The smell of certain gaseous emissions from planktonic algae appears to tell some Antarctic birds that tasty morsels of food may be lurking under the ocean's surface, say researchers from the University of California, Davis, and University of Washington. Identifying naturally occurring, biologically made aromatics has proven elusive, says Gabrielle Nevitt, an assistant professor of neurobiology, physiology and behavior at UC Davis. The study provides clues for understanding how natural odor landscapes aid birds. The study is published in the Aug. 24 issue of the weekly journal Nature by Nevitt and University of Washington researchers Richard Veit and Peter Kareiva. Attracting odor-sensitive seabirds, such as white-chinned petrels and prions, is a chemical known as dimethyl sulfide. It smells vaguely of oysters and the ocean, and may be mildly nauseating to humans. The sulfurous chemical is a metabolic byproduct of phytoplankton, tiny marine algae at the base of the global food chain. More specifically, this scent is produced in response to grazing by zooplankton, slightly larger organisms that feed on the algae. In turn, seabirds and other animals eat the zooplankton -- particularly Antarctic krill. Dimethyl sulfide is better known for its role in global climate regulation and has been intensively studied by many atmospheric scientists and chemists. "To our knowledge, this is the first evidence that dimethyl sulfide is part of the natural olfactory landscape overlying the southern oceans," says Nevitt, who studies the olfactory behavior of ocean animals, including seabirds, salmon and lobster. In controlled behavioral studies conducted on the British icebreaker RRS James Clark Ross, Nevitt and her colleagues found that dimethyl sulfide attracted many species of birds, including white-chinned petrels, prions and two species of storm petrels. In experiments where Nevitt deployed dimethyl-sulfide-scented vegetable oil slicks and control unscented vegetable oil slicks, for example, Wilson's storm petrels were sighted flying twice as often into the scented slicks compared to the control slicks. Just as noteworthy, Nevitt says, is the apparent olfactory apathy of albatrosses and cape petrels, indicated by their landing with equal frequency on the scented and unscented slicks. A second set of experiments compared the response of white-chinned petrels and black-browed albatrosses to aerosol plumes of the sulfurous algae gas. Consistent with the oil slick results, these petrels flew like bloodhounds tracking a scent, and the albatrosseses did not seem to track the scent, Nevitt says. These results also seem to fit well with other aspects of the different species' behaviors and characteristics suggested by other scientists. Albatrosses and cape petrels may be more visual hunters, often foraging by spotting groups of seals, whales and other visually conspicuous seabirds. On the other hand, the storm petrels, white-chinned petrels and prions attracted to the dimethyl sulfide are cryptically colored, visually inconspicuous birds that often hunt at night; anatomical studies by other researchers suggest that smell is more important than sight to these birds. Now that the chemical has been identified as a potent olfactory cue for foraging, Nevitt and Veit hope to further explore its role in other behaviors, such as navigating. Extensive studies of dimethyl sulfide have provided detailed information about seasonal and daily fluctuations of the odorant around the globe. Nevitt has been studying the olfactory landscape of the southern oceans since 1990, when she completed her doctoral degree at the University of Washington, taking regular hiatus from postdoctoral fellowships at Cornell University and the University of Oregon. The breakthrough in her efforts to identify important smells came from a chance meeting of Tim Bates, an atmospheric scientist with the National Oceanic and Atmospheric Administration who had wondered if animals could smell dimethyl sulfide. Bates' charts of dimethyl sulfide concentrations around the globe suggested to Nevitt a "smellscape" of olfactory peaks and valleys that seabirds and other animals may use to find food and perhaps navigate. The research was funded by a National Science Foundation Polar Programs grant.