina, photographed in Colombia, illustrates the kind of behavior that made the study possible. The animal is shown guarding eggs on a disk-shaped mud nest, a scene that could easily be overlooked as a routine wildlife image. Yet such a photograph can record a specific reproductive strategy, identify a previously undocumented association between an adult and a clutch, and help distinguish parental care from simple proximity. When many images are combined with expert verification, isolated observations become evidence for testing broad hypotheses about evolution.
The biological advantages of paternal care may differ among species. A male that guards eggs can reduce predation and increase the number of offspring that survive, but remaining with a clutch may also limit opportunities to feed, mate again or avoid environmental danger. The costs and benefits depend on factors such as nest structure, predator pressure, the availability of food and whether females produce eggs that are especially valuable or vulnerable. In species where females mate with multiple males, males may also gain an advantage by guarding eggs that are likely to be their own. These pressures can help explain why parental care appears in some closely related harvestmen but not in others.
The study’s broader message extends far beyond arachnids. Citizen-science platforms are often viewed as tools for mapping species distributions or tracking seasonal changes, but photographs can also preserve evidence of behavior that is rarely witnessed by professional researchers. A single image may reveal courtship, migration, predation, nest construction or parental investment. Because harvestmen are frequently photographed from multiple angles and encountered across a wide geographic area, iNaturalist provided a large, geographically distributed source of observations that traditional expeditions alone could not easily match.
Machado and his colleagues emphasize that contributors did not need to know they were documenting an important discovery. Their observations became scientifically valuable because they were made carefully, linked to locations and dates, and shared where specialists could examine them. The researchers say that photographing wildlife from several angles and uploading images to platforms such as iNaturalist can help uncover behaviors that remain invisible in formal surveys. In tropical ecosystems, where biodiversity is immense and many species are active at night or in difficult terrain, this distributed approach could transform the study of animal behavior.
The findings also highlight a persistent challenge in evolutionary biology: behavior is often harder to document than anatomy. A species may be known from preserved specimens while its reproductive habits remain entirely mysterious. By integrating community photographs, field observations, laboratory work and phylogenetic analysis, the researchers converted scattered evidence into a testable account of how parental care evolved. What began as thousands of casual encounters with long-legged arachnids ultimately revealed repeated evolutionary experiments in motherhood, fatherhood and abandonment—showing how curiosity, when shared, can become a powerful scientific instrument.
Subject of Research: Evolution of parental care in harvestmen, including maternal and paternal egg-guarding behavior.
Article Title: One small step for citizens, one giant leap for science: iNaturalist records boost our understanding of the evolution of parental care in a clade of arachnids
Web References: Smithsonian Tropical Research Institute website: https://stri.si.edu/ ; STRI video channel: https://www.youtube.com/user/TheSmithsonianPanama ; iNaturalist: https://www.inaturalist.org/
References: Machado, G., Buzatto, B. A., Caetano, D. S., Flores, E., García-Hernández, S., Grob, O., … & Villarreal, O. 2026. One small step for citizens, one giant leap for science: iNaturalist records boost our understanding of the evolution of parental care in a clade of arachnids. Zoological Journal of the Linnean Society, 207(2), zlag061.
Image Credits: John A. Uribe Rosas
Keywords: harvestmen, daddy longlegs, parental care, paternal care, maternal care, arachnid evolution, citizen science, iNaturalist, Opiliones, Gonyleptoidea, animal behavior, tropical biodiversity
