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Of packrat poop, creosote bush and juniper-fed lamb

Scientists find that gut bacteria can help animals digest toxic plants.


How can herbivores survive munching on poisonous plants? A new study helps prove something that scientists have long suspected -- that it’s not just enzymes in an animal's liver that break down toxic compounds in plants, but that bacteria in its guts also play a major role.

As an aside, if you’re wondering why an animal would deliberately eat a naturally toxic plant, that happens for several reasons – perhaps the plant contains a lot of nutrients, or it tastes good, or both (for example, larkspur and locoweed, which plague Western cattle growers). Or maybe better-quality forage isn’t available and the animal is desperate.

The new University of Utah study focused on the woodrat (Neotoma lepida, also called packrat), found in Western deserts. In desert areas where creosote bushes grow, the bushy-tailed rodents eat it and thrive, despite a toxic resin in the leaves that can cause kidney cysts and cancer. Apparently the creatures acquired this talent some 17,000 years ago, according to a press release about the study:

“In a natural climatic event at the end of the last glacial period, the Southwest dried out and our major deserts were formed,” the study’s senior author, Denise Dearing, professor and chair of biology, says. Creosote, which was native to Mexico, moved north into the Mojave Desert and replaced juniper there, but did not go farther north into Great Basin deserts. Desert woodrats in the Mojave started eating creosote bushes, while desert woodrats in the Great Basin kept eating toxic juniper, to which they had adapted earlier."

Eventually, scientists believe, those ancient Mojave woodrats somehow acquired toxin-degrading microbes that helped them adapt to eating the creosote invading  their habitat. To test that idea, the researchers gave antibiotics to captive creosote-eating woodrats, and found that once the microbes in their guts were dead, the animals lost their ability to detoxify creosote resin. They regained it when they ingested feces from other woodrats that could handle eating creosote. Apparently eating each others’ poop is something that woodrats normally do; such feces-eating transfers microbes from one woodrat digestive system to another. When woodrats that had never encountered creosote ate the feces of creosote-eaters, they also became able to detoxify and digest the plants.

The study, funded by the National Science Foundation, raises some important questions, note its authors:

"– Endangered species may lose diversity of their gut microbes when they are bred in captivity. When they are released to the wild, does that leave them unable to consume toxic plants that once were on their menu?

– Livestock like cows often are fed antibiotics to promote growth. Does that impair their ability to eat toxic plants like locoweed when drought reduces pasture grass?

– The study of woodrats someday might impact farming practices in arid regions, where toxic plants like creosote and juniper are abundant. Livestock now can’t graze on these cheap food sources. Could interspecies transplants of gut microbes help livestock expand their dining menu? Kevin Kohl, a researcher at the University of Utah, says he’d like to transplant woodrat gut microbes into sheep or goats to find out if that increases their tolerance to toxic foods. 'Juniper is expanding its range, and ecologists and land managers are concerned,' he says. 'Farmers are interested in getting their sheep and goats to eat juniper.' "

Photograph of a woodrat courtesy of University of Utah.

The woodrat study is just one more bit of evidence showing that an organism’s  microbiome functions in many ways essential to its digestion, immunity and general well-being. The average human contains up to three pounds of bacteria and other microorganisms, which colonize our skin and gut. Scientists are continually learning more about those microbes; for example, Science News just reported on new research showing a link between sugar-loving gut microbes and colon cancer.  And UCLA researchers recently found that bacteria in food can change brain function, perhaps suggesting eventual treatments for autism, Alzheimer’s and Parkinson’s diseases.

On the wildlife front, HCN reported last year on researchers trying to fight fungal diseases with beneficial bacteria. Scientists are experimenting with using bacteria to slow the growth of chytrid fungus, which has killed millions of frogs and toads across the globe, and to combat white-nose fungus, which is destroying bat colonies in the U.S.

It’d be a boon for ranchers indeed if their livestock could suddenly start chowing down on juniper. And who knows, maybe that’d infuse the meat with an interesting taste? Perhaps juniper-fed lamb might become as sought-after as grass-fed beef.

Jodi Peterson is the managing editor of High Country News. She Tweets @Peterson_Jodi.