Staghorn ferns act like bees in a beehive
KC Burns’ favorite research days are when he packs his backpack and walks in the wilderness without a diary. While hiking on Lord Howe’s Island in Australia, he came across a cluster of staghorn ferns. They are common potted plants, but in nature they grow in dense colonies that cling to the treetops. In the stunted forest of the volcanic island, the treetops are at eye level.
“I almost looked beyond it,” said Dr Burns, a biologist at Victoria University of Wellington in New Zealand. Then he took a closer look and realized that the plants in the colony were doing different jobs to survive. Ferns that grew taller had waxy fronds that seemed to direct rainwater toward the center of the colony. Lower down, the ferns grew leaves that were more spongy and moist to the touch. Some plants weren’t reproducing at all – they seemed to have devoted their lives to collecting water for the tangled roots of their neighbors.
It struck Dr. Burns that the ferns worked together like some kind of superorganism, maybe like bees in a beehive.
“I sat down and thought, oh my god” he said. In an article published last month in Ecology, Dr Burns and his coauthors argued that colonies of staghorn ferns Platycerium bifurcatum exhibit a kind of collective behavior known as eusociality. Until now, scientists have only recognized eusociality in certain species of animals such as bees or ants that live in colonies and divide their work.
To measure the work distribution of the ferns, the researchers sampled plants growing at different heights in 24 colonies. They counted two types of leaves on each plant. One type, which they called the nest fronds, is rounded and mostly brown, clasping the tree like cupped hands. The other fronds, which are long, green and forked like antlers, may develop spores on their underside which will become the next generation of ferns.
Plants closer to the top of each colony had more spore-forming fronds. Plants near the bottom had more sectional non-breeding nest fronds. About 40 percent of individual plants did not reproduce at all, such as worker bees.
Next, the scientists cut quarters out of the fronds of the nest, dried them, and then soaked them in water to measure how much water they were absorbing. They found that the fronds of the nest at the bottom of a colony were more absorbent.
Since the colony’s roots developed in a tangled web, these spongy leaves could help the entire colony stay hydrated. Scientists found that the largest colonies (the largest they studied contained 58 individual ferns) had more spore-bearing fronds per capita. Living in a large group could therefore improve the physical condition of ferns.
For the most part, the groups are families. “We quickly realized that genetics are important,” said Dr Burns, as eusocial animals live in closely related groups.
When the researchers analyzed the DNA of 11 fern colonies, they found that most of the plants in a colony were as closely related as possible – they were clones. New plants are born from the buds in the root systems of others, Dr. Burns said.
Being clones “means that different individuals have genetically aligned interests,” said Guy Cooper, an evolutionary biologist at the University of Oxford. By helping a neighboring clone, a plant also helps its own genes to survive.
Dr Cooper said he would like to know more about the life cycle of a colony and how individual ferns depend on each other.
Even though staghorn ferns are not as social as bees, “it was very cool to see that there could be similar types of complex social behaviors occurring in plants,” he said.
He also pointed out that some plants that were propagated by cloning were seen as a single individual, not several. For example, aspens produce massive groves of clones from a root network. An aspen forest in Utah nicknamed Pando is sometimes referred to as the world’s largest organism, spanning 106 acres.
“Then you have to ask yourself more philosophical questions about whether they are different individuals to begin with,” Dr. Cooper said of ferns. Perhaps the ferns in a colony look more like limbs on a body than bees in a beehive.
However, cloning does not explain the whole story of staghorn ferns. In some colonies of Lord Howe Island, Dr. Burns and his colleagues have found unrelated plants. They don’t know how these ferns became part of the treetop communities.
Plants are among the most flexible living things on Earth, said Karen Kapheim, a biologist at Utah State University who studies the evolution of social behavior in bees. Perhaps it’s no surprise that a fern can also evolve into social trends, she said.
More and more science is revealing how plants behave and communicate, said Dr Kapheim. “I think adding social behavior to that fits with this new emerging understanding of plants.”