Scientists uncover the secret pollinators behind southern Africa’s paintbrush lilies

Hannah Butler and Steven Johnson have uncovered that several species of Haemanthus, also known as paintbrush lilies, rely on sunbirds to pollinate their flowers and produce seeds. The research, published in the South African Journal of Botany, uses camera traps and selective flower exclusions to show that sunbirds play a crucial role in pollinating these beautiful blooms despite their flowers lacking some typical bird pollination features. This study challenges prior assumptions that paintbrush lilies like Haemanthus deformis and Haemanthus coccineus are pollinated by insects or rodents. 

Sunbirds essential for pollination and seed production

The researchers discovered that two Haemanthus species require sunbirds for pollination. They did this by using cages around the flowers to keep sunbirds out.

When they covered the red flowers of Haemanthus coccineus and the white blooms of Haemanthus deformis with mesh cages that kept birds away, these flowers produced significantly fewer fruits and seeds. For Haemanthus coccineus, the fruits had no seeds at all. Haemanthus deformis had fewer seeds, and these were eliminated when the scientists put bags around the blooms, indicating that insects play a small role in pollination.

In contrast, the light pink Haemanthus humilis did manage to set some seed when sunbirds were excluded. This suggests it can use insect pollinators to a degree. However, open, bird-accessible flowers still produced significantly more fruit. So, while Haemanthus humilis enjoys mixed pollination, sunbirds remain important contributors.

Sunbirds get creative to access nectar

The research team set up motion-activated camera traps to directly record how sunbirds interacted with the flowers. The footage revealed frequent visits from multiple sunbird species across all the Haemanthus plants.

The cameras documented how the birds adapted their behaviour to access nectar from different shaped blossoms. For instance, the short flower stalks of Haemanthus deformis place the nectar right at ground level. But that didn’t deter the sunbirds – they simply perched on the earth while feeding! This surprised the researchers, who predicted rodents might pollinate the low geoflorous blooms.

It was interesting to see that sunbirds pollinated both red and white Haemanthus species. This result shows that flower colour does not always reliably indicate a plant’s pollination strategy. Sunbirds fed from the bright red blooms of Haemanthus coccineus, matching the “classic” bird pollination syndrome. But they also readily visited the white flowers of Haemanthus deformis – a colour more associated with insect pollination. This shows pollinators like sunbirds forage based on nectar rewards rather than colour cues alone. It emphasises the need for observational and experimental studies to determine plant-pollinator relationships accurately.

Paintbrush blossoms conceal specialised pollination

The research focused on four species of Haemanthus native to South Africa. These plants produce dense clusters of small tubular blooms packed into rounded inflorescences. Though each inflorescence is an individual flower, they act together as a unit, which Butler & Johnson refer to as blossoms.

Their blossoms evoke little paintbrushes, which inspired both their scientific name (Haemanthus means “blood flower”) and one of their common names, paintbrush lilies. The compact flower heads offer pollinators easy access to nectar. But their small individual blooms lack some typical bird-adapted features like large landing platforms. This made their pollination strategy unclear without detailed study.

Prior research on some Haemanthus species suggested they were pollinated by insects like bees or butterflies. Their flowers offer accessible nectar but lack traits like large size and perches that suit bird pollinators. However, the densely packed inflorescences resembled those of other Southern African plants known to be bird-pollinated. This discrepancy prompted the researchers to look closely through observations and experiments.

The paintbrush structure seen in Haemanthus is also found in other genera like Scadoxus. A prior study on Scadoxus puniceus showed it relied on sunbirds, indicating this blossom form represents a specialised adaptation to bird pollination. This evidence hinted that sunbirds may likewise be important pollinators for Haemanthus species despite their small individual flower size.

Peeling back the petals on paintbrush pollination

To unravel the pollination puzzle of Haemanthus, the researchers combined observational data with exclusion experiments. They recorded floral visitors in the wild and analysed traits like nectar properties. This allowed them to see which animals interacted with the blossoms.

However, observations alone couldn’t reveal if frequent floral visitors actually pollinated the plants. So, the team did selective exclusion experiments at the study sites. They covered inflorescences with mesh cages that prevented bird access while allowing smaller insects to still reach flowers.

Butler & Johnson then assessed the pollination contributions of birds versus insects by comparing the caged plants with uncaged controls. Caged inflorescences that produced significantly less fruit and seeds clearly relied on sunbirds. Differences in seed output with and without sunbirds definitively demonstrated their key pollination role.

Analysing floral traits also provided clues to adaptation. All species offered nectar suited to bird beaks. This specialisation for sunbird pollination likely contributed to the evolution of Haemanthus’ unique paintbrush blossoms.

Shedding light on specialised pollination systems

By unravelling the pollination biology of multiple Haemanthus species, this research provides new insight into specialised paintbrush blossoms. It shows this floral adaptation suits sunbird pollination, even without large blossoms. The findings also expand knowledge of avian pollination diversity in southern Africa.

Sunbirds play a broader and more flexible role than expected. Their nectar-feeding specialisation allows them to service plants with divergent shapes and colours. Yet specialised floral traits like paintbrush inflorescences still cater to sunbird pollination. These mutually adapted relationships likely coevolved and diversified in tandem.

Understanding these interdependent pollination systems helps inform conservation efforts for rare plant species and threatened bird populations. The research team’s meticulous experiments remind us that even the smallest flowers may conceal complex, specialised ecosystems worth protecting.

READ THE ARTICLE
Butler, H.C. and Johnson, S.D. (2023) “Evidence for sunbird pollination in African blood lilies: Haemanthus (Amaryllidaceae),” South African Journal of Botany, 162, pp. 154–159. Available at: https://doi.org/10.1016/j.sajb.2023.09.001.

Cover: Haemanthus coccineus flowers. Image: Canva.

Updated November 3, 2023 to clear up a mess in reporting the experiment results. See comments.