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Burying beetles are good parents.

Burying beetles mix a special growth potion for their young: one part dead mice, many parts bacteria

GRONINGEN, THE NETHERLANDS—Some parents will go to any length to help their children. But even the best helicopter moms and dads have nothing on the burying beetle. A team at the University of Cambridge in the United Kingdom has discovered that the reclusive 15-millimeter insect, which lays its eggs underground next to small animal corpses, uses a slime cocktail to manipulate the number and kinds of microbes its young are exposed to as they eat the provided meat. Over time, that controlled exposure leads to larger, healthier larvae. This is one of the first examples of a beetle tweaking its local microbiome, and it could shed light on how our own immune system interacts with gut bacteria.

“We tend to think of bacteria as the enemy, but actually they are not,” says Jon Richardson, a graduate student at the University of Edinburgh, who studies how burying beetle parents care for their young in different circumstances. The new work, he adds, supports the growing appreciation of the beneficial roles microbes play.

Burying beetles (Nicrophorus vespilloides) live in European forests and in North American bogs, where they scour the landscape for dead mice and birds. After stripping the carcasses of feathers or fur, they roll them into neat balls which they bury along with their eggs and themselves in an underground nest. The parents also eat the mouse or bird guts and cover the “meatballs” in a special antimicrobial slime cocktail, which scientists suspect slows decay. When the eggs hatch, the larvae munch on the ball while the parents feed them and protect the nest from other beetles.

Curious about these antimicrobial behaviors, evolutionary biologist Rebecca Kilner at Cambridge, along with her postdoc Ana Duarte, assessed the numbers and kinds of bacteria on the carcasses. Many times over, Duarte put a dead mouse in a small box with a pair of burying beetles. A week later, nothing remained but bones and the larvae. In between, at both 24 and 72 hours after the start of the experiment, the duo sampled what bacteria were present in what amounts. As a control, they also sampled boxes with dead rodents but without any beetles inside.   

“To our complete astonishment, beetle-prepared carcasses had a greater bacterial load” than carcasses left to rot on their own, Kilner reported this week here at the XIV Congress of the European Society of Evolutionary Biology. When she and Duarte analyzed the microbes present, they realized the slime selectively killed some microbes, including fungi, but promoted the growth of others, perhaps to keep bacteria dangerous to the beetles or their larvae from settling on the carcass or to help the larvae better digest the meat. Some of the favored microbes come from the beetles’ own gut. Others came from the mouse gut bacteria, suggesting adult beetles regurgitate some of the microbes they consumed from the carcass. These results also appeared last month in the Journal of Animal Ecology.

“The work fits well in the growing consensus that microbes provide crucial and often unexpected benefits to their hosts,” says Daniel Rozen, an evolutionary biologist at Leiden University in the Netherlands. “Burying beetles are ideal for studying [this] because beetle parents and larvae are intimately and unavoidably associated with bacteria and fungi throughout their development.”

At the meeting, Kilner also reported some new results: Provisioning the beetle larvae with the dead mouse gut microbes seems to help them grow. When she and Duarte raised the burying beetle larvae with germ-free mice, which lack any gut bacteria, the young did not grow as large as those buried with regular mice. “This is very puzzling,” says Rozen, “and [it] poses a clear challenge to the simple notion that bacteria are uniformly bad for beetle larvae.”

His own work looking at the microbes in the guts of burying beetle larvae further challenge this notion. As Rozen’s team reported on 29 June in a preprint on bioRxiv, bacteria native to the beetle larvae guts protect it from pathogens. “The upshot is that it’s all far more complicated than we’ve thought,” he says. “It isn’t just beetles versus microbes, because some microbial species are required to keep larvae healthy. The challenge is to identify which ones and why.”

Kilner adds that there are potential parallels with the way the human immune system interacts with the gut microbiota: “The language in this field is now more about management than warfare.”