(Photograph = Black-eyed Bulbul on Aloe vryheidensis, Martin Heigen)
Introduction
Plants and their pollinators go together, like lock and key. The usual pollinators are insects and birds. Aloes are generally bird pollinated. But there is specificity. Just any bird won’t do. It has to be the right bird(s) so that pollen is transferred.
The Details
Birds that pollinate flowers usually drink nectar, which is a watery solution of sugars and amino acids. The birds drink the nectar for nutrients, and perhaps because it is often sweet. Most aloes have clear, sweet nectar. The birds drink the nectar but in doing so are dusted with pollen, which they transfer to other flowers. In this way, aloes ensure that pollination is achieved, that seeds are made, and that they will have descendants.
A number of bird species visit the flowers of Aloe vryheidensis, but only a few of them reliably transfer pollen to other A vryheidensis flowers. Some birds come to sample the nectar and then fly off on other business and never transfer the pollen to another flower of A. vryheidensis.
A. vryheidensis is unlike most aloes because it has dark, bitter nectar. After trying it once, most birds are repelled, especially sunbirds which don’t “fit” the flowers well and which are not good pollinators. However, some species of birds fit the flowers of A. vryheidensis just right and these species don’t mind the bitter nectar. In fact, they like it. In effect, A. vryheidensis “filters” out the wrong nectar sippers and encourages the right ones.
One way to interpret this information is that the bitter nectar evolved to help the Aloe reject unsuitable species of birds. The nectar also evolved to be palatable to the “right” birds. The right birds evolved to enjoy the bitter nectar, and so they get more nectar because they don’t have to compete with most other species of birds.
In any event, the arrangement seems like a win-win situation.
Additional Reading: Bitter-Tasting Nectar Functions as a Filter of Flower Visitors
