Looking for lunch

AS EVERY schoolboy knows, insects pollinate flowers, while birds and mammals disperse seeds by eating fruits or transporting burs on their feathers and fur. These are examples of co-evolution, a phenomenon first described by Charles Darwin in “The Descent of Man, and Selection in Relation to Sex”, published in 1871.

Co-evolution, in which two species have evolved together in response to adaptations that each has imposed on the other, can be extremely subtle. For instance, some animals may help to transfer pollen from one plant to another without acting directly as the carrier themselves. Instead, they make it easier for the pollen to be dispersed by other creatures. Mats Olsson and Richard Shine, of the University of Sydney, and Elisabeth Bak-Olsson, of the University of Gothenburg in Sweden, have discovered, apparently for the first time, such a mutually beneficial arrangement between a reptile and a plant.

For most of the year the Tasmanian snow skink—a lizard confined to mountain tops in that island—is catholic in what it eats. But when the honey-bush is in bloom, the skinks make a point of tearing off the tough, red petals that enclose the rest of the flower. They chew them to extract the nectar and spit out what is left. At first sight, this skink-savaging may not seem all that helpful to the honey-bush. But it is. It exposes the reproductive parts of the flowers, thereby allowing pollinating insects to get at them. Dr Olsson and his colleagues found that flowers with the petals left intact never produced seed. But, according to their paper in the Biological Journal of the Linnean Society, 87% of flowers with the petals torn off did so.

To check that it was the skinks alone that were the plants' little helpers, the researchers placed cages around some honey-bushes. Virtually all of the petals were removed from the flowers on bushes without cages. Only 16% of the caged bushes had their petals ripped off, possibly by high winds battering them.

Dr Olsson found no evidence of pollen being carried on a skink and so concluded that the lizards play no role in transporting honey-bush pollen from one flower to another. But he did see a range of insects—wasps, flies, bumblebees and others—feeding from honey-bush flowers that had no petals covering them. Without the petals removed, it was impossible for insects to do any pollinating.

Why did the honey-bush evolve such a complicated procedure for its pollination? Dr Olsson thinks the answer lies in upland Tasmania's awful—and unpredictable—weather. It is only worthwhile for insect-pollinated plants to open their flowers when the air is warm enough for insects to be flying. Most plants outside the tropics rely on the predictability of seasonal weather to achieve this.

The honey-bush employs the cold-blooded lizards as its thermometers instead. The snow skink needs warm, sunny weather to become active, so the honey-bush's petals are torn off only in such weather. The bush and the skink have thus co-evolved to help each other. In effect, the plants use hired help, paying in nectar for the lizard's labour.