Staghorn ferns are popular houseplants, sporting long, antler-like fronds that poke out from a brown, tissue-papery base. They may also be the first known example of a plant that exhibits a type of social organization—that is, the first plant thought to be eusocial.
In the dry forests of Lord Howe Island in in the Tasman Sea, wild staghorns grow in colonies clinging to the trunks of trees. A recent study in Ecology finds that nests of these ferns arguably share characteristics with those of ants or termites; they’re composed of closely-related individuals that form morphologically distinct castes with different roles. All previous examples of eusociality are in animals. “Plants were completely off the radar,” says co-author K.C. Burns, an evolutionary biologist at Victoria University of Wellington, New Zealand. Finding a putative example of a eusocial fern shatters the notion that animals are somehow more socially complex, he says, and suggests that “the pathway to eusociality is open to both plants and animals.”
Burns noticed the fern colonies while hiking through the stunted tropical dry forests of Lord Howe Island, where trees are short and arboreal epiphytes grow at eye level. Each fern had two kinds of fronds. Strap fronds stand out, growing long and green from the center of each individual plant. Nest fronds, by contrast, are round and flat, spreading like green pancakes from the middle of each plant; the fronds senesce to form the tissue-papery base.
Ferns growing at the top of a colony have upright strap fronds, shaped like rain gutters that channel water and nutrients into the core of the group. Their nest fronds are waxy, and appear to deflect water and leaf litter into the nest below. Ferns lower in the colony have strap fronds hanging slack toward the ground and nest fronds that are circular and spongy.
When Burns quantified the absorbency of nest fronds from 10 different colonies, he found that those growing at the bottom had thicker, more absorbent tissue than those at the top. “It’s plain as the nose on your face that they’re subdividing labor,” Burns says. “[Ferns] at top seem to be water and nutrient capturers; ones below seem to store water.”
What’s more, when he randomly sampled five individual ferns across 10 colonies, Burns found that the number of reproductive fronds increased with the height of the colony, and 40% percent of ferns didn’t reproduce at all. Reproductive division of labor—typically a reproductive queen and nonreproductive workers and soldiers—is one classic criterion for eusociality. DNA analysis of the 10 fern colonies confirmed that the majority contained genetically identical individuals, though two nests held more than one genotype.
Eusociality typically requires two other conditions. One is overlapping generations, meaning one generation co-occurs with the next. The other is cooperative brood care, meaning collectively feeding and supporting offspring through division of labor. It’s unclear if those conditions were satisfied in the case of the ferns, says evolutionary biologist Sandra Rehan, at York University in Toronto Canada. There was no generational data in this study, she notes. And since the ferns spread asexually on shared roots, they don’t actually exhibit an active system of resource acquisition typical of brood care. But Uli Ernst, a behavioral ecologist at the the Apicultural State Institute at the University of Hohenheim, Germany, adds that since older and younger ferns (their clones) live together sharing water and nutrients, one could technically call these overlapping generations and brood care.
One key question is what defines an individual fern. If a colony can begin with a small plume of strap fronds sticking up from a few nest fronds and then spread asexually on the same roots, perhaps it is a single plant, Rehan says. Strawberries, too, spread on stolons, and a whole field can represent a summer’s growth of one plant.
The difference, Burns says, is that the whole strawberry patch looks the same. The ferns, by contrast, differ markedly in morphology and reproduction depending on their role in the colony. The next step, he adds, is to sample 40 more wild nests, quantifying their life history traits more thoroughly.
Drawing conclusions about staghorn social organization may ultimately hinge on the nuances of eusociality—some definitions frame the concept as more of a spectrum, notes evolutionary biologist Guy Cooper, at the University of Oxford in the UK, who was not involved in the fern study. The most extreme form, obligate eusociality, occurs when castes are permanent and irreversible, such that no individual could survive and reproduce alone. In these cases, natural selection shifts from acting on individuals, to driving adaptations at the colony level. At the other end of the spectrum is facultative eusociality, in which castes can be impermanent and individuals can survive and reproduce alone. Future work has yet to clarify on which end of the spectrum this staghorn falls. Either way, the fern is exciting, Cooper says. “It’s clearly eusocial in some sense of the word.”
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