Paternal care – a father tending eggs or young on his own – is one of the rarest arrangements in the animal kingdom. Yet in a group of long-legged arachnids called harvestmen, it has appeared again and again. A new study shows how, and it owes much of its evidence to ordinary nature lovers armed with smartphones.
An international team led by a University of São Paulo biologist combined nearly three decades of field research with tens of thousands of observations uploaded to iNaturalist, a popular platform where anyone can post photos of wildlife. The result, published in the Zoological Journal of the Linnean Society, more than doubled the documented cases of parental care in harvestmen. Since 1936, scientists had recorded such behavior in 80 species; the new work added dozens more, with 62 fresh records drawn from iNaturalist alone.
That expanded catalogue let the researchers reconstruct, for the first time, how maternal and paternal care evolved across the superfamily Gonyleptoidea. The pattern is anything but tidy: guarding behavior arose, vanished and re-emerged repeatedly over the group's history. Maternal care always evolved from no care at all – the same route seen in insects. Paternal care, by contrast, sprang from either no care or from existing maternal care.
Why harvestmen matter
That flexibility makes harvestmen an unusually rich natural laboratory. With more than 6,900 known species, they make up just 0.6 percent of arthropod diversity, yet account for more than half of the independent origins of paternal care in the animal world. When males alone guard the eggs, the researchers note, it often reflects sexual selection – females favoring caring fathers, an idea known as the "enhanced fecundity" hypothesis.
"It's very rare in nature, paternal care, and this behavior evolved many times independently," said lead author Glauco Machado. Studying harvestmen, he added, lets scientists probe the forces that push a species toward it.
The findings are also a quiet vindication of citizen science: photographs snapped by curious amateurs, taken together, revealed an evolutionary story that decades of specialist fieldwork alone could not.