By Athayde Tonhasca
The peacock flower (aka red bird of paradise or pride of Barbados, Caesalpinia pulcherrima), originally from tropical regions, and the flame azalea (Rhododendron calendulaceum), an American native, are favourites among British gardeners and landscapers. These plants’ species epithets explain their popularity: pulcherrima, Latin for ‘prettiest’, and calendulaceum, ‘of the colour of calendula’, refer to their flamboyant inflorescences of yellow, red, and orange tones.
These plants are not related, but if you examine their flowers, you will notice they share one feature: their anthers (male parts) and stigma (female parts) are elongated and at a considerable distance from one another. Botanists call this spatial separation between male and female structures herkogamy. This feature, just like dichogamy (when male and female reproductive organs mature at different times), reduce the interactions between pollen and stigma of the same plant, therefore reducing the chances of inbreeding.
Herkogamy would be a headache for plants relying on bees or flies for pollination. Considering the size of the flowers and the way their reproductive structures are spread out, it’s unlikely that these insects would come into contact with both anthers and stigma. The peacock flower and the flame azalea need pollinators capable of handling these awkward morphological quirks. They need butterflies.
Butterflies are familiar, prevalent and showy flower visitors, but visitation alone does not make a pollinator. Pollination requires the transport of a sufficient number of viable pollen grains to the right stigma at the right time. And, generally speaking, butterflies are not good at it. They are not particularly ‘hairy’ and lack specialized pollen-collecting structures. Most species don’t feed on pollen, and are quite skilful in avoiding it by slurping nectar while perched on their long legs. So psychophily (pollination by butterflies) has been demonstrated here and there, such as for some orchids and in the cotton fields of the American Gulf Coast. But cases are relatively rare, and mostly in the tropics. Many an uncharitable ecologist has categorised butterflies as ‘parasitic nectar thieves’ because they deplete flowers of a metabolically expensive product without contributing to their pollination.
To the peacock flower and flame azalea, butterflies are anything but parasitic. They deal with structures that stick out like bristles on an old brush by fluttering over the flower, inspecting it before landing to sip its nectar. By doing so, they touch the anthers, and pollen grains get attached to the ventral surface of their wings. When they repeat these manoeuvres on another plant, some of the pollen will fall on or be brushed off by the pistils, enabling cross-pollination.
Size is important in this pollen-gathering mechanism, as reaching both anthers and stigma requires wings with large areas. So big swallowtails (family Papilionidae) are the main pollinators of these herkogamic flowers. And the more they flap their wings, the better. In the case of the flame azalea in its native Appalachian Mountains, the great spangled fritillary (Speyeria cybele) gathers pollen in its wings, but not nearly as much as the eastern tiger swallowtail (Papilio glaucus). The former stays still once it lands on the flower, while the latter keeps fanning its wings, thus increasing the chance of contact with anther and stigma. The outcome of wing pollination has been measured experimentally: when butterflies were excluded from flame azaleas, their fruit set was almost nil.
An unusual flower shape may be a hint of adaptation to wing pollination. Another garden favourite, the South African blood lily – also aptly known as ball lily, fireball lily, and powderpuff lily (Scadoxus multiflorus) – seems to have been designed to make life difficult for pollinators. But butterflies are not deterred by its porcupine look. They flap their wings and get their nectar, picking up some pollen while they’re at it.
These herkogamic species do their bit to facilitate butterfly pollination. Their yellow/red colours are particularly attractive to these insects, so flowers are easily located. And their pollen is sticky and relatively flattened, so it adheres to the scales of a butterfly wing.
In his ‘butterfly effect’ theory, meteorologist Edward Lorenz (1917-2008) asked whether the flap of a butterfly’s wings in Brazil could set off a tornado in Texas. This metaphor suggests that small events can lead to a ripple effect with a momentous result over time. Studies with herkogamic flowers reveal a new angle of the butterfly effect: by flapping their wings, butterflies may have much to contribute to the maintenance of biodiversity.