So that's why the koala crossed the road

The mystery of why koalas snub some eucalyptus trees and favour others, often crossing busy highways to reach them, may have been solved: they are seeking out leaves containing low quantities of a chemical that makes them feel sick, according to research.

The mystery of why koalas snub some eucalyptus trees and favour others, often crossing busy highways to reach them, may have been solved: they are seeking out leaves containing low quantities of a chemical that makes them feel sick, according to research.

Zoologists have long been baffled as to why koalas eat from particular trees, avoiding others of the same species. Now scientists at the Australian National University in Canberra have established that levels of formyl phloroglucinol compounds - known as FPCs - play a role in determining which leaves are eaten.

The digestive systems of koalas enable them to metabolise some of the poisons found in gum leaves, but the FPCs damage their small intestines and cause nausea. One of the aromatic oils that give eucalyptus trees their distinctive smell is said to alert the animals to the presence of the chemical.

William Foley, a scientist at the university, said: "We have identified this chemical in particular eucalyptus trees which stopped the koalas from eating them."

Koalas are said to be the fussiest eaters of all animal species - they sift through 20lb of leaves a day in order to find the 1lb of sustenance that they need and fancy - which is partly why their numbers have declined to perilously low levels. There are fewer than 100,000 koalas left in Australia.