New research shows that red flower
colouring could have evolved to deter,
rather than attract, animals.
Image: iStockphoto

Australian native plants have turned to chemical warfare to ward off flower-loving herbivores, with a new study revealing that bright red flowers evolved not to attract animals – but to turn them away.
Researchers from Curtin University and Plymouth and Portsmouth universities in the United Kingdom have overturned the belief held by many naturalists, including Charles Darwin, that flowers evolved purely to attract birds and insects as a way of helping them cross-pollinate. 

The latest work published in the current issue of New Phytologist suggests flower-eating animals also could influence floral characteristics. The flowers are not just an evolutionary response to pollinators, but also to herbivores.

The biologists studied 51 hakea species to show how their evolution reflects the need to defend their flowers against herbivores, including emus and kangaroos.

Plants were collected from South-Western WA between October 2003 and June 2005 and were divided into two groups.

Insect-pollinated species sported mainly tiny, white flowers surrounded by dense spiny leaves, which prevented animals such as emus and cockatoos from eating the flower.

Bird-pollinated species have open spineless leaves, usually adorned by bright red flowers, which also attracted emus and cockatoos.

The team discovered consistently higher levels of cyanide in the red flowers of bird-pollinated species compared with the spiny, white-flowered plants. The researchers found that plants with red flowers contained about 10 milligrams of cyanide per gram, which is enough to make an animal sick.

Cyanide is commonly used by plants to deter herbivores, so did this evolve in flowers as a response to herbivore attack?

The researchers believe so. Animals that eat the plants may learn to associate the colour red with the bitter taste produced by the cyanide.

WA plants may have not only developed a remarkable defence against flower predators, but they also clearly advertised the fact, they say.

The research was funded by a British Ecological Society small ecological project grant. The Society is a registered charity established in 1913 by academics to promote and foster the study of ecology.