Plants Pollinated by Opossums

In the Atlantic coastal forests of south-central Brazil grows a peculiar holoparasitic plant that resembles a fungus in appearance: Scybalium fungiforme, a member of the family Balanophoraceae. This plant emerges above the soil surface only during its flowering stage. At all other times it remains hidden underground, deriving nutrients entirely from the roots of its host plants. Its reproductive structure forms a disk-shaped inflorescence wrapped in scale-like bracts, and the species is dioecious, with separate male and female individuals. Because of its unusual morphology, the plant attracted the attention of researchers, who began investigating a fundamental question: what kind of animal is responsible for pollinating it?

Most known species within Balanophoraceae are pollinated by insects of the order Hymenoptera, particularly bees and wasps. Other visitors such as beetles, cockroaches, flies, and even harvestmen have also been reported participating in pollination in this family. Based on the floral traits and flowering behavior of Scybalium fungiforme, however, researchers proposed an unconventional hypothesis in the 1990s: the plant might be pollinated by opossums or small rodents. This prediction arose from several floral characteristics commonly associated with pollination by non-flying mammals. The flowers open at night, possess dull coloration, and produce abundant nectar close to ground level—features frequently found in plants adapted to nocturnal terrestrial pollinators. Opossums, in particular, are common nocturnal ground-dwelling mammals in the region.

The nectar production pattern of Scybalium fungiforme further supports this possibility. Nectar secretion occurs mainly at night, and a single inflorescence can produce between 113 and 1,858 microliters of nectar with a sugar concentration of approximately 8–10%. Even during the daytime, more than 200 microliters of nectar may remain available. Nevertheless, daytime temperatures are higher and can accelerate nectar evaporation, suggesting that the plant likely relies primarily on nocturnal visitors for pollination.

Sketch of an opossum drinking nectar from Scybalium fungiforme（感謝Jaime Somora/Arquivo Pessoal提供）

To test the hypothesis, researchers conducted preliminary field experiments during the 1990s. They applied fluorescent powder to the inflorescences of Scybalium fungiforme and placed baited traps nearby to capture animals that approached the plants. During these trials they captured a white-eared opossum, Didelphis albiventris, whose snout was coated with fluorescent powder, indicating that it had contacted the flowers. Although this observation demonstrated interaction between the opossum and the inflorescences, the researchers did not directly observe pollen transfer between different plants at that time.

Nearly three decades later, in 2019, researchers revisited the question at a second forest site located about 50 kilometers from the original study area. This time they used infrared-equipped video cameras to record nocturnal activity around the plants. The recordings revealed that the big-eared opossum, Didelphis aurita, frequently visited the inflorescences of Scybalium fungiforme. Over four nights of observation, fourteen visits were documented, each lasting between 25 and 70 seconds. The visits followed a consistent pattern in which the opossum moved between different plants roughly every forty minutes, indicating a trap-lining foraging strategy in which an animal repeatedly follows a predictable route among food resources.

The cameras also recorded occasional visits by a rodent species, the montane grass mouse, Akodon montensis. However, only three visits were observed, all to a single plant, suggesting that rodents likely play a minor role compared with opossums.

https://facebook.com/61568286094421/videos/1236458250993624

（影片：紀錄負鼠取蜜。感謝Amorim FW et al提供。）

Daytime observations revealed additional potential pollinators. One surprising visitor was the Violet-capped Woodnymph hummingbird, Thalurania glaucopis. These hummingbirds visited both male and female inflorescences and often moved among multiple plants during a single feeding bout. Individual visits lasted between 20 and 150 seconds. Although the floral morphology of Scybalium fungiforme does not match the classic traits associated with hummingbird pollination, the reddish bracts surrounding the inflorescences may attract nectar-feeding birds. Hummingbirds are known to exploit a wide range of nectar sources, including flowers that are not specialized for bird pollination, and may therefore act as secondary pollinators.

A hummingbird feeding on nectar from Scybalium fungiforme（感謝Amorim FW et al提供）

In addition to birds, numerous insect visitors were recorded during the day. The most frequent were hymenopterans, including the stingless bee Trigona spinipes, the honeybee Apis mellifera, and the social wasp Agelaia angulata. Wasps were particularly abundant and often remained on the inflorescences for extended periods while feeding on nectar or collecting pollen.

Despite the diversity of daytime visitors, the evidence indicates that opossums are the primary pollinators of Scybalium fungiforme. Their bodies can carry substantial quantities of pollen within their fur, and their size allows them to move effectively between different plants. Most importantly, opossums possess an anatomical feature that enables them to access the flowers: opposable toes on their hind feet. These toes allow them to grasp and peel away the scale-like bracts that cover the inflorescences. Only after these bracts are mechanically removed are the flowers exposed and accessible for nectar feeding and pollen transfer.

This requirement for mechanical removal of bracts is unusual among flowering plants and may represent a specialized evolutionary adaptation associated with pollination by non-flying mammals. Opossums, with their dexterous feet and climbing ability, are particularly well suited to perform this action. Although hummingbirds and insects frequently visit the flowers and may contribute to pollination, their roles appear to be secondary compared with that of opossums.

The discovery that Scybalium fungiforme relies primarily on opossums for pollination highlights the remarkable diversity of pollination strategies found in flowering plants. It also suggests that non-flying mammals may play a more significant role in pollination within the Balanophoraceae family than previously recognized, opening new avenues for future research into plant–animal interactions and the evolution of pollination systems.

Author: Shui-Ye You

Reference:

Amorim FW et al. (2020). Good heavens what animal can pollinate it? A fungus-like holoparasitic plant potentially pollinated by opossums. Ecology.