Sunbird surprise: A test of the predictive power of the syndrome concept

- Literature suggests that Thonningia sanguinea is pollinated by flies.
- This is not supported by the floral traits (pollination syndrome).
- We predicted that rodents would be important pollinators.
- We found that sunbirds were the most important pollinators.
- This paper highlights the usefulness and frailties of the syndrome concept.

Floral syndromes are thought to be the product of convergent evolution, where the floral characters of unrelated species have evolved similar forms in response to shared pollinators. A contentious corollary is that floral form should be predictive of pollinators. In the forests of Uganda, we came upon the inflorescences of Thonningia sanguinea, a parasitic plant which previous literature suggested was part of a brood site pollination mutualism with flies (Calliphoridae and Muscidae). The general phenotype of the inflorescence suggested pollination by vertebrates, and the phenotypic similarity with several species of rodent pollinated Protea suggested to us that terrestrial mammals may be important pollinators. Pollinator observations and quantifications of pollen loads demonstrated that T. sanguinea is not visited by mammals, and that sunbirds are likely the most effective pollinators. The fact that the syndrome concept drove us to question the published literature on fly pollination demonstrates the usefulness of the concept. However, due to several phenotypic traits which did not conform to the classic sunbird pollination syndrome, sunbird visitation came as a surprise. While the syndrome concept is very useful, pollinator predictions based on syndrome traits should always be treated as working hypotheses.