Day 76: Woodrats eat poison to live

A woodrat in Utah's Great Basin is surrounded by toxic juniper leaves, which make much of its diet. Credit: Denise Dearing, University of Utah

Thousands of years ago, the Earth went through a warming phase that rivaled the drama of modern climate change predictions. And at least one species adapted its diet to survive.

Between 18,000 and 10,000 years ago, an ice age was ending. Juniper trees vanished from what is now the Mojave Desert, in the southern reaches of Utah, Nevada and California, and toxic creosote bushes grew in their wake. 

The woodrat, a species that had used juniper as a staple food source, faced a challenge. 

“It was either eat it or move out,” says biologist Denise Dearing of the University of Utah. The small critters adapted, and they remain common in many western habitats to this day — including the Mojave.

Dearing and her colleagues have narrowed the hunt for detox genes that let the rodents eat the creosote. Their results appeared this week in the journal Molecular Ecology.

Dearing and her colleagues got interested in the work because “we don’t really know how herbivores can feed on toxic diets,” she said. “If we can understand it, we may be able to learn how they will deal with climate change.”

Creosote could make more toxins under increased carbon dioxide conditions, for example.

During the study, eight woodrats were captured from each of two western regions: the Mojave Desert, where creosote still dominates, and the cooler Great Basin — where juniper trees held on. Rats from both areas were fed rabbit chow mixed with either creosote or juniper.

The scientists then scanned the rodents’ genetic blueprints to look for active genes known as “biotransformation genes” because they produce liver enzymes to detoxify the poisons in creosote and the less-toxic juniper.

“We found 24 genes in woodrats from the Mojave Desert that could be key in allowing them to consume leaves from creosote bushes,” Dearing said. “The leaves are coated with a toxic resin that can comprise up to 24 percent of the dry weight of the plant.”

Even with detoxification genes, creosote bush is so toxic the woodrats can eat only so much. When they eat it exclusively, in winter, they lose weight. In spring, they gain weight when they also eat other plants.

Juniper is also toxic, but not as much as creosote.

Dearing says that as creosote invaded thousands of years ago, either some woodrats already had genes to let them eat it, or there was a mutation in existing detox genes that allowed for creosote consumption.

Over time, Mojave woodrats with those genes were more likely to survive on creosote, while those in the Great Basin stuck to juniper.

Sources: A Eurekalert press release and the study abstract.