The swift-running, flightless Australian emu is helping scientists piece together the puzzle on the respective roles that humans and natural climatic changes have had on the Australian continent over the millennia. Emus eat a variety of different plants, which carry different compositions of carbon isotopes. The isotopes stay intact once metabolized and are found in the shells of the eggs that the animals lay. This information is used by the scientists as a marker to reveal aspects of the past climate and vegetation of Australia.

Scientists Beverly J. Johnson and Marilyn L. Fogel of the Geophysical Laboratory of the Carnegie Institution of Washington, G. H. Miller of the University of Colorado and others* report their findings in the May 14, 1999, issue of Science. The researchers establish, using contemporary eggshell samples, that the carbon isotopes varied in response to the inclusion of certain grasses in the diet of the emu. The grasses of Australia fall into two distinct sets based on their photosynthetic pathways. One set converts carbon dioxide to a compound with 3 carbon atoms ( C3) during the process of photosynthesis, while the other changes the CO2 into a compound with four atoms of carbon (C4). The distribution of the C3 and C4 grasses over the continent depends on the season of rainfall-C3 grasses dominate southern Australia where it rains in the winter and has cooler temperatures, while C4 grasses are dominant in northern Australia with summer precipitation and warmer temperatures.

After dating and analyzing fossilized eggshell samples from the Lake Eyre Basin of Australia, the investigators conclude that between 65,000 and 45,000 years ago the C4 grasses were abundant and readily available for the emu to consume during its breeding season. The summer rainy season influenced by the monsoon from the north, they argue, was more extensive at this time, so that C4 grasses grown in summer were still abundant when emu were laying their eggs in
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Contact: Marylin Fogel
Fogel@gl.ciw.edu
202 686-2410 x 2480
Carnegie Institution
14-May-1999