Not long after the sun goes down, pairs of burying beetles, or Nicrophorus orbicollis, begin looking for corpses.
For these beetles, this is not some macabre activity; it’s house-hunting, and they are in search of the perfect corpse to start a family in. They can sense a good find from miles away, because carrion serves as a food source for countless members of nature’s clean-up crew. But because these beetles want to live in these corpses, they don’t want to share their discovery. As a result, burying beetles have clever ways of claiming their decaying prize all for themselves. In new research published in The American Naturalist, researchers from UConn and The University of Bayreuth have found these beetles recruit microbes to help throw rivals off the scent.
Immediately following the death of an organism, decomposition begins. The body’s building processes cease, and microbes begin un-building and recycling the corpse. Proteins, composed of amino acid building blocks, are broken down and release egg-y, onion-y, and other well-recognized scents that signal death to some, and opportunity to others. For burying beetles, the scents mean one thing: “home, sweet home.”
“Burying beetles are really unusual because of their parental care, which is uncommon in beetles, and carcass preparation is just one expression of their parental care,” says lead author and UConn professor of Ecology and Evolutionary Biology Stephen Trumbo.
The problem is, rival carrion feeders sense these same malodorous cues, and the researchers were interested to see how burying beetles contend with this problem.
“Of course, when the beetles find the carcass they are using these odor cues, these same cues are being given off while they begin to use this resource,” says Trumbo.
Trumbo explains that the team became interested in this odor controlling behavior and explored the idea that perhaps the beetles were influencing the microbes somehow through the course of processing and preparing the carcass, which he says is one aspect of parental care that has not been considered before.
These experiments began with some puzzling fieldwork. The initial studies were centered about putting various compounds into tubes, poking small holes in the tubes, and placing the tubes next to a dead mouse to see if the compounds in the tubes influenced whether insects found the carcass. Trumbo explains that they started by using compounds known to attract carrion feeding insects and therefore expected those compounds to attract more insects than others,
“We knew a few of the compounds that animals use to find the dead animal, they’re mainly sulfur-based compounds and we tested some of them and we weren’t getting great results. The ‘major compounds’ did not have a great effect.”
The team decided to let the beetles dig deeper into the question for them by exploring differences between human-prepared and beetle-prepared carcasses (see side bar). Samples of microbially-derived volatile chemicals given off during decomposition were collected and analyzed by colleagues in Germany using gas chromatography-mass spectrometry. The differences seen with the beetle-prepared carcasses were surprising,
“We found a couple of compounds that really knocked scent down tremendously and these weren’t known to be important before. So in a sense we let the beetles tell us what is important because the beetle is saying ‘Ok I need to knock these odors down a lot’.”
The researchers found a 20-fold reduction in a compound called methyl thiocyanate, an attractant, when the carcasses were prepared by beetle parents-to-be. They also found that these carcasses emitted an increased amount of dimethyl tri-sulfide (DMTS), a deterrent. In all, the burying beetle-prepared carrion not only emitted less of the attractant, but emitted more of the deterrent leading the researchers to conclude the beetles are actively deceiving their counterpart undertakers.
“We used to think the beetles were sterilizing the mouse with their preparation process, but that is not what they’re doing at all, they’re completely changing the microbial community and therefore the odors that are released. They are changing and in a sense controlling the more aggressive microbes,” says Trumbo.
“The beetles are able to fool the rivals by manipulating the odor profile so they do not recognize the carcass. They put out information to misdirect rival individuals.”
Burying beetles are resource specialists, meaning they need to have the right conditions to carry out their life cycle — so without flexibility in conditions, Trumbo says they engage in an active disinformation campaign to mislead rival carrion feeders. By ensuring they have the carcass all to themselves, the parents are better able to provide their young with enough food and safety from other insects who may want to eat their young, and therefore increase the likelihood their brood will make it to adulthood.
“We are finding all these elaborate details about our gut microbes and co-adaptation with gut microbes and specializations. Anytime you have a resource specialist, like the burying beetle, they are going to encounter a similar microbial community every time they find their resource, then they might have some really complex adaptations as well,” says Trumbo.
Further work is needed to discern which microbes are recruited or controlled in the preparation process, but the results show that preparation alters the microbial-derived cues to mask the information from other beetles. Trumbo and his colleagues are now looking at cues in different species of burying beetles, and he says they have been approached by another research team hoping to help conserve the endangered species of American burying beetle, where conservationists may be able to use the chemical cues Trumbo’s team found for conservation efforts.
“We know microbes are producing volatile chemicals that signal all sorts of things,” says Trumbo. “These microbial volatiles are all over the place but we do not know much about how animals manipulate them. This is an interesting case of deception in the animal world.”
Collaborators include Paula Philbrick of UConn and Sandra Steiger and Johannes Stökl of The University of Bayreuth. The research was supported by The University of Connecticut Research Foundation and by the German Research Foundation.