This post is written by BEACON managing director Danielle Whittaker
Danielle Whittaker holding a black and white warbler at Mountain Lake Biological Station, Virginia
When we fall in love with someone else, is it because they are our soul mates… or is it because we like the way their microbes smell? We think a lot about the importance of physical appearance and the content of what we say. But when it comes to attraction, we may have less control over our preferences than we think.
Just like humans, birds are thought to rely on sight and sound as their primary senses, yet smell turns out to play an important role in choosing a mate. For the last decade, I have been studying how birds use odors as indicators of a potential mate’s suitability. Dark-eyed juncos (Junco hyemalis) are songbirds found throughout North America that spend the summer breeding in habitats with cooler temperatures, especially in the mountains or far north in Canada. Like most birds, they produce preen oil from their uropygial gland, located above the base of the tail. This oil, which they rub into their feathers while preening, is a source of odor that transmits information about an individual’s species, sex, breeding condition, and hormone levels. This odor also relates to individual reproductive success: males that smell more “male-like” have more offspring, as do females that smell more “female-like,” suggesting that these compounds are also communicating information about reproductive health or ability.
The preen gland is located above the base of the tail
Recently, my collaborator Kevin Theis (former MSU postdoc, now assistant professor at Wayne State) and I have been studying the source of these odors. While at MSU, Kevin studied the bacteria in hyena scent pouches. These bacteria produce the odors that hyenas use to communicate with each other. He suggested to me that symbiotic bacteria in the preen gland could also be responsible for producing junco odors. We decided to test this hypothesis by sampling the bacteria in and around the preen gland, and determining whether any of the bacteria present were capable of producing these compounds. We found that the preen gland is home to a very rich and diverse microbial community. Even better, using the Microbial Volatile Organic Compound database, we discovered that many of these junco bacteria are known odor producers. Two genera in particular, Burkholderia and Pseudomonas, are capable of producing over half of the compounds in junco chemical signals – and these two bacteria were very common and abundant in our samples.
Scanning electron microscope image of bacteria in a junco preen oil sample
Our next step was to test whether removing these bacteria actually changed the juncos’ smell. I injected a broad-spectrum antibiotic into captive juncos’ preen glands, and sampled them before and after treatment. Compared to control birds that were injected with only saline, birds receiving antibiotics had significantly lower levels of three volatile compounds – 2-tridecanone, 2-tetradecanone, and 2-pentadecanone. These three compounds are the same ones that are correlated with reproductive success, suggesting that symbiotic bacteria could be responsible for a chemical signal that’s important in junco mate choice. We are now in the process of sequencing the bacterial swabs from the birds in this study, to examine which bacteria were killed by the antibiotics and to identify candidates responsible for producing the compounds.
Female junco incubating eggs at Mountain Lake Biological Station, Virginia
We are also now studying how these symbiotic microbes are transmitted between individuals. We have found that nestling juncos have bacterial communities very similar to their mothers, and less similar to their fathers. This pattern makes sense because it’s only mothers that sit on the nest and keep the nestlings warm as they are growing, and microbes are shared through physical contact. We also found that the adult male and female pairs were more similar to each other than they were to other adults of the same sex – again, physical contact is the likely explanation. Our next steps are to examine more closely the effects of social behavior on individual microbial communities, and whether an individual’s odor reflects their social patterns.
So the next time you find someone attractive, stop for a moment and wonder why. Is it the way their blue eyes sparkle when they say something witty? Or could it be the scent of bacteria… maybe even bacteria they got from somebody else?
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