In a new kind of plant trickery, this yam fools birds with fake berries

Deception and intrigue are not limited to people or even animals. Plants, too, have evolved ways to fool their pollinators, their enemies and even the organisms that disperse their seeds. Now an international team has uncovered trickery in a climbing vine that fooled even them. The black-bulb yam (Dioscorea melanophyma) makes fake berries that help the species spread to new locations, the researchers report January 12 in the Proceedings of the National Academy of Sciences.

The story “feels refreshingly new,” says Kenji Suetsugu, an evolutionary ecologist at Kobe University in Japan who was not involved in the work. These yams have lost the ability to reproduce seeds via sexual reproduction and must clone themselves. Plants that make clones — lilies and begonias, for instance — typically reproduce with detachable buds called bulbils, which tend to fall off and sprout near their parents. But by transforming the buds into fake berries that some birds eat, this yam now has a way to spread far and wide, a hedge against their local environment changing. “It’s a clever evolutionary workaround,” Suetsugu says.

Gao Chen, an ecological biologist at the Kunming Institute of Botany, part of the Chinese Academy of Sciences, and his team mistakenly picked up these bulbils thinking they were berries while they were collecting seeds in Southwest China in 2019. Seeds are usually inside berries, but there were none when they cut open this one. He thought, “They can cheat me, then, I think they can cheat birds.”

Bulbils are usually white or dull colored, not black and shiny like the yam’s, but proving these ones mimic berries took a lot of work. Chen’s team analyzed and compared the appearance and color of berries found near the yam and found 15 species where bulbils and berries were indistinguishable. Three years’ worth of camera trap photos showed that 22 bird species visit these bulbils and a few even eat them.

In the lab, Chen discovered that the most frequently fooled visitor, a bird called the brown-breasted bulbul (Pycnonotus xanthorrhous), will pick a berry over a bulbil most of the time. But when berries get scarce, say in the winter, the birds frequently eat the bulbil. The bulbil passes through the gut unharmed in about a half hour, during which time the bird may have transported it 750 meters or more, he calculated.

“The results extend the mimicry concept to nonreproductive structures of the plant,” says Pedro Jordano, an ecologist with the Spanish Research Council at the University of Sevilla who was not involved with the work. Other known examples come from sexually producing species. Japanese dogsbane lures grass flies with flowers that smell like dying ants, while a South American vine can change its leaves to match its host plant.

Biologists as far back as Charles Darwin noted that there are certain seeds that look like they are encased in the same fleshy fruit as other species but really are bare and offer no food reward to animals that eat and transport them. Black beans practice this kind of deception, Chen and his colleagues reported March 2025 in Plant Diversity.

“The birds are foxed into dispersing the bulbils because of their resemblance to fruits they are used to eating,” says John Pannell, a plant evolutionary biologist at the University of Lausanne in Switzerland, who was also not involved with the work. The birds get nothing in return. That these bulbils have evolved to look like berries, says Jordano, “is amazing for any sensible naturalist.”