They have identified a pheromone - a powerful chemical signal - given out by male tree frogs during the breeding season to attract females.

The researchers, writing in the journal Nature, believe it is the first sex pheremone to be identified in frogs. It follows closely on research published two weeks ago which identified the chemical signal a salamander uses to woo his mate. That was thought to be the first identification of a vertebrate pheromone which affects female receptivity.

In this new work, Professor John Bowie and colleagues, of the University of Adelaide, South Australia, studied the male magnificent tree frog, Litoria splendida.

They discovered that males have glands in their heads from which they are able to squirt a molecule into water where the creatures breed.

Splendid chemical

This water soluble molecule, which the researchers have named splendipherin, instantly attracts female L. splendida frogs - even in tiny amounts.

The team tested it out by putting pads soaked in the perfume into special water tanks. When they put a female frog in the tank, she instantly homed in on the chemical from as far away as a metre and went and sat on the pad.

The female frogs moved on to the pads

Just 40 nanograms (billionths of a gram) of splendipherin was sufficient to get a response. Smaller quantities of the pheromone got no reaction and larger ones seemed to confuse the animals.

Pheromones are now a popular subject for research. They are thought to play important roles in species recognition, reproduction and other behaviours.

Quite a few pheromones have been identified for insects, but only now are scientists beginning to describe these signalling molecules in vertebrate animal species.

Even in humans, pheromones are at work. Research has shown that the females of our species who spend a great deal of time in one another's company often send out chemical signals that help them synchronise their menstrual cycles.