Desert woodrat, sitting in front of a creosote bush. Credit: Kohl et al, 2014.
Desert woodrat, sitting in front of a creosote bush. Credit: Kohl et al, 2014.

Desert Rat Relies on Microbes To Detoxify Its Deadly Meals

In the Mojave Desert of the western United States, the adorable desert woodrat fills its stomach with deadly poison. This rodent feeds on the creosote bush, a low-lying shrub whose leaves are covered in a toxic resin. The resin is a cocktail of hundred of chemicals and chief among them is nordihydroguaiaretic acid or NDGA—a chemical that wrecks the liver and kidneys of lab mice. But the woodrat doesn’t seem to mind. Every single day, it swallows resin in amounts that would kill a normal mouse.

When Kevin Kohl learned about the woodrat, he wondered if bacteria in the rodent’s gut might help it to tolerate its otherwise lethal diet. After all, bacteria are such biochemical virtuosos that they can detoxify everything from crude oil to uranium. And since every mammal has trillions of microbes in our guts, it’s perfectly plausible that some of these could help plant-eaters to neutralise the poisons that they ingest. The idea made sense. Kohl just needed to test it.

Nature gave him a hand. Around 17,000 years ago, changing climates allow the creosote bush to move up from Mexico and expand its range into the southern United States. When it reached the Mojave desert, it came within the jaws of the desert woodrat, which started eating it. But the bush never made it into the northern Great Basin desert, and the woodrats that live there have never encountered it before.

Kohl showed the experienced Mojave woodrats have different gut microbes than the naive Great Basin ones. These bacterial communities also react differently to an influx of creosote toxins: the naive ones shrink away, while the experienced one become more diverse and abundant, and switch on genes that help them to break down the toxins in the resin.

To confirm that these microbes are important, Kohl killed them with antibiotics. Afterwards, the woodrats could all still eat normal laboratory chow. But when they were fed with creosote, they couldn’t tolerate the resin and lost a lot of weight. Within two weeks, all of them had lost 10 percent of their weight and were removed from the experiment. When Kohl removed their microbes, the experienced woodrats couldn’t even handle the tiny levels of creosote that their naive cousins can. “[It] effectively removed 17,000 years of ecological and evolutionary experience with creosote compounds,” he wrote.

Conversely, Kohl managed to transform naive woodrats into creosote-busters by infusing them with the microbes of their more experienced cousins. He did this by grinding up the faeces of the experienced individuals and feeding it to the naive ones, mimicking what the rodents naturally do in the wild.

The transplanted microbes successfully colonised the guts of their new hosts, and imparted their detoxifying powers. Kohl could quickly tell: creosote toxins discolour the urine of a woodrat and make it darker, but these recipients rodents were destroying enough of the toxins that within three days of the transplant, their wee was a normal colour. And sure enough, they put on more weight and survived for longer than other naive rats.

Like a lot of other research on gut microbes, these experiments bring new evidence to old ideas. Forty years ago, ecologists W. J. Freeland and Daniel Janzen hypothesised that plant-eating mammals might rely on their microbes to break down toxins in their diet. In the decades since, other researchers have found examples that support Freeland and Janzen’s idea.

Reindeer, for example, can eat a type of lichen called “reindeer moss”, which contains a powerful poison called usnic acid. The deer swallow lot of usnic acid, but there’s barely a trace of it in their waste—something must be breaking it down. Gut microbes are the obvious candidates, but no one knows if they are truly responsible.

There’s better evidence that bacteria help cud-chewing animals from Central and South America to detoxify mimosine, a compound produced by a local tree called Leucaena. The microbe responsible is called Synergistes jonesii, after Australian scientist Raymond Jones, who worked tirelessly to identify and study it. Thanks to his efforts, farmers can now buy S.jonesii to feed to their cattle, allowing them to happily much away on a diet of easily grown Leucaena.

These cattle, just like the woodrats that received the microbe transplants, could instantly eat a new type of food. If this change was due to mutations in the animals’ own genomes, it would have taken many generations to evolve. But by relying on microbes that already have the right ability, the animals could immediately broaden their diets.

Perhaps this happens in the wild all the time? This is all conjecture, but imagine a woodrat moving into a new area filled with unfamiliar plants. It takes a mouthful, and picks up microbes that live on the surface of the plants—maybe these have already evolved to deal with any chemical defences within. They find bits of faeces on the ground, and eat them—maybe they pick up gut microbes from local animals that have already adapted to deal with local plants.

Reference: Kohl, Weiss, Cox, Dale & Dearing. 2014. Gut microbes of mammalian herbivores facilitate intake of plant toxins. Ecology Letters

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