Paternal care is rare, but harvestmen made it common
From the photographs of curious amateurs scattered across the globe, a team of researchers has reconstructed one of evolution's quieter dramas: how harvestmen — ancient, spider-like arachnids — came to care for their young, and why mothers and fathers arrived at that tenderness by different roads. In two days on iNaturalist, citizen scientists contributed what a century of formal fieldwork had not, doubling the known record of parental guarding in a single week. The finding reminds us that the boundaries of scientific knowledge are drawn not only by expertise, but by who is invited to look — and that evolution rarely tells a single story, even within one family of creatures.
- A century of published research on harvestman parental care was effectively doubled in forty-eight hours when citizen scientists uploaded sixty-two new records to iNaturalist, exposing how much the natural world still hides in plain sight.
- The discovery created a productive tension: the sheer volume of amateur observations outpaced formal science, forcing researchers to reckon with both the power and the limits of crowd-sourced data.
- Mothers and fathers in harvestmen did not arrive at caregiving the same way — maternal care emerged only from non-caring lineages, while paternal care took two distinct evolutionary routes, one of which appears driven by female mate preference.
- The team is working to correct a sampling bias built into the dataset, since egg-guarding animals are far easier to photograph than those that provide no care, meaning absence of records does not equal absence of behavior.
- Harvestmen, with over 6,900 species and more than half of all known independent origins of paternal care among arthropods, are now positioned as a global model organism for studying the evolution of parental behavior.
A University of São Paulo researcher and an international team have untangled a long-standing puzzle in evolutionary biology — how parental care arose in harvestmen, the eight-legged arachnids often mistaken for spiders — and they did it not through museum collections or costly expeditions, but through photographs uploaded by amateur naturalists to iNaturalist.
The numbers are striking. Between 1936 and 2025, published science documented parental guarding in eighty harvestman species. In two days of searching iNaturalist, the team found sixty-two new records, more than doubling a century of accumulated knowledge. That expanded dataset allowed them to trace, for the first time, how both maternal and paternal care evolved within the superfamily Gonyleptoidea.
The story that emerged was not a straight line. Parental care has appeared independently multiple times in harvestmen, disappeared from some lineages, and reappeared in others. But mothers and fathers took different evolutionary paths. Maternal care arose only from non-caring species — a pattern also seen in insects. Paternal care, by contrast, evolved either directly from non-caring species or from lineages where females were already guarding eggs. In the latter case, the researchers suggest sexual selection was at work: females preferring males already engaged in egg-tending, a phenomenon called enhanced fecundity.
Harvestmen are unusually well-suited to this kind of inquiry. More than 6,900 species have been identified, and they account for more than half of all independently evolved examples of paternal care known among arthropods — a remarkable concentration of a genuinely rare behavior. Lead author Glauco Machado noted that in many harvestman species where males alone care for offspring, that behavior appears to be sexually selected, meaning females actively choose caregiving males.
The researchers are careful to acknowledge that citizen science does not replace expertise. Correctly identifying species, determining the sex of a caregiving individual, and distinguishing true parental care from mate guarding all require specialized knowledge. Machado was direct: species cannot be protected without names, and names come from taxonomists, whose role in modern science is more vital than ever. A sampling bias also exists — animals guarding eggs are easier to photograph than those providing no care — but the team argues the approach still closes significant gaps, and they expect it to reshape research on parental behavior across insects, frogs, and many other animal groups worldwide.
A University of São Paulo researcher and an international team have solved a puzzle that field biologists had been picking at for nearly three decades: how parental care actually evolved in harvestmen, those eight-legged arachnids that look like spiders but aren't. The answer came not from dusty museum collections or grueling expeditions, but from photographs and observations uploaded by amateur naturalists to iNaturalist, a global platform where anyone with a camera and curiosity can log what they see in the wild.
The numbers tell the story. Between 1936 and 2025, published scientific papers documented parental guarding behavior in eighty harvestman species. In just two days of searching iNaturalist, the research team found sixty-two new records. They more than doubled a century of accumulated knowledge in forty-eight hours. The expanded dataset—nearly thirty years of field research combined with these citizen contributions—allowed the scientists to trace, for the first time, how both mothers and fathers came to care for their eggs within the superfamily Gonyleptoidea.
