Desert woodrats switch one dietary poison f...(plants may make more toxins under increa...)

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 bush.

Before creosote came to the Southwest from South America, woodrats ate juniper throughout what is now the Great Basin of Utah and Nevada and the Mojave Desert of southwestern Utah, southern Nevada and inland southeastern California.

But things changed as the last Ice Age waned.

As the glaciers receded, creosote bush invaded. The Southwestern deserts formed as the land became hotter and drier, and creosote bush replaced juniper trees in those areas.

Dearing says that as creosote invaded, some woodrats already had genes to let them eat creosote, or there was a mutation in existing detoxification 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.

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