Humans trust dogs to help find missing people and bees to pollinate their crops. But scientists still don't fully understand how these animals use their extraordinary sense of smell to locate the source of an odor.
New CU Boulder research reveals that the wind itself plays an important role. As an odor travels, the turbulent air changes the timing of odor whiffs in systematic ways, creating patterns that may be useful to animals navigating toward an odor source.
The findings are part of the international Odor2Action research network, which brings together an interdisciplinary team of scientists from 16 institutions to tackle a core problem in neuroscience: how animals use odors to guide natural behaviors.
Elle Stark, lead author of the study into how wind and air affect odor plumes, poses in front of a computer screen displaying some of the data from the research in a lab at the University of Colorado Boulder. (Credit: Scott Franz)
“By understanding how air and wind transform odor signals, we can better understand animal navigation behavior and eventually mimic it, in applications from search and rescue to locating hazardous chemical leaks,” said Elle Stark, a postdoctoral researcher in the Department of Civil, Environmental and Architectural Engineering and lead author of the study.
Published on the cover of the journal PRX Life, the study bridges physics with neuroscience through a collaboration between CU Boulder engineers and Professor Jonathan Victor, a neuroscientist at Weill Cornell Medicine.
For the study, the team categorized odor signals in terms of frequencies they contain. Kind of like how people can identify songs by recognizing specific frequencies, Stark said, the “frequency content” of an odor signal may help animals to find the source.
The study identified three fundamental ways turbulence transforms odor signals as air carries an odor away from its source. The turbulence filters out some frequencies, spreads others around, and generates new ones.
Imagine, for example, a potent plume coming from a wild Bergamot flower. As odor leaves the flower with the hope of attracting a pollinator, Stark explains, turbulent eddies stretch this plume into filaments that fold, spread, rotate and mix.
When the plume finally reaches a bee, the odor signal is complex and dynamic. “There might be a burst of odor, a short whiff, and then empty air for a while, and animals somehow make sense of all of that, to be able to navigate to the source,” she said.
Scientists call this process “olfactory navigation”, and animals have been getting better at it through evolutionary adaptation for millions of years.
The research team hypothesizes that animals make use of the systematic changes in frequency that happen between the time that an odor leaves a flower and reaches a nose.
“These transformations are likely important ingredients in how animals interpret odor signals to determine distance and direction to the odor source,” said John Crimaldi, professor of civil, environmental and architectural engineering, co-author of the study and lead principal investigator of the Odor2Action network.
Making the invisible visible
Odor plumes are invisible, making them hard to visualize and even harder to study. But what if you could see them?
Crimaldi’s group uses both computer simulations and laboratory experiments to do just that. By applying advanced techniques in experimental fluid mechanics–the branch of physics that studies how liquids and gases move–they have measured the intricate details of how air and odor move together within a plume.
In other work, the team generated real-world odor plumes in a wind tunnel, then shined lasers on them to make the invisible odor visible. When you watch a video of it, it resembles a column of smoke spiraling through the air after a candle wick is extinguished.
The technique allows them to not only visualize the flow and odor evolution but also to convert the images to quantitative datasets that help to shed light on the complex dynamics in these plumes.
In other work, the team generated real-world odor plumes in a wind tunnel, then shined lasers on them to make the invisible odor visible. (Courtesy/Elle Stark)
Sniffing the future
Stark and Crimaldi say understanding how turbulence transforms odor signals is only one step in the sensory process.
“Animals engage in what we call active sensing, which involves moving around through the plume, flicking their antenna, sniffing or other behaviors that modify the odor signal,” Stark said.
The signal is then modified again during neural processing.
“Future research can analyze how each step serves to transform the odor signal and then we can understand how animals use them to navigate.”
By understanding each stage of that process, researchers hope eventually to develop artificial systems that can do more than simply detect odors. Today's electronic noses can determine that a chemical is present, but they generally cannot determine where it originated.
"Our work suggests that by the time an odor reaches an animal, the airflow has already transformed the signal," Crimaldi said. "The next challenge is understanding how the nervous system takes advantage of that transformed information."