What emerged was not a simple story of evolution marching in one direction. Parental care in harvestmen has appeared multiple times independently, vanished from some lineages, and then reappeared in others. But the paths taken by mothers and fathers diverged sharply. Maternal care evolved only from species that showed no parental care at all—a pattern that matches what researchers have seen in insects. Paternal care, by contrast, took two different routes. It either arose directly from non-caring species, or it evolved from species where females were already guarding eggs. The researchers propose that when paternal care emerged from maternal care, it likely reflected a form of sexual selection: females favoring males who were already tending to eggs, a phenomenon known as enhanced fecundity.
Harvestmen are unusually valuable for this kind of investigation. More than sixty-nine hundred species have been identified, making them one of the most diverse groups of arachnids. Yet they represent more than half of all independently evolved examples of paternal care known among arthropods—a remarkable concentration of this rare behavior. Glauco Machado, the lead author, explained the significance: paternal care is genuinely uncommon in nature, and the fact that it has evolved so many times independently in harvestmen makes them an exceptional window into understanding what drives such behavior. In many species where males alone care for offspring, that caregiving is sexually selected, meaning females actively prefer males engaged in egg-guarding.
The speed of the discovery hinged on a shift in how science gathers data. Citizen science projects have already helped monitor bird populations, rediscover lost species, and even uncover ancient cave art. iNaturalist, the platform that made this harvestmen study possible, allows volunteers worldwide to upload georeferenced observations of plants and animals from virtually anywhere. Machado's team turned to it after hearing about citizen science applications in bird research. What they found was not just the volume of observations but their accessibility. A researcher in the Global South no longer needs to visit museums around the world, spending months and substantial money. The search took one week, with the iNaturalist portion completed in two days.
Yet the researchers are careful to note that citizen science does not replace expertise. Identifying species correctly, determining whether a caregiving individual is male or female, and distinguishing true parental care from behaviors like mate guarding all require specialized knowledge. Taxonomists remain indispensable. Machado emphasized the point: species cannot be protected if they lack names, and names come from taxonomists. Their role in modern science, he argued, is more important than ever.
The study does have limitations. Animals actively guarding eggs are far easier to photograph than species providing no care, introducing sampling bias into the dataset. But the researchers argue that work like this closes significant gaps in understanding which species exhibit parental care and which do not. With more than half of the records analyzed newly documented through citizen contributions, Machado believes this approach will continue expanding research on parental behavior across insects, frogs, and countless other animal groups where both maternal and paternal care exist.
Citações Notáveis
Paternal care is very rare in nature, and this behavior evolved many times independently in harvestmen, making them exceptional for studying the evolution of parenting.— Glauco Machado, lead researcher
Taxonomists' role in modern science is more important than ever. We cannot preserve a species that doesn't have a name, and names are provided by taxonomists.— Glauco Machado
A Conversa do Hearth Outra perspectiva sobre a história
Why does it matter that maternal and paternal care followed different evolutionary paths?
Because it tells us that evolution doesn't have a single solution to the problem of caring for young. The fact that mothers and fathers arrived at caregiving through different routes suggests they faced different pressures, different opportunities. Understanding those pressures helps us understand what makes parental care possible at all.
You mentioned that paternal care sometimes evolved from maternal care. How does that even happen?
The researchers think it's about female choice. If a female sees a male already caring for eggs—maybe his own from a previous mating—she might prefer him as a partner. He's proven he can do the work. Over time, that preference becomes strong enough to shape evolution. The males who care get more mates.
So citizen scientists just took photos and uploaded them, and that solved a thirty-year-old puzzle?
Not quite. The photos and observations provided the raw material—the data. But it took expert taxonomists to verify what species each observation was, to confirm the sex of the caregiving individual, to distinguish real parental care from other behaviors. The citizen scientists expanded the dataset; the experts made sense of it.
Why harvestmen specifically? Why not study this in something more familiar?
Because harvestmen are unusual. They're only a tiny fraction of arthropod diversity, but they contain more than half of all known cases of paternal care in arthropods. It's like finding a forest where half the trees are a species that's rare everywhere else. You study where the phenomenon is concentrated.
What's the risk of relying on citizen science for something this technical?
Sampling bias is real. People photograph animals doing visible things—guarding eggs is visible. They don't photograph absence. So the dataset skews toward species that care, not species that don't. But the researchers acknowledge that openly. It doesn't invalidate the findings; it just means you have to interpret them carefully.