The art of deception: Why morphology matters in flowers' pulling power

Ecologists know a great deal about sexually deceptive orchids' remarkable olfactory mimicry. Much less is known, however, about whether an orchid flower's morphology matters.

According to lead author of the study, Dr Marinus De Jager of Stellenbosch University: "Since the discovery that the floral scent of these remarkable orchids mimics female sex pheromones, many of these compounds have been made in the laboratory. When you apply these chemicals to objects -- even objects that look nothing like female insects -- males will attempt to mate with them, so many people assumed that scent is all that matters."

Together with Professor Rod Peakall of Australian National University, he decided to test this assumption by taking advantage of an experiment already designed by nature.

Australia is home to most of the world's species of sexually deceptive orchids. Among them are two different, but closely related, species of orchid pollinated by two different species of wasp. But although the flowers of these two orchids look different, they both produce the same chemical to attract the male wasps.

The species in question are the broad-lipped bird orchid (Chiloglottis trapeziformis), which is pollinated by a wasp called Neozeleboria cryptoides, and the large bird orchid (C. valida), which is pollinated by N. monticola.

These small orchids live in the moist forests of south-eastern Australia, and have very complex and strikingly different flower morphologies. The broad-lipped bird orchid has a small, diamond-shaped lip with a central insect-like 'callus' that resembles the female of its wasp pollinator. The large bird orchid has a larger, heart-shaped lip and many smaller calli.

To find out whether floral morphology was deceiving the wasps, De Jager presented males of each wasp species with both orchids -- the one it normally pollinates and the one it does not. He then recorded the wasps' behaviour, and whether or not they tried to mate with the orchid.

He found that even though both orchids produce the same chemical attractant, the male wasps of both species behave very differently towards each orchid. In both cases, the males tried to mate more often, and for longer, with the orchid they normally pollinate.

According to De Jager: "We found convincing evidence that morphology matters in sexual deception. This is a novel result as it is the first comprehensive study to reveal an important role for flower morphology in sexual deception."

"This means that future studies on sexually deceptive pollination should also look at pollinator behaviour as a driver of floral trait evolution, as the role of flower morphology may play a much larger role in the pollination of these intriguing orchids than was previously realised."