Naturalists, including Charles Darwin, have always thought that the appearance of flowers is dictated purely by the need to attract the various insects, mammals and birds needed for cross-pollination.
But the new research, which has been published in the current issue of New Phytologist, suggests that animals who may potentially eat the plants can also exert an important influence on floral characteristics.
Biologists from the three universities studied plants in Western Australia to show how the need to defend flowers against herbivores such as emus is also a factor in their evolution.
The scientists studied hakeas, a group of plants which are noted for their diverse appearance. Although many species produce relatively small, white flowers encircled by dense spiny leaves, others possess large, red flowers which are sparsely surrounded by spineless leaves.
Earlier observations suggested that while the former are predominantly pollinated by insects, the showy and accessible flowers of the latter are usually bird-pollinated. However, showiness and accessibility bring with them a potential problem – if pollinators like birds can easily locate and access a flower, then so can large vertebrate herbivores.
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They found consistently higher levels of cyanide in the large, red flowers of bird-pollinated species compared with the spiny, white-flowered plants. As cyanide is commonly employed by plants to deter herbivores, the obvious conclusion was that this has evolved in bird-pollinated flowers as a response to herbivore attack.
The close association between red flowers, bird-pollination and high floral cyanide levels also suggests a second major revision of how biologists must now view floral evolution.
Red flowers have traditionally been seen as a means of attracting bird pollinators to the plant. But Dr Hanley believes that the animals that eat the plants may learn to associate the colour with the bitter taste produced by the cyanide.
“The colour red acts as a warning to large vertebrate herbivores like emus, parrots and kangaroos that the flower contains distasteful or even poisonous cyanogenic compounds. It seems that Western Australian plants have not only developed a remarkable defence against would-be flower predators, but that they also clearly advertise the fact,” he said.