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Eating Creosote: A Poisonous Response to Climate Change

Genetic mutations are allowing woodrats to eat toxic creosote as climate change makes their preferred diet of juniper harder to find.

The old saying "pick your poison" has never been more apt: Woodrats in the Mojave Desert have taken to eating toxic creosote bushes after the disappearance from the region of juniper trees — once the staple of their diet.

Now, University of Utah biologists are hunting for the so-called "detoxification genes" that enable the rodents to eat toxic substances, in the hope that other animals might be able to make a similar adjustment as climate change comes to dictate their diet.

"We found 24 genes in woodrats from the Mojave Desert that could be key in allowing them to consume leaves from creosote bushes," said biology professor Denise Dearing in a press release announcing her study, published in the journal Molecular Ecology. "The leaves are coated with a toxic resin that can comprise up to 24 percent of the dry weight of the plant."

The researchers captured eight woodrats from the Mojave Desert — which stretches from southwestern Utah to southeastern California — and the cooler Great Basin of Utah and Nevada, and fed them rabbit chow mixed either with creosote or juniper. Using complex microarrays, scanning pieces of 40,000 lab rat genes, the scientists then tried to identify the "biotransformation genes" that produce liver enzymes to detoxify the ingested poisons. If the creosote-detoxification genes are pinpointed, they might someday be used to modify other grazing animals — including cattle or sheep — so they, too, could eat creosote.

It's important to note that the creosote munched by woodrats is a resin, different from the preservative humans use to coat power poles or bridges. It's still toxic enough to the packrats that they lose weight in winter, when creosote bush represents their only food source, and regain it in the spring, when they can supplement their creosote cravings. (Juniper, like many plants that seek to ward off predators, is also toxic, but slightly less so.)

Woodrats make ideal study subjects because their ancient middens — nests composed of plant and food fragments, dung and small rocks — are extremely well preserved. Juniper pollen in middens throughout the Southwest shows that it formed the woodrats' main diet before the climate warmed, but creosote pollen began appearing about 10,000 years ago, indicating the bushes had begun to displace the juniper trees by then.

"I see this as a tale of two poisons or a tale of two diets — two populations of the same species that feed on very different staple foods: creosote and juniper," Dearing said. "It's like suddenly having your food coated with a different poison than you are used to. It's interesting to see the response of these animals to a natural climate change event where they were forced to change their diet and adapt to a new type of toxin."